BRIGGS OPPOSED TWIN PERFORMANCE PARTS&TIPS

Shizz, I can see the head ripping off before any other kind of destruction, Maybe the piston would crack or whatever. <----Total Myth, especially with a mower.

ive setup a pair of mikuni 28mm carbs on a 17hp oppy hard part was the throttle but this engien had billet rods the weak spot is the stock rods upgrade them before the dual carbs this engine i built was for semi stock mower pulling the engine didnt last long piston cracked on the right side due to the owner never tuned the right ran the right side lean.

Alright thanks

UPDATE:Added stroker info on 40ci/46ci engines.

are you sure that using the stock 42 ci crank and block with 40 ci rods is going to stroke it? I don't see how that would work

I know I am new here, but I have been working on small engines for over 25 years and I have been lurking around various sites for performance ideas for my 46 CI, opposed vertical shaft engine that crapped the bed a couple of years ago. Since I have to rebuild it anyway, I'd like to up the performance a bit.

With that said and with all due respect to MowBandit, I have to agree with 1997Murray, it is physically impossible to "stroke" a 42 CI engine using a 42 CI crankshaft. By changing the connecting rods, which is what is stated on the first post of this thread, all you're doing is pushing the stroke higher or lower in the cylinder, but the piston is still physically moving the same distance. Stroker by definition means the piston moves higher upward and downward in the cylinder, resulting in a longer stroke.

Another discrepancy I found in this thread is the differences in cranks. Granted, the engine manual I have is for vertical crankshaft, opposed 42 and 46 CI engines (I have one of each and thought how great it was that I did not have to buy a 42 CI crank), MB's information may only pertain to horizontal crank engines, but my manual covers both sizes of engines and says that the stroke is the same for the 42 CI and the 46 CI engine, the only difference between the two engines is the bore diameter. It seems putting a 42 CI crank in a 46 CI engine (vertical crank) will not make a stroker, either. Or, perhaps MB's information is based on older engines where there might have been a difference? I don't want to call out MB, I am just saying that there are discrepancies in what is written compared to what I have been able to find.

Also wondering if any of Briggs' other L-head engines have compatible connecting rods, since this seems to be the weak link on these engines. I have not been able to find specs on the relevant details of the connecting rods, such as the center to center distance. The reason for this is that there may be other engines that have high performance rods that are compatible with the oppy's. I have a hard time thinking Briggs' would mass produce engines but give each one their own unique connecting rods, etc. Seems like connecting rods and pistons would be two items that they could have standardized and used in several engines.

yeah, we (mowbandit and I) have had this discussion... The definitions of "stroker" was discussed, and I'm in the process of rewriting this (with his permission and assistance) to correct the confusion. Given the problems with spelling, grammar, and terminology, it does make for a confusing read. Any of us that have been working with engines for 20 and in my case 35 years, will spot some glaring issues.

Sorry i havnt been on but there was some info left out the 40ci opposed twins have a smaller crank then the rest of the opposed twins but has longer rods and yes i know that the crank can not and will not make a engine a stroker only the rods the reason i said a 42crank is because is you attempt to pair a set of 40ci rods to a 40ci crank its jsut gonna be a stock 40ci engine now if you throw 42ci rods on a 40ci crank then its pretty much destroking the engine. the reason i say use a 42ci crank is the 42ci engine is the most common engine and the 40ci engine is based on a 80's briggs the 40ci 42ci and 46ci all have the same bore UNLESS and only unless you happen to find a big bore opposed twin now people say that there just a rumor but there not the rare big bore come with a 3.57" bore unlike the common opposed twins that come with a 3.44" bore now if you have a 46ci engine and you wanna make it a stroker just need a set of 40ci rods since the 42ci and 46ci have the same crank if im not misstaken correct me if i am wrong.

Also note that i say block when i refer to stroking a 46ci i didnt say engine im just saying if you just have a 46ci block laying around lol

On a side note... Porting and polishing should only be done on the exhaust port. The intake will benefit from just a port. Leaving the rough surface will help the air/fuel mixture mix from the turbulence.

2014-11-2511:34:39from a very new member what I see is not more cu. in. but moving the ring lands closer to top of block which should increase comp. on the up stroke and piston suck sooner on the down stroke resulting in more saturation and possibly a little more power.

20HPOPPY wrote:

I know I am new here, but I have been working on small engines for over 25 years and I have been lurking around various sites for performance ideas for my 46 CI, opposed vertical shaft engine that crapped the bed a couple of years ago.  Since I have to rebuild it anyway, I'd like to up the performance a bit.  

With that said and with all due respect to MowBandit, I have to agree with 1997Murray, it is physically impossible to "stroke" a 42 CI engine using a 42 CI crankshaft.  By changing the connecting rods, which is what is stated on the first post of this thread, all you're doing is pushing the stroke higher or lower in the cylinder, but the piston is still physically moving the same distance.  Stroker by definition means the piston moves higher upward and downward in the cylinder, resulting in a longer stroke.

Another discrepancy I found in this thread is the differences in cranks.  Granted, the engine manual I have is for vertical crankshaft, opposed 42 and 46 CI engines (I have one of each and thought how great it was that I did not have to buy a 42 CI crank), MB's information may only pertain to horizontal crank engines, but my manual covers both sizes of engines and says that the stroke is the same for the 42 CI and the 46 CI engine, the only difference between the two engines is the bore diameter.  It seems putting a 42 CI crank in a 46 CI engine (vertical crank) will not make a stroker, either.  Or, perhaps MB's information is based on older engines where there might have been a difference?  I don't want to call out MB, I am just saying that there are discrepancies in what is written compared to what I have been able to find.

Also wondering if any of Briggs' other L-head engines have compatible connecting rods, since this seems to be the weak link on these engines.  I have not been able to find specs on the relevant details of the connecting rods, such as the center to center distance.  The reason for this is that there may be other engines that have high performance rods that are compatible with the oppy's.  I have a hard time thinking Briggs' would mass produce engines but give each one their own unique connecting rods, etc.  Seems like connecting rods and pistons would be two items that they could have standardized and used in several engines.

New to the thread but not these engines i have been working on just the opposed briggs for almost 9 yrs mostly in pulling tractors and i can keep up with the best bc i can do things to get them to run top notch. That being said i will say this the 42 and 46 cubic in motors ARE NOT the same stroke if you lay the cranks next to each other they are most deff different not to mention the rods are diff length so they have diff cranks. Plus the blocks are machined out more on the inside to allow for the bigger stroke so idk why you think they are the same cause there most deff not, if they were than why is one 42 cubic in and the other 46 cubic in yet the bores are the same? Something must be diff.As far as strokeing one the last time i tried the rod journals are diff size from engine to engine so they dont swap unless you machine them to match. I had tried to put 18hp rods on a 16hp crank and they wont fit now unless i happen to have some messed up parts they woudnt work. Both crank and rods came from running engines, i have all the extra stuff to test out this theory and will try it when i get time. I am currently building a 20hp horizontal opposed for my current pulling tractor i have converted these motors before and unless you need to run a pto its very easy to do.