Piston to cylinder wall clearance.

[member3480]

Just to make yo feel comfortable I have been using Cruzinimage for over 20 years was owned by someone else before. Plus I have built more than 15 with his overbore kits and stock kits and my buddy has been riding his bike for a little over 10 yrs with those pistons. Plus I did a bike with parts bought by customer from a well know XS650 parts place in Florida and guess where those pistons double the price came from. Lol they didn’t remove the paperwork. Contact them threw web site not eBay.

 

[arcticXS]

Oversizes are in 1/4mm increments... or .25mm. Do you have a vernier that will measure down to that?

If a vernier caliper (not to mention dial or digital caliper) cannot measure a quarter millimeter reliably, it is totally junk. My Mitutoyo from 82 is still able to measure correctly down to 0.10 mm every time, and down to 0.05 most of the time.

 

[G_YamTech_314]

Cool! You guys are making me feel better. I'm almost positive the bore size is factory.

The only other question regarding the bore itself is whether or not the "010" stamped on each cylinder means anything.

 

[Jim]

whether or not the "010" stamped on each cylinder means anything.

That's what it measured at the factory.... 75.010mm.

 

[Jan_P]

A few opinions
If the pistons are stock there can be digits on top of the crown indicating the nominal play installed
I have driven 2 of the exact same model and year 1980 .The first ran better than the second although worse compression
So within limits the carburetor and ignition are more important than cylinder.
Compression in my view.

There is also a routine to check the wear with the pistons in .. measuring the installed piston ring gap.
fex look in a Clymer manual.
Take into consideration that rings are also worn. As well as the bore.
Standard clearance Piston to Cylinder are ( not sure if for all years ) is 0.050 mm to 0.055
That is small --- and in my opinion it is difficult to measure with any instrument.
I cant do it.
I dont believe vernier caliper is the right tool for this.
I recently did a ( Cheap ) Rebuild and took the parts to a shop for inspection.
A rebore here is expensive. Especially if it is shop with reputation of competence.
Perhaps a kit with all included is the better option. Taken into consideration all.
I might cost more but new parts are New.
But sure depending what bike or what plan when it should be done.

 

[G_YamTech_314]

Revisiting this topic after quite some time...

I took both pistons and the cylinders to the machinist finally (he's been quite busy) he said my piston to cylinder clearance is right around 2 thousandths (which if I recall correctly is pretty good.) I also had him check out of round and he said it's a bit out of round but nothing in need of a boring. He's going to hone them both, and advised that I use the original pistons and out all new rings in. I'm FINALLY ready to see the top end of the motor come back together once I order parts.

 

[G_YamTech_314]

Now I just need to know what size rings to buy... If it's stock size could I just buy rings for a stock motor???

 

[5twins]

Yes, and Cruzinimage has reasonably priced stock sized ring sets. I don't recall if you reported what pistons you found in your motor when you tore it down. If stock, they will have a 3 digit number stamped on the top. That represents the fractional portion of the size, as in 74.xxx mm. The 3 digits would replace the x's. The stock pistons are just under 75mm in size. This one would be 74.953mm .....

 

[G_YamTech_314]

I believe mine were stock! I'll double check as soon as I get home. I'm assuming I'd buy a ring set that was a little bigger and just grind the gap?

 

[5twins]

Well, that's another option. You can buy 1st oversize rings and file the end gaps to get them at the minimum gap spec. But, Cruzinimage doesn't sell 1st oversize rings or pistons for that matter. They skip right up to 2nd oversize. But, if your cylinder clearances are as tight as you say, stock sized rings should be fine. In fact, the originals may still be OK (in spec). Have you measured their end gaps? .....

Although I haven't found it too often, there were a couple times over the years that I was able to reuse the original rings because they still measured in spec.

 

[G_YamTech_314]

Well, that's another option. You can buy 1st oversize rings and file the end gaps to get them at the minimum gap spec. But, Cruzinimage doesn't sell 1st oversize rings or pistons for that matter. They skip right up to 2nd oversize. But, if your cylinder clearances are as tight as you say, stock sized rings should be fine. In fact, the originals may still be OK (in spec). Have you measured their end gaps? .....

Although I haven't found it too often, there were a couple times over the years that I was able to reuse the original rings because they still measured in spec.

I have not done so recently. Which rings do I measure? Just the scraper and compression ring?

I don't know how I'd even measure an oil ring lol. Which makes me wonder how I'd know it was any good.

 

[5twins]

Well, you can measure the 2 oil ring "rails" but it's a bit fiddly because they're so thin. But yes, you measure the top 2 rings for sure. Use a clean piston, upside down, to push the ring down squarely in the bore about an inch. To easily get the ring compressed into the bore, here's a little trick I learned from my dealer - insert the ring with it vertical, then simply rotate it horizontal.