Briggs and stratton Model 5s Modification

I've been collecting these engines for quite a few years and always wanted to try modifying one. Now, I have probably 8 or more of them and a decent amount of parts to work with.
First of all, what the heck is a model 5s?
Made between 1949-1957
Single cylinder cast iron block.
2" bore X 1.5" Stroke.
1hp @ 3600rpm

This is the first one I bought and restored (been sitting for a couple years now). The engine is based on a family of 2" bore engines, all with basicly the same block since the 1930's. Model WM, WMB, and a few others which I don't remember. The 's' stands for suction carburetor.


By the time production ended in 1957, Briggs and stratton had turned out 1,740,106 of them. One and three quarter MILLION engines. That being said, there are a decent number of them still laying around. I think the majority of them were used to power reel type lawn mowers. The one I restored is on a small walk behind cultivator.


The carbs are extremely simple. A small tab is pulled for a choke. Only a single jet and a tube that sticks down into the gas tank which works on vaccum. Except for the throttle, there are not really any moving parts. Pretty much all of them that I know of had an air vane governor. 5s's had a bunch of different options while the engine block itself is always the same. Aluminum and cast iron heads, sumps, clear plastic or tin oil bath air filters, oil fill caps. One I have even has a small oil pump instead of a dipper on the rod.


Here I have some of the collection dug out. A couple pictured are not 5s's, but 6s's, which we'll get to in a minute. Notice the engine to the far right has an aluminum head and the two closest on the left have cast iron heads. Also, on the far left, some didn't come with a flywheel fin screen. Some of the variation in parts doesn't make sense from a mass production standpoint. For instance the aluminum and cast iron heads. When I look up the production date from the serial number, the parts are all over the place. It's not like they decided to switch from cast to aluminum half way through production. I've found aluminum heads on earlier engines and later ones. Some of the late production 5s's did have recoil starters, but I think these are rare and I've never seen one in person.


Here we have a model 5s block on the right and a model 6s block on the left. The model 6s is almost the same engine except it has 1/2" longer stroke. 2" bore X 2" stroke instead of 2" X 1.5". The block is taller, crank throw longer, and longer connecting rod. Also I think the valves are longer. Otherwise, all the other parts are interchangable as far as I know. I have a few of these also, but only about four total and maybe only one complete.



When I get a chance, the idea is to build one of these engines up and stick it on a restored reel mower. I've had a lot of ideas, possibly converting to overhead valves, dual plug flathead using an opposed twin coil, weld up my own performance cam, mini supercharger. The engine blocks are super rugged for how small they are and I think could stand up to quite a bit. The good news is that some parts are interchangable with the 3-3.5hp briggs flatheads. The mounting holes are the same on the 5s, 3.5hp, and 5hp flathead briggs engines. Also, the crankshaft taper is the same on all three of those engines. Meaning......I can buy a billet flywheel to fit this. These have point ignition, but the coil is the same size as newer engines and you can convert them over as easy as you can bolt on another coil. I've thought of getting one bored out some and doing some machine work, but I need to come up with a plan first. If I'm not going to bore one out, I need to find the best block I have to start with. These things are from the days of never throwing things out, so many of them are really worn out and use more oil than gas. Thought of trying to make a stroker with a 6s crank and a shorter 5s rod. Not quite a perfect fit. It's almost possible to swap in a 3hp crank and rod, but that's a story by itself. Once I get more pictures, I'll explain what fits and what doesn't

Those look very similar to my two Suffolk engines that I have. They were made later ('60s) and built in England. I think they were based on the 5s.

I have one in bits which I was going to convert to run on compressed air but never got round to welding up two extra lobes on the camshaft to make it a 2 stroke.

I can post pics if you want.

Yeah if you could take a few pics. It would be interesting to see them.

Sorry for the wait for the pics.

The complete one and the block of the other one:


The complete one (minus carb)


Here are bits of the Work in Progress compressed air conversion:



Any questions, feel free to ask. I look forward to seeing progress on your engine.

Thanks for posting the pics. That's pretty neat, looks like a lot of similarities. Blocks are real close. I'll have to do some research on those and see what the deal is. Do you know what model they are or if there are any numbers anywhere? I haven't had much time to get back on this project yet, been working on other things.

I found the rare base with the oil pump in it. In the top of the picture is a normal base which is aluminum. The one with the oil pump is cast iron. It's geared off the cam and has a small sump with a screen over it. Just sprays the oil straight up at the crank.


Your basic 5s block from the output side. The hole to the right is for models with a kick starter or hand lever starter. On those models the hole would have a large pin pressed in. Compared to newer aluminum blocks, these are way more rugged than they need to be for 1hp. Should be able to handle pretty much anything you want to jam into it.


