10hp Briggs Flathead Build (non-SynchroBalanced)

This past summer I picked up a stripped down but not seized flathead from the salvage yard. No flywheel, pulley, coil or head but I figured it would be hard to go wrong for $10. When I opened it up for the first time last night I nearly shat myself when I saw the best case scenario right in front of me... an internally balanced flathead with no risk of sending SynchroBalance plates through the block! Here it is beside a SynchroBalanced 11hp for comparison.

They have the same deck height and externally the only difference I could find were the cooling fins beside the exhaust port, the 11hp are rounded and the 10hp are squared off. These engines are only a year or two apart so age shouldn't be the difference. I'm hoping those squared ports will be a identifier for future trips to scrap yards...

After a bunch of research I was starting to suspect that it was probably a 10hp of the 22xxxx engine family (22CI), I looked at a pic of the tractor I got it from and sure enough that's a 1982ish MTD Lawn Flite 10/34. That along with the "22" on the connecting rod and the mystery is solved!

This whole engine is in really good shape with minimal wear anywhere. The bore is really decent and within spec, this pic shows the worst of the scuffing.

The crankshaft "bearings" and surfaces are excellent as are the crankshaft and connecting rod journals. Im not sure about the valve clearances because I didn't measure before I took the sump off but hopefully they just need a good cleaning and lapping in. The valve heads, seats and contact patch(es) are looking decent for being 40 years old! Lol

Internally I've found that the 11hp cam has about a 0.030" taller lobe on the intake as measured from the stem of the cam, not very scientific I know but it actually looked taller by eye and because I've ground the base circle (sanded actually) of that cam down a hair I couldn't trust a base circle-lobe tip reading. Really I'll need to install them one at a time in the same block and rig something up with a dial gauge to try and quantify the differences. They do have different casting numbers as well, hopefully you can see in this pic what I saw when I looked at them side by side.

How they get their displacement differences in blocks with the same deck height is by stroke and rod length! The pistons compression heights are identical as is their diameter, the 10hp rod is longer by approximately 1/8" (quick measurement) to account for the shorter stroke of the crankshaft. These 2 engines are comparably comparable to the Chevy 327 and 350 as they have the same main similarities/differences and would exhibit the same differences in their characteristics, the smaller revving higher and the larger producing more torque with everything else being equal so there's a fun fact for you!

Then there's that beauty of a balanced crankshaft from the 10hp... I'm still in awe of it. Lol.

Well I'm pretty tired and have to be up early but there's a big little intro to this engine for yous! Over the next few weeks and months I plan on doing and sharing alot of research and building but first things first I need to clean it up, put it together as a basic stock engine using only parts from the 11hp that are required to get it running!

That's all for now!

I'd be interested to know the weight differences between both engines full assembled. Those balancers have some heft to 'em.

Very nice!  I wonder if your detective work on that squared end will turn out to be true?  I bet you are right.  Honestly the 10hp looks beefier in the top side than the 11.

Waiting patiently for your first fire video in the future. 

Me too @AllisKidD21 and that's something I plan on covering with the cranks at the very least, everything else SHOULD weigh up the same. I know the 10hp felt a hair lighter so hopefully it's not my mind playing tricks on me!

Hopefully this weekend @MightyRaze!

Today I started cleaning some crud off the block and doing another inspection, all looks well but this plug under the flywheel is something that intrigued me when building the 11hp and again now...

As you can see it lines up perfectly with a hole going into the "plain" bearing and has me thinking that MAYBE, I could utilize it to deliver a little bit of oil to the bearing using crankcase pressure by installing a small fitting in the bottom of the oil sump? I'll experiment on the dead 11hp and if it looks like it can be done and be easily reversed I think I'll go ahead with it! I mean, it can't hurt to get a little extra oil up to the tipity top of the engine right!?

I'll have a little extra space under the flywheel to try and run a small (copper or nylon) line after I remove the starter ring gear and magnets, this engine will be pull start only so I don't need them or the charge/light coil. The less weight and drag on the engine the better!

I don’t see why you couldn’t, but really the advantage of pressure lube is lubrication from the crankshaft to the connecting rod. On oppys, the upper rod especially. I’d bet that hole is left over from older points ignition setups. The points and condenser on older singles were under the flywheel.

@RichieRichOverdrive for sure it is and it's not like I'd ditch the oil slinger in favor of it I'm just thinking maybe it's a way to get extra oil way up there, that bearing will see alot of extra abuse it wasn't intended to handle.

I respecfully disagree that it has anything to do with points and I understand why you mention that but I'm fairly certain it's just a plug to fill the hole from when they drilled a hole to the upper bearing "column". It's on an angle and in a completely different position than an actual points engine as seen here on my late 60's 7hp, also on my 1977 16hp single the points are on the side of the block and not under the flywheel.

