Topic: cb400f 76 factory honda piston dimensions

[Redline it]

so if Honda list the standard piston dia as 51.00mm, then 25 over would be 51.25 right? that's 1/4mm and converted to inches it's 2.017717 minus the last 2 numbers 2.0177

piston to bore clearance is suggested to be held between .0004 to .0008 inches which would be in the range of 2.0181-2.0185 inches. is that the way the cylinders are cut, after boring and hone.

and decking would be determined if it needs to be? or just do it.

this one motor that was an ebay purchase, first the head and valve cover, they looked alright so i got the rest of the motor for 300 bucks or something like that. nothing was wrong with it, guy said he was gonna ring it and he lost interest. almost identical to a motor i ran for 31,000miles, same gear teeth chipped and cylinders show no obvious differences.

measuring those cylinders with a central tool co inside micrometer i get 2.009, i can't find any taper or out of round that i can measure. no ridge, some combustion coloring is all. should i just put it back to gether and run it? if i got 10,000 miles out of it, i'd be gone before that mile.

if that was accurate then bore size 2.0181 minus existing bore 2.0080 = .0101 to cut or 10 thousands/ 101 hundredths. is that enough meat to work with to use the 51.25 pistons? doesn't seem like a lot. 

[Bodi]

Normally on rebores I give the barrel section and new pistons to the machine shop, they bore and hone the cylinders to fit, and number the pistons. I just gap and install the rings then install the pistons in the correct bores.

With what you describe I would run it as is if I had the pistons identified for the bores they were in. If not, on an original engine I would do a light hone and use new rings (gapped of course). The factory did not individually fit pistons. You could re-use your rings if you need to but I would check gaps (risking breakage) unless the cylinders measured very very close to identical diameters. New rings and bores honed to a nice crosshatch is preferable.

[Redline it]

that's the best news of the day, i think i do have the pistons marked. i checked my original cylinders (#4 ring broke at around 30k miles),it's a solid .001 smaller all across. and that was a hard 30k too. so i bought the honda pistons thinking to rebuild that top end. as the no.4 compression ring groove is englarged a bit. i had about 11 miles to go, when the ring broke, i didn't even stop, i just rode it home. and i bought the 466 pistons a few year back. your suggestion will keep me out of out of society for a needed break. i got all the parts except rings. i was gonna go to APE? with 3 projects. but my luck isn't worth a crap right now. that always kind of was hard to understand putting new pistons and pins on old rods. thanks Bodi.

[bryanj]

Never seen a Honda rod small end wear unless siezed

[Bodi]

A .25mm first rebore is not always enough to cover broken ring scratches, it should eliminate wear and taper from a normally used and just worn engine with low compression and high leakdown.
466cc is the maximum safe overbore at 54.5mm (IIRC) and leaves the sleeves thin but strong enough. The sleeve's bottom inside taper is mostly gone though, so working rings in without compressors is less easy than with the stock bore - but still possible.