Six head bolts. 1/2" exhaust port like a 3hp briggs. Tiny intake port, could be bored out much bigger if you used a better cam and ran higher rpm. I think on 3.5hp briggs flatheads, the intake valve is slightly bigger than the exhaust valve. On 5s's, both valves are the same size as the 3.5hp exhaust valve.


Here's the difference between 3hp parts (on the left) and 5s parts (on the right). The 3hp crank is from an older 3hp, newer 3.5hp engines should be pretty much the same thing (3hp from 1st year 1958? to 3.5hp last year 1999? is pretty much the same engine block). You can see the 3hp piston has a much larger bore, but both engines have about the same stroke. Rods have same wrist pin size but 3hp crank journal is larger. 3hp crank is slightly heavier. No way you can bore the 5s block to fit a 3hp piston. Your almost right in line with the outside of the casting of the cylinder. Main bearings on both engines are the same size.


So the wrist pins and rod length are the same on both the 3hp and the 5s. 3hp rods are a bit heavier duty but the big end bearing is too big to fit the 5s crank. Well, you say, just slap in the 3hp crank and then you can use the 3hp rod and a stock 5s piston and have a bullet proof setup with stock parts. WRONG! Two reasons why we can't do that. First, as you can see better in the above picture, the 3hp crank and cam have straight cut gears while the 5s's have angled cam gears. And cams can't be swapped because the 5s cam is hollow and rides on a pin while the 3hp cam is like most modern small engines you guys have seen, solid with dowels at each end.
The second reason is that while the main bearings are the same size, the 3hp crank is shorter slightly at the ends. When slid into the 5s block, it rides in the bearings ok, but it doesn't reach out far enough to hit the oil seals. On the flywheel side the issue is even worse. The tapper for the flywheel doesn't even stick out far enough to get the flywheel on. So with all that junk, the 3hp crank is obviously not going to be an option. The lines drawn are supposed to show where the edges are, but you can't see it that great.


5s rod on the left, 3hp rod on the right. The 3hp one is a decent amount heavier. I've revved the heck out of a couple 3hp engines without too much problem, so I figured if we could use that rod, with the much lighter 5s piston, you'd have a pretty bullet proof setup for maybe 5000-6000rpm maybe. Who knows, maybe the stock rod would take a lot, but I don't really want to blow up a good 5s block to find out. They used to use similar engines in quarter midgets in the 50's. I might be able to do some research and find out what they did with them if anything. Also look closely and you'll see the 5s rod end cap bolts on flat while the 3hp end cap has notches to help keep it straight.


Not only is the webbing thicker, but the beam widens out more at the ends of the 3hp rod (on the right).


The journal is just slightly larger on the 3hp rod. I was thinking you might be able to build up that much if you used babbit and made a bearing from that. If you could get it sized down to fit the stock 5s crank, then it would bolt right in. Another option would be getting someone to mill out a custom billet rod for it, which I think wouldn't cost more than $200. No idea. Need to call around and find some place that does machine work.


And also matthewd, looked up some video's of those suffolk engines running. Seems like they made a 70 something cc one and 90 something. These 5s's are about 74-75 If I remember correctly. I checked one a long time ago. Strange how similar the blocks are.

Suffolk actually based their original design on the 5s which is why they are so similar. I done some research and the only difference I can find in the block is that "Suffolk Iron Foundry" is stamped into the side of the block of the Suffolk engines. There is a plate on the side of the shroud that indicates the engine size etc. I'll upload a picture of that soon.

And Mr.Modified, they made 75cc, 98cc and (I think) 114cc models. Both my engines are 75cc.

As far as I know, some of the later Suffolk engines came with a recoil starter.

I got these two engines from a friend of mine and I think he still has the recoil that came with one of them. They were both sitting in his grandad's shed for 30 years.

Maybe the larger one is also based on the briggs 6s and just has a longer stroke. You think they are pretty much only in England though? Would be neat to check one out.

I got a picture of the tag on the side of the shroud.


Maybe the larger ones were based on the 6s, I don't know. They were made in Suffolk, England so I'd expect you'd only find them in Britan but they may have been exported. Not sure.

Suffolk started off making the 75cc model. The early ones were extremely similar to the 5s. The only difference being the fuel tank and carb

Those carbs look pretty strange compared to anything you'd see here in the US. Kind of a wild looking design.

The carb is a Zenith 13TCA-2 although the earlier ones came with a Zenith 13TCA or a 13TC.

I can't find the little manifold for mine and the carb is quite badly damaged so I'm looking to get a decent second hand one.

