19.5HP B&S Opposed Twin Stroker

So I was searching around the forum a few days ago before i applied here and saw something on building an opposed twin stroker. I was curious if anyone has done it? I believe the 19.5hp is a 42ci. (What I have) and i couldnt recall if the piston had to be shaved down 0.060. I believe you can run original pistons with 18hp 40ci rods and 20hp 46ci head. Please correct me if I'm wrong. Once again thanks for the warm welcomes.

I think 18hp-19.5 was 42ci and 16hp down was 40ci and 20hp up was 46ci?   I'm pretty certain the first 2 numbers in the model are "ci".  I think they all had the same block (except the ones with oil filter)

I will let the more knowledgeable chime in on the mixing of the parts.  

Yeah Mighty's right about the displacments. Also the blocks were indeed the same I believe, just different stroke from different crank, rods, etc. Unfortunately though I'm not sure exactly which parts would would interchange in order to make a stroker, but I like the idea!

The first 2 numbers would make sense. My first 2 are 42. Yea, figured I would get into small engines a little bit. I also have a small turbo from a subaru my brother wants me to try and hook up to it haha.

Curios on what crank you would be using?

I should be able to use the same crank right? I really just want to see if I can get it all to work and push some extra ponys and if it does. I will buy billet parts for racing I just don't want to spend all the money on parts that idk if it will even work.

To stroke out a motor means you are putting in a crank with a bigger throw, you dont use the same crank to stroke a motor. You would use the same crank if you are boring out the cylinder bore for a bigger piston, you would reuse the crank and rod.

To stroke out a motor, you need a crank with a bigger throw, wich means a shorter rod to reuse the same piston. Reusing the stock engine parts is hard because you need all the specs on all the parts to make a combo kinda work. If you are buying billet parts for this, call them up, they did the research already and will pass it on to you just to sell you the parts. Trust them first before anybody else. They most likely have a combo already to bolt in, you just pay them to buy the parts and tech. Well worth the money.

So maybe it wouldn't be a true stroker. But isn't the concept the same? Running a shorter rod on stock crank with a taller head leaving more room as if it had a bigger throw?

You're still going to be moving the same out of air, unless you bore it over. The concept is not the same.

And to what mighty was saying, I think a rare few 42cis were 16hp. But for most engines, he has it right. 12-16 is 40ci, 16.5-19.5 42ci, 20-21.5 is 46.

So from what I'm gaining here is running a shorter 40ci rod and 46ci head with a stock crank will pose no performance enhancement?

If you just use a shorter rod with the stock crank the compression ratio will go down significantly.

Yep. Best you cando is get a longer rod and machine it or the head to fit. I wouldn't recommend turning up the compression like that on these though. You'll have to go to copper head gaskets if you do. If you wanna build a little snottier engine without a crank swap, just open up the ports and go to dual slide carbs. Flow is the enemy of performance, especially on flatheads, and especially on these flatheads.

Maybe thats what ill do for now, race days in 13 days. See how she runs then just order a stroker kit if im not satisfied with the port and dual carb set up.

Yah, what they said. Usually a stroker kit would come with everything engineered to work from the manufacturer, all you need is a good engine block for it. You would most likely pay like $600 plus for full race kit for a gain of a couple of inches on specific engine blocks too. You could get the same hp gain from a good induction setup at atleast 1/4 of the cost. That would be cam, intake, and carb(s). Billet rods maybe, if you plan to rev it high, but not necessary if you plan your induction for a power band under the stock rods limitations.

Yea, whats yer guys take on flywheels? I know many say you need a billet flywheel, but my brother inlaw raced his 12.5hp and that sucker was pushin 6k. Never blew his up.

I think lighter flywheels give a snappier throttle response. As far as blowing up? I don’t think so. They’re usually balanced to start with.

As long as someone hasn't beat on it with a ball peen hammer to get it off the crank at one point, it's most likely fine. If they're cracked or damaged in any way, that's when they can be an issue.

Oh, and sticking with the original heads would be a good idea. These heads are all the same design and putting a head from a bigger engine on an engine that is smaller would only drop the compression. Modding the original heads would be ideal but I never see these engines survive long with just higher compression. Only thing I would do to the heads would be to unshroud the valves the best you can. High compression alone doesn't make hp. High compression goes hand in hand with a camshaft that has alot of duration to keep the compression the same. The longer duration on a cam drops the compression a little, so shaving the heads a little will bring up the compression to NORMAL ratios. This is where a compression tester becomes your best friend.

I would try to mod one of those small aluminum flywheels from a, I think a 5hp Briggs push mower, Would have to use the coils from the 5hp too, I would think you would have to make brackets to hold them. I think it might bolt up to the oppey crank, maybe.

Thanks. New to this game and I know everyone makes mistakes but id like to make the mistake of forgetting to put gas in her than have low compression because I went out on a limb and tried something without learning about it.

Maybe look up the engine and see if you can get any kind of technical manuals on the motor straight from Briggs and Straton Co. You would be looking for info on the diameters of the rod bearings, bore, piston size, rod length, and so forth. Even info on buying the parts would be helpful because the parts will have detailed info, like a piston rod would be this long or bearing size would be this diameter and so forth. With a little math you would be able to figure out if what you want is doable from stock parts and would let you know if you need a custom part to make it work. This is an old school way of doing this.

I have 3 of these oppeys myself waiting for rebuilds because they been sitting around. I'll be rebuilding one hipo and the other 2 will be for work probably.

You have to have the rods out of the 40ci. Put them rods on the42ci crank in the 42 block. I used 46ci heads also ported.. used the 46ci cam. Had to mill like .075 off pistons... The shorter stroke 40ci. Has a longer connecting rod. And when you add that to the already longer stroke 42ci crank and block well then u see why u have to mill the pistons.. it is ALL DONE WITH FACTORY PARTS.. hell mine is done with all used parts and it is a beast.. i have two 28mm round slide carbs..no gov at all idid alot of work i wont mention of on the inside of it but it is real it can be built and will stay together if you know how to run it..