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What is the maximum piston/cylinder clearance for a '70 CB450 with cast pistons? Can't find it any of my manuals. Thanks, sorry for the newby questions. Also what is the maximum ring end gap?
 

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Sensei
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Standard piston to cylinder clearance is .001 to .002" Max is .004"
Ring gaps: Top and second std.. .012 to .020"... Max .031"
Oil ring std ... .008 to .016" max .031"
 

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Steve's info is correct - this is standard machine shop values, it's (the cylinder to piston clearance) not documented in any service manual that I've ever seen.
Ring gaps and cylinder specs are in the manual.

Most Hondas (especially the 450) have very small cylinder "taper" tolerances - that usually drives my decision to re-bore, in the absence of any obvious damage.
 

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Alright, so I finally got my bores and pistons measured today. I was hoping for just a hone, but it looks like I'm right on the edge.

Right cylinder: 2.758"
Right piston: 2.754" (skirt) - 2.750" (top)

There is some corrosion on the bore. The machinist thinks it would take about 0.002" to clean it up. That puts me right on the edge of the 2.760" service limit. It is also 0.001" out of round.

Left cylinder: 2.756"
Left piston: 2.754" (skirt) - 2.750" (top)

The left is a lot cleaner and should only take 0.001" or less to clean up.

I don't really have the $150+ for new pistons, but if it is required, I guess I'll have to bite the bullet.

He said the heads of the pistons were narrower because they would heat up more and expand. So I'm guessing the pistons are measured at the skirt?

So I was planning on having the machinist hone the right cylinder until it is clean. Then measure it. If it is greater than 2.760", I will have to bore it out. If not, I'll get the left side honed too and put some new rings in it. If it does end up being 2.760" that means my piston clearance is 0.006". Or is it half of that, 0.003" each side?

Sound like a good plan?

I think most of my compression problem was from the cruddy valve seats and partly from the corrosion on the cylinder wall.
 

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Flugtechnik said:
Is piston clearance 1/2 of (bore Dia. - piston Dia.)?
No, the piston/wall clearance is the TOTAL clearance. So, if your piston measures 2.745 and the cylinder measures 2.750 then you have a .005" clearance. It's the same as saying you have .0025" on each side, or .005" total.
 
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