Zenith is a pretty well known brand I guess. On older stuff anyway. I picked up a pumper carb with a single diaphram that I think would be about the right size for the super mod 5s. Forgot to take a picture, but I imagine it was off an old chain saw because it has one of those paper/honeycomb type air filters on it. Could even use the air filter too if I could clean it out, it's packed with grease. Has a high and low speed adjustment. Pretty sure it's a lauson.

I don't see an easy way to make an overhead valve or over head cam setup for this thing at the moment with the tools I have to work with right now. So, I think the best idea is to see what we can do with a flathead first. I'm thinking first we need a half decent running bone stock 5s for comparison, maybe make a really crude dyno up for them. So I'll get one running that doesn't smoke super bad and is more or less intact so we can find out what stock ignition timing is ect. Then the Modified one, stock crank, 3hp rod with babbit bearing, stock piston, 5hp flathead light weight racing wrist pin, weld up and grind a cam with some more duration and lift, maybe newer style valves to replace the old pin retainers, heavier springs, vertical shaft mower flywheel with electronic ignition coil, and re-configure the head some. Not a whole lot you can do with the head I don't think. Fairly small combustion chamber as it is. Maybe start off with an aluminum head and lap it in for no head gasket.

Looking into running methanol or nitromethane. Nitro supposed to be somewhat tough to get your hands on. R/C nitro fuel is available in gallons, but it's pretty expensive. Methanol is cheaper I guess, but I wouldn't know where exactly to get that in large enough quantities to use off hand. I forget the perticulars, but there's quite a bit more power to be hand with the same engine just by jetting for methanol or nitro and maybe ignition timing changes. Something like up to around 15% increase with methanol if I remember.

This is the carb I wanted to use. It's a little bigger bore than the stock 5s intake, but I think it would work ok. Fancy vintage air filter on there. I think they used them on chainsaws. Could clean it out and use it if you wanted to stay with a vintage theme. Maybe rinse it out with gas, packed with dirt and grease. Probably remove that ugly tin shield. Might have been a heat shield or something for the fuel line.



I also have another carb which is about the same exact thing except the other one is a little newer and cheaper. It has plastic jets. Venturi is also slightly smaller than this one even though the throttle plate is the same. I tried that other one on a couple different small engines but couldn't get them to run right. I might be able to mix and match parts from both to make something that works.


Sometime soon I'm going to have to get some run video's of a stock model 5s. I came across a small hydraulic pump I may be able to use to make a dyno for these things. I don't have any small water pumps to use for a water brake dynamometer, so I thought maybe a hydraulic pump would do the same job. But to check things at higher rpm, oil might foam up and have issues like that, but I think I could just use thinner oil. What I need is the pump hooked to the engine with a closed loop running through a valve so I can restrict the flow. A gauge to measure how much pressure builds up (and how much load is on the engine). Then I just need an accurate tachometer and I can measure the difference between modifications.

Decided to start off with the project by trying to make a better cam. Here is the stock intake at full lift. Not super impressive. The bottom of the valve almost doesn't even clear the deck of the block. I'm not too concerened with extra lift as I am duration. I'm going to stick a crank in this block and check out the stock cam specs so I have something to compare to.


Carb I intended to use might be a bit too big even with higher rpm's and modified cam.


Both intake and exhaust valves are the same. Notice the area under the seat is about as wide as the seat is. Probably remove the bottom up to the bottom edge of the seat. Also hard to see is the hole in the bottom for the pin type retainer. Not sure if the pins will hold up with stiffer valve springs. I think stock 3hp briggs springs might be a lot heavier and good enough for what I'm doing. 3hp valves are longer and therefore heavier. The heavier the valves, the stiffer springs you have to use to control that extra momentum and prevent float. I'll cut down the stock springs as much as possible.


The huge amount of metal around valve guide takes up most of the port. That could be cut down some if I can find a way to grind it.


Valves are farther away from the cylinder than they could be. Sort of wastes space in the combustion chamber. I guess they needed room in the bottom to space the cam out from the crankshaft. Not much we can do about it. I'm going to try either modifying a stock head or making my own with areas cut out over the valves so when they are at full lift, the bottom of the valve is almost flush with the bottom of the head. At least that was my idea to get the head down a bit lower for more compression. Not all that much room for improvement with a flathead. Maybe keep the head closer around the back of the valves (opposite of the cylinder) where there isn't any flow. You can see where the stock head ends up if you look at where the carbon deposits are.

To get an accurate reading on things, I can't just have the crankshaft hanging in the bearing on one side of the engine. It moves around too much and messes up your readings. So I dug out a side cover for the flywheel side of the engine and cut the bottom off so the engine block can sit flat while I'm changing things around. Bolts onto the block with four bolts. The area around the center is where the ignition points used to be of course.