Good thinking though man and I'll definitely also be doing some other oiling mods to the engine like opening up and chamfering the hole in the connecting rod and possibly the drain back hole in the block as well.

Just throwing it out there.  I had an 8hp Briggs that had points under the flywheel.  It's plunger was about where that plug is on your pictures.  Nothing was on the side.


Delivering oil through that hole sounds pretty cool.  Wouldn't have to put out much more than a drip from time to time.

Hate to throw a wrench in your gears @MightyRaze but I'd be willing to bet your 8hp was like my 7hp pictured above, I think I've caused a little confusion because of the angle of the engine in the photo. That plug, when considering the direction of the cylinder head up (12 o'clock position) is at about 7 o'clock compared to the points plunger on the 7hp at 12 o'clock.

This should clear it up, heres the 11hp which is identical to the 10hp, you can actually see the provisions for bolting on the points and the condensor but there's a difference at the 12 o'clock position where the plunger would be. Please also note that with the charge coil in place there would be no room for points.

Was your 8hp horizontal or vertical shaft and about what year? My 7hp horizontal does not have that plug so I'm thinking maybe it was either an update to the oiling system over the years or maybe it's a vertical shaft thing to ensure more oil makes it to the bearing? Let's try to figure this out together!


Heres close ups of both for side by side comparison:

Early 80's vertical shaft

Late 60's horizontal shaft


***Edit*** I found this diagram a few days later while cruising the manual for the engine.

@Brianator I have 12.5 flathead that's is identical to them

@Brianator  It was very early 70's and the plunger was at a 12 o'clock with the head being 12 if I remember right.  Not 7.  I wanted to say it was a horizontal.  I was very young.  I remember it being one of the first engines I played with filing the points it seemed all the time.  LOL.

@Hillbilly mowers not quite identical but darn close, yours is actually a little newer and overall better. What's the model and type? I can't read it in the photo. If that's a decent engine then hold onto it! If it's not I may want a couple of parts off it, let me know.

@MightyRaze I had that problem too until I learned that a good polishing of the surafces and wiping some dielectric grease on them kept them good for a long time! You live, you learn right!? Lol.

@Brianator man what do u need man and I'll ship it to u free of charge man cause I don't mess it and everything on it is in good shape but I don't know about the piston head cause when I got it in on a trade it has the wrong spark plug and when I removed the plugs the tip was smashed up alot where if u open the gap it would broke off but man other that it has very good compression.

@Brianator there the stamp plate of number

@Hillbilly mowers that's a very generous offer but I wouldn't let you ship it for free, the cost to ship that hunk would be quite costly! I will download and inspect the manual later though to see what parts I could actually use if you're willing to strip it down? I'd pay shipping and could return the favour by sending and or making/sending parts you may need for your build.

I know the crankshaft and counter weights are useless to me which are actually half the amount of the weight or more (not taking flywheels into consideration) but I think some of the other parts could be useful, I'll get back to you later about that.

I'm guessing that is the hole in this parts man for a 11hp.



I wonder if it was used to line up that bearing sleeve?

It very well could be part of it @MightyRaze but I am pretty confident that its main purpose is to get oil to the upper half of the bearing and why I thought it could be a difference between vertical and horizontal shaft engines? The horizontal shaft wouldn't have that issue because it would be constantly getting drenched but in a vertical shaft that's a long way up to sling oil!

Here's a couple more close ups.

There's also a drain hole that would drop oil on top of the crankshaft counterweights.

@Brianator man I'm here if u need anything and man I don't care one bit to strip and send u what u need man cause I may need ur your help on some parts some day and man I wouldn't care to man anytimes

The "Silver Bullet" Weight Loss Program (Phase 1) - The Engine

Time to put this thing on a diet! Less weight + less drag = more power = more speed am I right!? Hang on a sec I need to call an old friend...

Hey @AllisKidD21 long time no talk man, how you been?! Merry Christmas and Happy New Years!

AllisKidD21 wrote:

I'd be interested to know the weight differences between both engines full assembled. Those balancers have some heft to 'em.

The results are finally in although not fully assembled (yet) but I have quantified the actual difference of the crankshafts and it's BIG so I'll start with that!

11hp crankshaft                                            7.21lbs
11hp SynchroBalance assemby                +7.44lbs 
11hp Total rotating assemby                 =14.65lbs

10hp crankshaft                                            8.01lbs
11hp rotating assemby                             -14.65lbs
Total weight loss                                        =-6.64lbs

Over 6-1/2 lb difference can you believe that!?


Now on to the rest of it I'm not done yet, now it's time to calculate the full rotational weight reduction:

11-10hp rotating assemby weight loss   6.64lbs
Starter ring gear and magnets               +1.10lbs
Total difference of rotating mass             7.74lbs

So I'm ditching close to 8lbs from nothing but the rotating assembly! Should help this little fella wind up pretty good and fast eh!?

But wait! There's more...

