New pistons

Puddle jumper

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Hi Guys,
Before I had the cylinder’s bored I turn up a mandrel (it was left in the chuck so running true at all times) to the exact size of the existing bore then mounted each cylinder on it using the tail stock with a pad to clamp the cylinder whilst machining, then faced the base (removing minimum amount to clean and match the cylinders). After this I also machined in an ‘O’ ring groove to help seal the base with out using a gasket.
The base is now true to the bore and can be used as a mounting face for boring.

Cheers Paul
 

Puddle jumper

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No, the bores were very good to start with no lip at the top and on a trial hone only took 0.001" to clean up, as long as the bore is perpendicular to the base it will be correct, there is more chance that the muff has moved over the years than the liner!!!!. And in my case the base was running within 0.003”( and that was a slight hollow ) before I machined them
Cheers Paul
 

timetraveller

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If your comment about muffs rotating was a 'reductio ad absurdam' then fine as there is no chance of the muffs rotating due to four studs passing through them. But for those with less experience I have seen lots of bikes where the liner has rotated, easily spotted by the lubrication hole on the rear no longer being on the rear. Machining out the muffs and fitting thicker liners to regain the original six thou interference fit is the answer to those with that problem or, of course, alloy liners with a ceramic coating inside.
 

craig

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Comet piston fit

I have 0.100" piston protrusion with a new piston fitted to a new liner. Is this okay? Are there any standard limits for this?
Craig
 

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timetraveller

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It is the same answer that I gave for the clearance between valves and pistons. Put some Plasticine round the top of the piston, put the cylinder head back on (you do not need to fit the tubes etc which cover the push rods) and turn the engine over a few times. Remove the cylinder head and see what has touched and where. Provided that you have at least 50 thou clearance every where you will be alright. Note that if you fit cylinder heads and pistons designed to give a squish band there must be at least 35 thou clearance to allow for expansion and stretch etc and not more than about 45 thou if the squish band is to do its job. (If the term squish band is not familiar to you then I think that the American expression is quench band but am not totally sure about that.)
 

craig

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new 8:1 piston with 0.100" liner top protrusion as stated earlier, plasticine check, no problem with clearance.
 
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