Now both main bearings support the crank and keep everything nice and straight. Sits flat no problem. Except I forgot that the flywheel would still hit... At least when the flywheel is off I don't have to pull the cover off to set it somewhere.


Since I had a second block on the table anyway, I just set it on top to work on it. The flywheel is a stock 5s flywheel but with all the fins removed. It had broken fins when I got it, so at the time I thought it would be cool to have one smooth like the Jr. dragster guys use. But...I didn't do a very good job and it doesn't look too smooth. All we need it for is checking cam shafts and marking crank degrees on it. You can see in this picture all the cooling air goes through that small opening to the left of where the coil is bolted.


Here I have top dead center marked on the flywheel and the backing plate. I actually filed a groove into the aluminum, but it's just colored with a marker so it's easier to see. These older engines had the coils held on with four bolts instead of two like the modern stuff. But the new style coils still fit using the lower two holes. If you want electronic ignition, all you have to do is bolt on the newer coil and flywheel from a 3.5hp briggs. Electronic ignition flywheels only have a single magnet, while the stock 5s one has two with a space in between. I tried once to see if the 5s flywheel would make a spark with an electronic coil but it doesn't work. However the trigger works isn't the same. You can see in this picture just how far ahead of the coil the magnets are when the piston is at TDC. The two holes to the right are where the plug wire fits through.


Nifty gauge that I found in a box of stuff a guy gave me. Makes it easy to find exactly where TDC is and exactly when valves start to open ect. I'm using a scuffed up junk piston without the rings for checking purposes. That way it slides right out of the bore without tools and it's easy to turn the engine over. So now pretty much everything is setup so I can accurately check the stock cam and then decide how to make the modified one. All I need to do is finish marking up the flywheel 360 degrees. The other thing I did to make checking the cam quicker is to drill out the mounting holes for the cam pin slightly bigger. The 5s cam is hollow and rides on a pin which is pressed into the block. I drilled the holes out just enough that the pin will slide in easy but not enough that the cam moves around. This way I can take the cam in and out without pressing the pin into place every time. Specs on the stock cam coming soon....

Looking around to find someone to make a few billet connecting rods and found these guys. Looks pretty high tech. I'll see if I can get an idea on prices and maybe send them a stock rod for measuring. Then they could just mill them out.
http://rrconnectingrods.com/

Stock 5s cam specs...Measured at the valve with both valves .004 clearance. 7°-37°/38°-11° That is, the intake opens 7° after top dead center and closes 37° after bottom dead center. The exhaust opens 38° before bottom dead center and closes 11° after top dead center. From my research, even most automotive engine stock cams have a bit more duration than this. Makes sense why you can get these engines to idle so low without stalling. Duration is so short that you don't have much issue with intake getting pushed backwards and exhaust sucked back in ect. Keep in mind that the figures I came up with are give or take a few degrees. I should be close, but I'm working with sort of primitive stuff here.

Supposedly about the hottest streetable cam for a car engine in general is something like 40°-70°/75°-35°
That being intake opens 40° BTDC closing 70° ABDC. The exhaust opens 75° BBDC and closes 35° ABDC. Confusing to visualize unless your looking at the engine, at least for me it is. Anyway, there's 36 teeth on the cam gear which amounts to 10° per tooth. I set the cam two teeth advanced in my test engine, which moves the opening points of the valves to about where they open on the imaginary street cam. My plan is to run the cam in this position and weld the lobes up on the flank, or "back" of the lobes to get more duration. I could try getting a bit more lift also, but we are going to have to cut reliefs in the head if we go too far. My idea is that when you weld up the lobe and then try to grind it back smooth with the primitive junk that I'm using, your never going to get it perfect. On the ramp or front of the lobes, of course there is going to be a lot of pressure because your working against the valve spring and trying to get the valve moving from a dead stop. On the flank, the cam is moving away from the valve, so I think there should be much less force on it. So with the weld only added on the back side of the lobes, the tappet is mostly only riding on the nice smooth factory part of the cam. The other thing is that the top of the lobe isn't flat. It's ground with a slight taper to one side to spin the tappet as it lifts it. This keeps wear evenly on everything so things don't get a big groove worn in one spot.