Rotating assembly weight reduction      7.74lbs
Charge coil                                                  0.71lbs
Starter                                                        +4.46lbs
Total                                                            12.91lbs  

After accounting for the pullstart by adding about 2lbs I'm looking at a total weight loss of around 11lbs on the engine alone! Most of that being rotating mass so I'm really happy about that but I'm also pleased about the loss of parasitic drag of removing the charge coil which on a car is a noticeable difference in the form of quicker throttle response. Overall this has to be worth at least 1hp right?!

I spent alot of time trying to format the calculations so they're all lined up nicely and easy to read,  if they're not check back later and they SHOULD be... ***after a few tweaks I got it, on mobile anyhow, hopefully it's good for everyone!***

I have a couple/few pics to come later but I have to get back to workin' on it now! Lol

Nice!  And it looks formatted well on computer.

It was pretty late by the time I finished up last night but I definitely got some good stuff done!

The starter ring gear came off the flywheel easier than I was expecting, I just had to drill 4 rivets and slowly pry around the ring until it popped off!


Funny how 2 magnets let go while firing the 11hp for the first time but the rest of them were stuck on there really well... I managed to get 6 of them off in one piece as spares for the future, the others that broke required a little heat from the propane torch to get off but thats no big deal and it cleaned up pretty well! I know the flywheels are dynamically balanced so I'm hoping I haven't thrown that off TOO much or I'll have to try and figure out a way to rebalance it which I'm sure will be a pain! Lol


I did a little "dry" fitting of the rotating assembly to check for things like valve lift, lash and head clearance. I removed the piston rings and used wd40 as lube for this part, actual assemby will be done with motor oil and Clevite assembly lube (because I already own it).


Here's some numbers to chew on, they are all approximate because I wasn't going to spend alot of time setting up a jig just to get a quick idea on what I'm dealing with.

Stock 10hp Camshaft
Intake ---- Lift .286"  Lash .005"
Exhaust - Lift .279"  Lash .012"

Modified 11hp Camshaft
Intake ---- Lift .339"  Lash .005"
Exhaust - Lift .280"  Lash .012"

How the valve lash turned out identical between 2 cams, one being modded is beyond me but I sure am happy with the difference in lift on the intake side!

With the cylinder head on with no gasket and not torqued down there was about .016" clearance from the top of the valve at it's highest point of lift to the roof of the combustion chamber so I should have plenty of "room" to surface the deck of the block and probably even run a thin head gasket to tighten up the quench (piston to head clearance).

The spark plug sits pretty far in the hole and that's not great for ignition but I can't put a longer standard plug in or the intake valve will smash it so I'll be shopping for a marine style or racing plug because they are designed to flush mount further in the hole and actually raise compression a tiny bit.

I know flatheads respond better to flow than compression so I'll be keeping that in mind as I go along!

What did you wind up doing with your camshaft to modify it? I'm thinking I will cut down the base diameter of the cam on my next oppy build to gain some lift. Challenging on the oppy because I have two match two sets of lobes, kind of doesn't matter much on a single.

@RichieRichOverdrive first of all the 11hp cam had alot more lift on the intake compared to the 10hp cam but based on calculations of keeping track of what I removed from the base circles and valve stems I guesstimate I gained .010 of lift. I "turned" down the base circle with a variety of sandpaper, by hand, over several hours and polished it in the process! While it's out I think I might round the edges of the lobe tip just a hair.. .  don't want to accidentally give it less duration!

This right HERE is a link that takes you to where I was working on it. The really tricky part is being able to gain enough length on the valve stem to make up for the loss of the base circle so you don't have excessive lash, what I did was lap the valves in, checked lash, sanded until the lash was getting loose, lapped some more, checked lash ect. ect. then touched up the ends of the valve stems to set final lash. It would be WAY easier to either get new valves or grind the existing and seats then go to town I'll tell you that!

But being in the spirit of being a low budget racing team (I'm the only member so far) I can't be dumping that kind money into it, it's all about what can be done on a shoestring in a dark grungy garage!  Lol

I see what you did, very nice! On my build I'm thinking about cutting like .04" off the base circle. May wind up having to clear the heads for the valves, not a problem. I am also planning on just building up the ends of the valves with weld and grinding them back down to get my lash back to where it should be.

Do it! Lol. I bet if you cut your seats and valves you'd gain all you would need and still need to grind the valve stems. That's a great opportunity for a 3 angle valve job which on it's own would be a big gain I'm sure but definitely check your valve to head clearance before you get TOO carried away! Lol.

I saw a video of a guy who welded and ground the lobes to create his own profiles but I don't feel that how he did was too proper.... you on the other hand could probably pull it off decently though! You're in the business... have any contacts that can spray weld? Not as in high heat/high feed "spray transfer" welding but actual spray welding, I'm sure there's a more technical term for it but I'm not sure what it is