Briggs 5hp Flat head build

I'm putting together a flat head 5hp briggs for a mini bike. I'd like to see how fast I can get one to go just using a centrifugal clutch and a single speed. A friend who has a bunch of mower parts and stuff gave me some used 5hp kart racing parts. I have a standard length Horstman H beam rod which some people say online is stronger than an ARC rod. I don't know about that but it's pretty beefy and should be fine for what I'm doing.


On the left is the Horstman H beam rod and a Briggs raptor 3 piston. On the right is a stock 5hp rod and piston. I don't have rings for the raptor piston and some people have said they burn a lot of oil. Rings are hard to find for the raptor piston anyway, so my plan is to just run a stock one for now. The raptor rings are really thin, at least the oil ring is. I assume they have it designed for less drag and oil consumption is not they're main concern. I don't really mind burning some oil either but I don't want to spend the $18 for rings...This is a budget build.


I have an ex kart racing block. Not sure if it started out as a regular factory I/C block or it was a raptor block. I don't know what the differences are or how to tell. But it's standard bore as far as I know (I have a second one .030 over) Steel sleeve bore, valve guides and lifter bores all have bronze inserts pressed in, and the lifter bore area has been welded up as seen in the picture (the weak point)


Someone did a nice job on the intake and exhaust ports. Top side of the block hasn't been touched. Not sure If I'll remove any material there or not just yet.


Block on the left is the ex race block. Center, a usable aluminum bore block, an older one which had points (pre 1982?). On the right, an aluminum bore block that has a scarred up cylinder. It was on a minibike and when I took it apart, I found the dipper had broken off and was crunched into a ball. Must have ran a bit without oil after that. I'll use the junk one on the right to line up the mounting holes and things in the mini bike frame.


I have plenty of stock parts for a head and side cover, plus tin work ect. And I have the H beam rod, a mild cam, used valves, a piston, good crank. All I need to get is some heavier valve springs and some beefed up automotive type retainers to use and I can start assembling. Pics as I go of course. I'll have some pics of the minibike build too but this will be focused on the engine since we are supposed to be in the engine section...

The race block looks like it has slanted valves. The one on the right looks like the same kind of block as the race block. What makes it a raptor engine I think is the output shaft has a bearing in it for durability. My 3hp on my mini bike is a raptor engine and has the bearing on the output shaft, also has a different head where it uses a Vanguard twin spark plug. Other then that it's just like a regular engine.

I have a 5hp flatty sittin out back I need to go through... interested in seeing what you come up with in your frankenmotor here

The valves on the stock ones are slanted slightly also, I think it just looks more pronounced on the ex race block in the pics. I was thinking the raptor engines maybe had those bronze guides from the factory, but I have no idea.

Pretty sure the bronze guides for the valves are standard and the bronze guides for the lifters were added. Never noticed them on the stock engines for the lifters and I rebuilt quite a few.

The cool bore engines, or whatever Briggs calls them (aluminum bore, non I/C) are just aluminum guides for both the valves and lifters. The I/C engines with steel sleeve have a bronze insert exhaust valve guide only. This is only what I have noticed on the older engines I have seen and there may be exceptions. Also, this is only for the smaller Briggs engines that I've looked at. Opposed twins and 12hp-12.5 singles may be a different story.

At any rate, I think your right about the lifter guides, I have to dig out my .030 over block and see what that one looks like before committing to this one for the build. If I remember correctly, that one has a nicer looking weld on the lifter area.

This should be fun... Id like to mod out a 5hp briggs for a minibike too!

As it turns out, my .030 over block has an uglier looking weld on the lifter bore than this one and also, it was a ball bearing block. I have no ball bearing cover and I have no idea if the cranks are the same. Looks like I'll be building the block pictured.

Another thing I found out, the crane cam has a smaller base circle on the lobes than stock, meaning it requires the use of longer than stock lifters. AND.....I found out some info on it. They were especially mild cams designed to fit WKA rules for something, not sure if it was like a stock class or what, but the cam is probably not a whole lot better than stock and only helps at around 4000rpm. So I'm thinking I could either buy a cam that gives some actual power or for now experiment with welding up my stock cams. I'm guessing the second option...

Knowing you, I think it'll be option two. Lol

The mini bike which the engine will go on, once it's lengthened out and the handlebars are lowered down.


Cam gear with some fancy weight reduction holes. Still a lot more holes to drill.

I've always wondered how much difference a lighter cam makes to the performance of the engine. Does it make it rev quicker because there is less weight?

Cool stuff mr.mod, bevel those holes the right way and you got an extra oil slinger! Haha. That cam gear kinda hard to drill through?

Thanks. The cam is actually pretty easy to drill through because it's cast iron and fairly soft. With a sharp bit you can cut through it pretty quick. I had the whole cam set in a block of wood so I can hold it straight in the drill press.

