Briggs parts interchangeability and other fun info

Recently i've been tearing apart some engines and figured I would post some parts that fit back and forth. At the moment i've got a 14hp opposed disassebled and I have an 18hp opposed also.


Before I take the 18hp apart, I took apart this 11hp single. Found out the piston is the same as in the 14hp opposed. The valves and lifters are different, although the valve springs appear to be the same. I don't have a way to check springs so i'm just guessing.


Turns out the pin for the "dog bone" rod that guides the balence weights is basicly a wrist pin that would fit many small briggs engines. 3hp-5hp flatheads for example.

I wonder if you can swap that single crank out for a traditional crank with castes counterweights on the crank.

Sure- if you could actually find a suitable one. I'm not so sure they exist. ARC racing sells brass counterweights that you can use on a SB crank- you simply take the counterweight eccentrics off the crank, and slip these on. Well- certain SM cranks anyways. Documentation suggests that SOME SB cranks don't have removable eccentrics so I think if you have one like that, you're pretty much screwed.

You can get those brass weights or arc has those billet dog bone rods too. Brass weights are about $80 from arc and the billet dog bone rod is $25. Actually, it broke in the engine in the picture and I had to replace it maybe 3-4 years ago when it was still on a grass cutter. Governor was kind of messed up and it would run up too high all the time. The weights banged around in there a little but they didn't break through the case.

Given the choice, I think the brass weights are a far superior choice. All the dogbone does, is replace the factory dogbone. You STILL end up with that ridiculous, iron, 1942-friggin-Buick swinging around in your crankcase. Ees no bueno.

I think the flywheel from the 11hp single fits the taper on the opposed twins, but it hits the case on the opposed. I was thinking of using the smaller diameter flywheel and either making an adapter to move the coil closer or using a different ignition system.

Finally started disassembling my 18hp opposed. The interesting two piece aluminum manifold was from the 14hp opposed. I didn't get the manifold bolts loose yet on the 18hp, but it looks like the ports are spaced out the same and manifolds should be interchangeable. The area where the carb mounts is the same.


The venturi of the 18hp carb is about 1/8" larger diameter than the one from the 14hp. The throttle plate and the rest of the carb looks to be about the same.


Heads and tin work are the same.


The 14 and 18hp share the same bore, but the 18hp has about 1/8" longer stroke. Valves appear to be the same also. Looks like the bores of my 18hp opposed have seen some rain water at some point. Oh well, that extra oil consumption will keep the exhaust valves lubricated...

What if you put the 14hp rods/pistons in the 18hp block, 1/4" more stroke?

That's cool you took the time and investigeated... Now all of us that read this know those parts are interchangeable

redlinemotorsportts wrote:

What if you put the 14hp rods/pistons in the 18hp block, 1/4" more stroke?

the stroke is determined by the crank. the rods do not.

Stroke is determined by the crank, rods, and pistons. He'd need longer rods with the 18's crank if it's deeper then the 14's, and the wrist pin on the piston needs to be high up towards the rings and then you'd get more stroke.  Least I think.

redlinemotorsportts wrote:

Stroke is determined by the crank, rods, and pistons. He'd need longer rods with the 18's crank if it's deeper then the 14's, and the wrist pin on the piston needs to be high up towards the rings and then you'd get more stroke.  Least I think.

Stroke is determined by the block / sleeve no? The crank merely controls the stroke, in my mind atleast

Ok so mostly if you put the 14hp pistions in the 18hp block... You get a more stroke... So mostly more go power.... DO IT!!!

Idea from redline to hook two opposed engines together at the base. They line up good. Even if you just made a fake one up it would look cool.


No clearance issues.


Exhausts are angled towards each other. Could easily merge the pipes together from the side by side cylinders. Single down draft carb in the middle maybe.

I could see this working good. Works better then i thought. Cranks dont really move around when the bottom case is off. Take both rods out of both blocks, cut the cranks and bevel em, mock them together and weld the crank inside the blocks.... custom cam extender from some tube, and your golden. I think

Yeah, but I've gotta use one of these engines in stock form. That beat 18hp block would be a good candidate for something like this. If you wanted to mount it to something, you would have to find a way to bolt it down of course. Also, you might need a couple more places to bolt the two blocks together since a couple of the holes would be impossible to use. Probably other engines with side covers would line up the same way. Might be some neat combinations with single cylinders.