At any rate, I'll see how close I can get to those specs and see how it goes. Maybe it will be too much. If you go to far, you lose all your bottom end. With a small engine that's rated for around 3600rpm max, everything is bottom end in comparison to automotive or motorcycle engines. Something to remember is that the valve isn't instantly at full lift right when it starts to open. So until you get it open enough, there won't be all that much flow. If it's really useless I can always grind some back off to cut it down to something usable. Hopefully the thing will run up to 6-7000 once everything is done.
Stock 5s cam               7°-37°/38°-11°
Custom modified cam 40°-70°/75°-35°

Pics of the hacked up cam once I lay some super weld beads on it!

I cut out the side of my test block so that I can quickly install and remove the cam without removing the flywheel, sidecover, rod, piston, and crankshaft. When I'm grinding the cam into shape, I'll need to check it often. This should make things go a lot quicker.


Painted up the test block so things look professional. Sent the custom connecting rod place an email, waiting to hear back.

i have one of these little guys with 5:1 gear reduction box on it

Probably pretty rare. I think I might have seen one with a gear reduction, but I'm not sure.

I mailed a connecting rod to R&R racing products in Florida. Now we wait and see how expensive custom rods will be.....

I was thinking about trying something like that on one of my oppys, the cam regrinding that is. That's a pretty mild cam your starting out with. Yah, no lift till after tdc, real mild. I think your going to loose idle though at 40 degrees btdc. Should sound pretty mean though. Thanks for sharing, I'd like to see how the cam welds up. Do you know if these cams are iron or steel?

The cam is cast iron. Yeah low speed running might not be so good, but on the other hand, the ramp is pretty slow. The valve may start to move at 40 BTDC, but there won't be much flow until maybe another 10-20 crank degrees. I'm basicly taking a shot in the dark with it. It was just some basic figures that were listed as what might be the hottest cam you would want to try in a street car. Racing cams get even more aggressive. I can always grind more off later if needed, but I didn't want to go to mild either because I'm trying to get some rpm out of it. You can send you stock cam out to a cam grinder and even if they don't make cams for your engine, they will weld your blank up and then grind it. All that sounds expensive though and not as much fun as trying it yourself. The good thing about single cylinders and hand grinding stuff is that you don't have to match it to the next cylinder. That's not to say that you couldn't do a twin cyl cam by hand, but there would be a lot of checking involved to get them matched up. I'm going to try casting my own aluminum cylinder head also, because I want to make some combustion chamber changes that I can't do by just grinding down a stock one. Should be trying that soon.

Welded up the exhaust adapter. Threads only on these engines and there isn't any place to tap holes to bolt a flange onto the port. Usually, I don't like to use thread in exhausts, but in this case I think the port will have more than enough flow. 3.5hp briggs engines have the same exhaust port and since these engines have much less displacement, I think it should have more than enough flow. The flanges are from two opposed twin mufflers. With opposed twin cylinders each at around 300cc if I remember correctly, you would think they would be pretty restrictive on the opposed. The 5s is only about 75cc. Of course I'll weld the rest of the exhaust system to the other flange and bolt them together with an opposed gasket. I'll have to wait to make the rest of the pipe until I start working on the reel mower that the engine will go on so I can see where to put a mount or two.

Got an email today that the stock con rod has made it to florida. Just have to wait to get a price on some billet ones. In the mean time, I decided to mess around with the cylinder head. Didn't get a chance to weld up the cam yet.

Here are both aluminum and cast iron heads. The aluminum heads have more cooling fins, but are pretty much the same. Those tiny threaded holes on the side of the aluminum head were for extra shroud brackets or in some cases a gas tank bracket. Some iron heads had those too. Not really important for this project. I like the look of the iron heads with the fins spaced out more, so even though they might not cool as well, I'm going to try casting one of those first. The wide fin spacing will hopefully be easier to cast because the sand mold will be thicker in between the fins and not be as likely to move when I pour the aluminum in.....I hope.


Combustion chambers pretty much the same. The aluminum head is much smoother and maybe slightly more compression. I've never tried casting anything before, so I'm not going to cast this side (mold will be much easier, only one sided). This side will just be how ever the top of the aluminum ends up and I'll grind it back out by hand.


This is the side obviously that will be face down in the sand mold. I JB welded all the head bolt holes so these areas will end up flat in my casting, then I can drill the holes back out. I ground the bottom of a spark plug flat and glued that up also.


Spark plug threaded in from the bottom to keep sand out of the plug hole and so I have something to hold onto when I pull the head back out of the sand. The aluminum will be from a junk 3.5hp briggs push mower block. I need about half a coke bottles worth of liquid for the head. My head is going to be slightly thicker than the stock ones to allow for slightly higher lift cam and also some other changes. Maybe I'll get some video of the process if I get time. I'm almost ready, just trying to mix up a good batch of casting sand.

Nice! Been a month and a bit since this thread was updated, any more progress? Would like to see how the casting goes