12 hp flat head build

thats right gents...its time...im going to be taking one of my 12 hp and building it...building it i mean possible boring it .20 some head work maybe some bigger valves if i can better cam...with that said what do u guys think...and should i use the flathead with older 1 piece carb set up or updated card 2 piece?? let me know and shoot me some ideas...

sounds good, are ya gonna profile your own cam or buy one? and if i were you, id use a older 1piece carb, that way you can tune it just right

Well I'm debating on the cam I might have a friend who has most parts I need and idk about the flywheel I'm wanting to loose that boat anchor but we'll see and I'd like...sorry using my phone it's hard to see wth I'm typing on it lol

so i was thinking...ok you guys know where some small block guys that build the high revving v8s for like 8k...well i know there build consist of shorter length rods in most applications...but do you guys think that i could use a shorter rod on my single 12 (if i can find one) and would have similar results? i do consider in the compression and everything else that plays the role its just a thought any ideas on that part and i do realize that high revving single needs to have a better oil lubrication system and maybe even a little head work..was considering throught the 11hp head on it with shaved eye brows and make some intake porting and polishing

As far as I know, rod or crank throw length and all that would only be and advantage when changing the stroke. As far as trying to make the rod hold up better by shortening it and accepting the piston won't reach top dead center, I think any possible gain in strength would definately not be worth the lost compression. If your going to get different rods anyway, you could buy billet rods either new, or possibly track down some used ones somewhere although you don't know how much hammering they've received from the previous owners. If you want to use stock rods, you can remove the casting imperfections and polish them. Supposedly vibrations usually start cracks at sharp edges or rough areas from the casting process. Then there's shot peening and all that to improve suface strength. I don't know if they do that with aluminum or not. How much all this helps with stock rods, I have no idea.
There's a link about it...
Polishing connecting rods

yea ill have to do some home work see what i can get out of this...might be worth a shot might not be if i cant find anything ill just do some mild porting and polishing and shave the eye brows on the 11hp head and just that 11hp head on 12 increases the compression ratio greatly how much idk need to check for compression first to see where im standing at

Regarding the heads- I think the 11hp has the same combustion chamber volume as the 12hp- so changing heads won't change a thing.

A shorter rod will locate the piston further down in the block- you'd have to match the rod length to the crank throw, to the piston height.

Carb- I ALWAYS prefer the pre-EPA, adjustable main jet carbs- whether the engine is modded or not. The ability to fine tune the mixture without having to redrill the jet, is golden. Now, that said- I'd rather go to a fully adjustable, more versatile carburetor in a situation like this- such as a Mikuni VM series or similar.

You mentioned "head work" a few times (twice?). What head work? On a flathead, all the work is in the block... no?

Somewhere around here is a thread where I started working a 12hp flatty, and kinda got busy with other stuff. But another user- TDub13, built one up. There might be some good info in there for you, lemme look for a link. Here it is--> https://www.atltf.com/t3011-briggs-flatty-low-buck-buildup One of these days I'll get around to finishing that engine...

From what I can remember about the heads: The 11hp head does indeed have a smaller combustion chamber. It was a major PITA to change over because it required different length bolts and the bosses for mounting fan shroud weren't long enough. I'm sure I wouldn't have gone through the trouble if they were the same inside.

The motor in my Scorpion has an 11 hp Briggs in it. I did all the goodies in it when I rebuilt it except for raising the compression in it. I actually lowered the compression in it for torque. I've done it time and time again from advise from and old engine builder, "If you build for torque, horsepower will follow". My advise would be to leave compression alone. too hard to change the ignition to take advantage of the higher compression with the loss of low end power because the engine is actually fighting the higher compression at low rpm's. I would suggest to upgrade induction and exhaust with weight loss from the flywheel. These flattie motors respond real well to these even though porting is limited due to thin casting walls and valve placement. The small radius sides in the runners, in the block, is the best spot to remove metal when porting. If you got the money, buy a billet piston rod which is about $150, if not then polish the stock piston rod. Need to remove as little metal as possible to keep strength.  Polishing the beam adds reliability, when being stretched the polished surface will keep the rod from cracking or pulling apart. Just keep rpm's at 4000, any higher then a billet rod is needed for reliability.

The idea on the length of the piston rod is, this goes for all piston driven engines-

      A short piston rod makes the piston move faster which makes the air/fuel charge move faster into the cylinder. The drawback with the the shorter piston rod is that the angle the piston rod is at mid stroke makes the piston rub on the cylinder wall harder to where it wears down the cylinder quicker and makes more drag on the piston(loosing hp), shorter engine life due to wear. That's why the 400 small block Chevy was a short lived motor.

      A long piston rod makes the piston move slower but this is favorable in many ways. Piston rod angle is straighter at mid stroke which means less drag of piston on the cylinder wall, less wear on the cylinder wall, and more hp is transferred from the piston to the crank. Plus since the piston is moving slower it will put less stress (less stretching force) on the piston rod,

There's more to it but that's the main idea of it. And to use either a shorter or longer piston rod means you need a piston to match the rod, this is done by moving the wrist pin in the piston.

These are low performance motors. Very low.

Compression in these motors is a joke. Changing from a 12hp head to an 11hp head will raise compression, but it will always be a low compression motor. It's inherent to a L head combustion chamber. Do the swap if you can. It will help make more power across the entire RPM range and you will still be nowhere close to being "too high". I run my modded 12 on 89 octane cheapo fuel and it's nowhere close to detonation or preignition.

Changing the ignition timing is cheap and a good way to get the most out or your motor. Every motor is different, and everyone's ideal power characteristic is different. Play with the timing to get the most power at the RPM you want it. In fact, you don't even need a key. The taper on the crank / flywheel is what keeps them locked together. The key is only there to align them during assembly (they're aluminum). You can file the key as Doc has indicated or eliminate it altogether and align the flywheel to the crank keyway by eye. Scribe a mark so you know where you set it to run best for future reference.

Changing out the old con rod for a newer "S" con rod will buy some insurance against the rod seeing daylight. Smoothing the outside of the rod and getting rid of casting flaws can help in mitigating stress risers, which in theory will make the rod less prone to failure.

I wouldn't suggest anyone waste their time with trying to engineer a longer or shorter rod into one of these motors.

I think the going rate for a different rod and matching piston is around $300.00 bucks when complete. Be easier to just get a bigger motor, LOL.