S&S 3 5/8" EVO piston?

[oneuptom]

S&S is listing a +0.010 over 3 5/8 piston #92-1901 for the EVO Big Twin. Piston has a 1.200" comp height, not the stock BT EVO 1.3750 comp height. I Think it should be what I need for my stock length S&S 3 5/8" EVO Sportster cylinders??

I can't seem to find a Sportster specific piston listed for what I need in their catalog. S&S EVO BT 3 5/8" cylinders are shorter than stock OEM cylinders to go with these shorter Compression height pistons.

[JBinNC]

Off the top of my head, I think you are correct, on the 1.20" compression height for evo XLs with stock flywheels and rods.

There should be listings for replacement pistons in the XL section of the S&S catalog.

Jim

[oneuptom]

Thanks Jim

I'm looking at their 2022 vintage catalog and don't see anything listed for Sportsters.

I guess for anything Sportster related now it would be best to call. Unless someone has a link for an older catalog?

[JBinNC]

In the current print catalog, S&S lists a 3 5/8, +.010 piston for XLs as 92-1941. That is a piston listed for stock (H-D) heads. I think the difference is more skirt clearance cut for piston-to-piston clearance with the shorter XL rods. These are for their 89 cu.in., 4 5/16 stroke motor kit. They offer other pistons for XLs with S&S heads.

Jim

[oneuptom]

Ok, starting to make more sense now, as in those 3 5/8 Sportster cylinders I have were originally part of a big bore stroker kit?.. Maybe S&S never offered the 3 5/8 cylinder/piston kit for the stock stroke?

I'd probably be better off using the longer skirt BT pistons with the my stock stroke crank, right?

[JBinNC]

Ok, starting to make more sense now, as in those 3 5/8 Sportster cylinders I have were originally part of a big bore stroker kit?.. Maybe S&S never offered the 3 5/8 cylinder/piston kit for the stock stroke?

I'd probably be better off using the longer skirt BT pistons with the my stock stroke crank, right?

With that 89 cu.in. stroker kit, in addition to the increased stroke, S&S used a longer rod so the piston skirts would not be so short. If you are using these cylinders & pistons on a stock evo XL flywheel assembly, you are going to have to cut piston-to-piston, and maybe flywheel-to-piston clearance in whatever piston you end up using.

I don't remember if S&S ever offered a 3 5/8 cylinder kit for a stock evo XL. Maybe someone with a better memory will comment.

Jim

[oneuptom]

Looks like the 92-1941 piston was also used in their Big Twin long stroke 3 5/8 x 5 103" kit so I'm assuming the shortest skirt of all them. Hopefully they can provide me with skirt measurements

[JBinNC]

You can use a 1200 XL piston as a guide. The only difference being bore size, there will need to be a cut on the front face of the rear piston for piston-to-piston clearance. Skirt length can be the same as the stock piston.

Jim

[oneuptom]

You can use a 1200 XL piston as a guide. The only difference being bore size, there will need to be a cut on the front face of the rear piston for piston-to-piston clearance. Skirt length can be the same as the stock piston.

Jim

I was thinking that.

I don't have the cylinders close to measure right at the moment, but as I recall they measured 0.007 longer than stock. I believe the spigots are noticeably longer though but can't remember exactly, so now I'm thinking most likely they are cut down stroker cylinders? Whoever shortened them did a fine job.

I only have access to some forged Ross 1200 pistons to measure. I have two XR1200 motors but they are all together

[oneuptom]

S&S cylinder spigot 0.175" longer

[oneuptom]

S&S cylinder spigot 0.175" longer

[ATTACH=CONFIG]109476[/ATTACH]

The bores are very nice, round & straight 3.6265.

What would the running clearance be with a standard S&S forged piston?

[oneuptom]

S&S got back to me and says a 2.5 to 3 thousands running clearance is ideal for this setup so +0.010" piston will need to be fit.

[JBinNC]

If your cylinders clean up nice and round, and you have skirt clearance under .006, I think you will be fine with the standard pistons. Ring life may be a little less.

Incidentally, you must measure aluminum cylinders with torque plates installed to have a hope of accurate measurements. (And that only approximates the actual bore in an assembled motor.)

Jim

[oneuptom]

If your cylinders clean up nice and round, and you have skirt clearance under .006, I think you will be fine with the standard pistons. Ring life may be a little less.

Incidentally, you must measure aluminum cylinders with torque plates installed to have a hope of accurate measurements. (And that only approximates the actual bore in an assembled motor.)

Jim

I would agree with the 0.006 also.

I can detect 2-3 tenths difference across the stud holes so that tells me they've been honed before with torque plates. I'm looking into the feasibility of adding piston oil jets also (4spd Evo XL) before deciding on the piston fit.

Thinking stripped down high compression, lumpy cam Lil' bar-hopper type bike. Got some mint early sand cast 883 closed chamber heads with big squish pads to perfectly match those sand cast cylinders. With 1200 valves installed CR would be over 11:1 with the flattop pistons.

[farmall]

That should be fun. I snagged a good running but cosmetically nasty 89" conversion and it's torquey fun to ride. This thread is useful in case I need to go inside it.