Scooterotica

Hi,
Im about to have a 150 cylinder bored out for a 175 conversion piston. What clearance should i ask for, and what is your reference? I have looked through the forum and saw 3 thou mentioned a few times, but no one mentioned where this number came from.

I'd lime to know if that is correct and how authoritative it is.

Tx in advance
David

here we go!!.....3.5 thou is good. I tend to do mine now at 3 thou (in old money). On some alloy cylinders I've done 2.5 thou.

Running in needs to be careful. I tend to do short sharp cycles. Cold run in for a couple of miles, then back to base. Cool down then another cycle. Give it some "welly". Repeat over and over. Don't be afraid of giving it some revs in all gears..

Chris

bristolmod wrote: Give it some "welly". Repeat over and over. Don't be afraid of giving it some revs in all gears..

Chris

Was told that way years ago by Dave Webster. It's never let me down either.Run in all my iron barrels that way.

Not forgetting ring gap 15thou.

Fantastic. Tx. Exactly the info i needed. Thunderbirds are go.

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What they all said about running in, absolutely.
10-11 thou ring gap seems to work. The pi(3.14159) theory kicks in, every .001 of piston to bore clearance gives .00314159 of ring gap = .009something , so 10-11 gives a little wiggle room. Bore to .003. without a doubt.

bristolmod wrote:here we go!!.....3.5 thou is good. I tend to do mine now at 3 thou (in old money). On some alloy cylinders I've done 2.5 thou.

Running in needs to be caereful. I tend to do short sharp cycles. Cold run in for a couple of miles, then back to base. Cool down then another cycle. Give it some "welly". Repeat over and over. Don't be afraid of giving it some revs in all gears..

Chris

How many times (ish) is 'over and over'? 10, 50, 200? Or do you still think in total miles?

dirtyhandslopez wrote:What they all said about running in, absolutely.
10-11 thou ring gap seems to work. The pi(3.14159) theory kicks in, every .001 of piston to bore clearance gives .00314159 of ring gap = .009something , so 10-11 gives a little wiggle room. Bore to .003. without a doubt.

Great calculation but another case of simple issue complication.
Would it not just be easier to adopt a rule of thumb approach and say ring gap should be 3.5 X piston/bore clearance?

hendy wrote:

bristolmod wrote:here we go!!.....3.5 thou is good. I tend to do mine now at 3 thou (in old money). On some alloy cylinders I've done 2.5 thou.

Running in needs to be caereful. I tend to do short sharp cycles. Cold run in for a couple of miles, then back to base. Cool down then another cycle. Give it some "welly". Repeat over and over. Don't be afraid of giving it some revs in all gears..

Chris

How many times (ish) is 'over and over'? 10, 50, 200? Or do you still think in total miles?

my bad- I tend to think in total miles but around 400.

Scooterdude wrote:

dirtyhandslopez wrote:What they all said about running in, absolutely.
10-11 thou ring gap seems to work. The pi(3.14159) theory kicks in, every .001 of piston to bore clearance gives .00314159 of ring gap = .009something , so 10-11 gives a little wiggle room. Bore to .003. without a doubt.

Great calculation but another case of simple issue complication.
Would it not just be easier to adopt a rule of thumb approach and say ring gap should be 3.5 X piston/bore clearance?

No, because the actual ring gap figure should be 3.14159 for every .001 piston/bore clearance. Going to be a tad difficult to read .00942477 on a feeler gauge though. Therefore nearly spot on would be .0095000 and, as said, .010 gives a little wiggle room for when you press on the file too hard when setting ring gap.
What the pi theory part of all does also, is let you know you have the correct rings for the piston