Checking my parts list for my '75 850 the Nrs. are close but you have to use the serial Nr. which could be on a plate between clamp bolts for searching.
Is it a high dome or low dome??? Does it have a power port??? Nobody offers a "piston kit" per say as you will need to purchase all parts separately. You can get a gasket kit....
Post a picture...
Yes, a motor s/n will verify this info. Don't purchase anything until you do. There appears to be only one Merc 800, 4 cyl. with nine s/n breaks. I checked and the pistons are common. 774-9137Axx. xx 12 is std., xx 13 is .015" OS, xx 14 is .030" OS.
The OEM pistons are NLA, but new old stock is available on eBay, etc. Sierra 18-4535 is the .030" OS version, and is available here at iBoats.
You can get them from WSM also in .010, .020, .030 and .040. Make sure you mikr the cylinders as these engines wear a taper around the ports and this causes idle and die in gear issues.
I couldn't find your engine manual on Boatinfo, but several manuals for similar motors state a .003" limit for taper, out of round, size. If you are over this, you need to bore & hone enough to straighten this up. .010" increments.
Your pistons are 2.875" Std. I don't know the hone finish size clearance spec. The small end of your tapered bore will be close to factory. Per Faztbullet the WSM link is below. $720. If you need to go .030" over, the iBoats Sierra part is also below. $472.
...my bad. Dyslexia. So he should buy OS pistons to match a 2.885", 2.895" or 2.905" bore clean up size and the pistons will be sized for correct clearance? I rebuilt my KAW 500 Triple 33 yrs ago, but I've forgotten the process.
Before you sink a lot of $$$ in this motor, if it sentimental or a full restore for a project...go for it. If it just a running motor to use for the weekend you can add a few hundred to repair/parts price and get a later model 3 or 4 cylinder looper...