As far as performance gains, quicker acceleration would be the only advantage if any. Some people in a Facebook group were talking about lightening cams and someone suggested they thought that a heavier cam gear might smooth out the valve operation some due to the flywheel effect it has. My own opinion is that lighter is still better. I think this person was thinking as the cam lobes get over center, the spring pressure pushes the cam faster than the crankshaft is turning it. I doubt that occurs, especially with stock springs. Another thing is slack in the timing gears is very minimal. Briggs makes plastic gears and lobes for newer engines from the factory, so it certainly can't hurt anything.

But my goal was just quicker acceleration. The gear being lighter means you don't have to spin up all that weight to speed. It's possible that any gains made from removing weight is lost from oil drag with the holes being there. Who knows. What's important mostly is that it looks really cool when we're done! And if it doesn't work it will always look good on the shelf.

How did you layout the holes?

My concern would be balance... a harmonic vibration in that gear could lead to cracks in the shaft

I eye balled it. Started with the inner ones. There is a factory hole drilled maybe for holding it when they grind it, not sure. Anyway, I made my first hole opposite of the factory one. Then I made two more between those holes. That gave me 4 inners, then did holes in between those, giving me the inside 8. From there I just marked out the outer ones. If you look close you can see what's left of my sharpie marks. I first mark a line from the inside out where I want the hole, then to keep them all about the same distance from center, I had a piece of copper wire I bent around the shaft so I could put a mark on it and turn it. That kept everything close. Once I had both marks with the sharpie, I carefully lined up the center punch and gave it a hit. They're off a bit but looks pretty good.

As far as balance, I don't think they have it balanced at all really from the factory, so I just figured keep the holes all spaced apart fairly evenly and that would be close enough. I don't think there's enough weight in play to have enough vibration to cause damage, but any vibration is lost horse power, maybe I could static balance it on the edges of some pieces of steel. What I really need is some better equipment here... Your supposed to have fancy knife edges or whatever and have things all leveled up to do that.

You need some calipers... lol

I do have calipers, not super high tech digital, but it does have a fancy Miami gauge on it.

For some reason, auto correct turns A n a l o g into miami.....why

Working on the test platform for the 5hp. For now I think I'll start with only a single engine, but I wanted room for two if I decided to use two later on. Also a longer wheelbase makes for a more stable machine at speed.


If I remember correctly, I think I added 10" to the frame.


One last section to finish in this picture.

Loookin' good mr.mod. Love the Briggs R/T on the blower shroud! I've never seen a Mopar Briggs before, hahaha.

Thanks. Lol yeah, I did that a long time ago. Turned out pretty decent, I think I free handed that.

That's a pretty sweet freehand. Good job.

Thanks.

I found out I had another standard size raptor 3 piston. As I may have already said, the standard size rings for the raptor piston are no longer available. So with my new mini lathe I recently purchased, I decided to modify the raptor piston to take stock sized rings which are a lot wider. The oil ring especially. I used a cut off blade to widen out the grooves.




Modified piston on the left, normal one on the right. I didn't do the top groove yet because I can't get that one out far enough in the lathe chuck to reach it. I had to shim it out just to do the middle groove. If I get a 4 jaw chuck I can flip the piston around and clamp it on the skirt. But with the 3 jaw, because of the odd shaped skirt which isn't round, I can't hold it backwards.


I lowered the handlebars on the bike and finished the frame with the sections removed from the handlebars.

Wow those grooves really are quite a bit wider. Real nice work on that piston mr.mod!

Thanks. I just thought about the pistons today and the engine I want to build first is an aluminum bore. I think the raptor pistons are only for cast iron sleeve bores as they don't have a chrome finish as far as I know. So for this engine I should probably use the stock piston.

The current engine sat outside at doug facklers for who knows how long and had water in it. Aluminum bore and when I took it apart, the dipper had broken off from someone running it at high rpm I would assume. I'm planning on lightening the valves, Lightened cam with the holes but stock lobes, stock piston that came from this engine, raptor 3 rod (which is pretty much a stock rod with a cast in dipper) milled head and a billet intake.

The idea is to just use it to test out the handling of the bike and get the exhaust and everything working. I ordered a cheap centrifugal clutch. When that comes in and the engine is put together, I can line up the chain and drill mounting holes in the mini bike frame.

Practice Tecumseh head, took off maybe an 1/8" and turned out nice and smooth. The swirl marks are deceiving but it's flat


I used a mill vice for the tecumseh head, but the 5hp briggs is too big and won't fit. So I have to clamp it to the table instead. I never did any of this kind of thing before so I'm just figuring things out as I go. So far I didn't break anything.


Head clamped to the table. I'll have to be more careful this time to not hit the clamps, with the tecumseh head they weren't in the way since I used the vice. Eventually I'll get a video of the operation.


The bit I am using now is the biggest one I have at the moment. Most people just use a fly cutter and make one pass across the whole head. This works good enough to use it, just takes a bit longer.