XS850 pistons & rings? Same as XS1100?

[XS850GUY]

Seeing lowish compression on my 81 XS850 (or the rental gauge from oriley giving spurious readings)

I'm looking at the service manual for both xs850 and xs1100...

XS1100H calls for piston 71.5x68.6 x qty4 #3H5-11631-00-46 (std),

XS850SH calls for piston 71.5x68.6 x qty3 #3J2-11631-00-46 (std).

Both diagrams call for the exact same wrist pin(2H7-11633-01-00), and clip(90468-12069-00)

How dissimilar are the 850 and 1100 pistons really? Can they be interchanged?

I called Wesco and they do not specifically offer a piston for the xs850, only the xs1100

I did some searching and couldn't find anything specifically discussing xs850/1100 piston crossover.

[cajun31]

Seeing lowish compression on my 81 XS850 (or the rental gauge from oriley giving spurious readings)

I'm looking at the service manual for both xs850 and xs1100...

XS1100H calls for piston 71.5x68.6 x qty4 #3H5-11631-00-46 (std),

XS850SH calls for piston 71.5x68.6 x qty3 #3J2-11631-00-46 (std).

Both diagrams call for the exact same wrist pin(2H7-11633-01-00), and clip(90468-12069-00)

How dissimilar are the 850 and 1100 pistons really? Can they be interchanged?

I called Wesco and they do not specifically offer a piston for the xs850, only the xs1100

I did some searching and couldn't find anything specifically discussing xs850/1100 piston crossover.

I think you just answered your own question. The first 3 number letter designations are for the model number and the part numbers are the same from there. If you are sure you are experiencing low compression it may just be time for a top end rebuild. If that is the case why not just go ahead and have the cylinders bored and just replace the pistons and rings. They are available form here. Search Results for “Yamaha xs850” | Page 2 | CRUZINIMAGE.NET

[Radioguylogs]

Did you have the engine warm and the throttle wide open when you measured the compression?

[XS850GUY]

I think you just answered your own question. The first 3 number letter designations are for the model number and the part numbers are the same from there. If you are sure you are experiencing low compression it may just be time for a top end rebuild. If that is the case why not just go ahead and have the cylinders bored and just replace the pistons and rings. They are available form here. Search Results for “Yamaha xs850” | Page 2 | CRUZINIMAGE.NET

Yes, overboring is an option, a very expensive option. Before I go that far, I was going to check the ring gap.. If mine are just worn, its possible I can get away with just new rings without needing to pay for machining.

I'm aware of the cruzin image kit.. and not super interested in going 3mm over unless I really have no other choice & would prefer a trusted vendor like wiseco.. Not trying to make it a race bike, just reliable.

I think there is a good chance the xs1100 pistons would work in the 850 head, despite having slightly different model numbers. Lending some hope, the guys at Scrambler Cycle are selling reproduction rings for 850 and 1100, and claim they are identical rings & pistons for either bike.

Still not certain I answered my own question & would need 1 of each side by side to confirm.

Did you have the engine warm and the throttle wide open when you measured the compression?

Room temp.. rather basement shop temp, so ~60F. Avg reading was abouut 90psi.
Not able to test warm because the bike wasn't running when i got it in 12/24, and needed electrical work to get the starter motor turning, etc.. besides a 1-2second test fire int he basement, not able to warm the bike up proper until the spring thaw.

My test process was definitely wrong. I didnt open the throttle, and I had the carb intakes taped up as well (ingress prevention as the airbox is MIA (for now)).

This was the first time I actually used a compression gauge (rental from oriley & already returned it, d`oh!) When I checked the compression gauge against my air compressor, the gauge read about 10psi below the compressor's gauge..

Could be a bad rental gauge, and was also definitely a bad test process. Will definitely re-check before going further.

Thanks!

[DiverRay]

From what you just posted, you did NOT do a compression test! Don't worry about the rings, and just get the bike running. This spring, once running, take it out for a 30 minute ride, let it set and cool for about 20 minutes, and THEN do a test. My guess is it will be fine. These bike WILL go over 100K miles without needing much.

[XS850GUY]

From what you just posted, you did NOT do a compression test! Don't worry about the rings, and just get the bike running. This spring, once running, take it out for a 30 minute ride, let it set and cool for about 20 minutes, and THEN do a test. My guess is it will be fine. These bike WILL go over 100K miles without needing much.

For sure, I will. This was mostly my plan & had no desire to tear anything down at this point.. Just doing some research, planning & prep while I wait for the thaw.