250sx piston & bore size

[Honda_Dude]

Okay, borrowed a caliper from a machinist @ work today that measures in inches and centimeters. I'm about to button it back up and wanted to measure the bore just to know what I have. (renewing cam chain & guides)

I may need a caliper w/ smaller increments, I don't know. The bore appears to measure 75.3 mm, or 2.960 inches. The manual shows a bore of 74mm, or 2.913 inches.

If I'm reading this right, the bore is .047 oversize ????? What size piston would you suggest in a bore this size ?? The cylinder wall looks GREAT, I was just considering a hone & new piston/rings possibly while it's apart.

The top of the piston is nicked pretty good where it had some piston/valve interference. Compression was still good so I might use it a bit yet (money's tight).


I'll play with some numbers in the manual & check what sizes pistons come in but info from this site would be a plus. :shiftyeyes:
THX

[Dirtcrasher]

There are plenty of guys here that can hook you up with the OEM service manual and specs.

You definitely need a caliper and more importantly a micrometer that reads .000 or "thousandths".

Not to mention that merely measuring the bore across the top or bottom of the cylinder jug sleeve in any different axis isn't a good indicator. You need to take it to a shop to be measured or use inside telescopic bore gauges and then measure those with a good (probably) 2"-3" micrometer.

Calipers get us by, they tell us the rough size of things but they are fairly useless as far as critical measuring goes.

Even if you just stop by an "old school" machine shop, hopefully you'll find an older guy that will be more than happy to take some measurements for you.

ALL cylinders wear egg shaped and the top and bottom that your measuring really don't say much about the middle. It may "look" straight but I'm sure that where the piston runs you'll find more wear.

It's an SX and they don't rev crazy and have great oil flow so hope for the best. BUT, depending on WHO touched it in the past, any of the oil galleries could have been partially blocked with gasket sealant and or missing o rings so who knows what you have there.

My 87 250SX is .080 over and a high comp piston with a webcam/springs/retainers 350X carb blah blah blah; fun little trike though :D

Hope this makes sense to you :beer

[Honda_Dude]

Thanks DC, makes plenty of sense. I have the service manual. You answered my question. That's pretty much what I wanted to know. I have an "old school" machine shop very close to me who's done work for me in the past (car engines). It sounds like I'll just take the cylinder to him. I thought a caliper may get me by on a 4-stroke Honda but yah looks like I'll need more accurate measurements. THX again