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1933 CJ 350 Piston clearance

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Hi I have obtained a NOS Hepolite 71 MM +.040" piston Cr 7.25 TO 1 

My cylinder is off for to have a cylinder liner fitted   and the machinist wants to know what size the bore is to be finished to ? Can anyone please advise ?

Regards

Philip

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Philip - if you look up 'clearance piston' in the search box, you should find what you need.

For the 500cc bikes (79 bore), it looks like .007" below top ring, .005" at skirt (so a feeler would have to go in from the top...) measured at 90 degrees to the pin.

I  make that about .00625 for 71 bore.  Unless someone else comes up with better, I'd stick to .007", having once enjoyed a cammy Norton engine seize up due to inadequate piston clearance.

 

 

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... are around .007" according to Edgar Franks. So I would always err on the larger side and give it.006" clearance at the skirt. Saves all that dreary running in as well.......

I had a 1931 Sunbeam which had been rebored before I took it over and had been returned to the vendor (a well known Suffolk dealer) by its previous owner as it had seized (as I discovered after I'd bought it). It did indeed tighten up as soon as I gave it any real work to do. I took it to my local rebore shop and had them hone .002" out and it was fine - no burning oil, and it would take full throttle with no signs of tightening up.

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Hi Philip,

   The way this works is all done for you, if you just follow the size stamped on the piston. If the piston is marked 71mm + .040" then this is the accurate size the cylinder should be bored to. The manufacturer has cam-ground the piston to be the correct clearance when fitted to the stamped size. If you add any bigger clearance to that you will have the equivalent of a fairly worn barrel before you start. The aluminium alloy of different makes of piston can vary in it's composition so go by the manufacturers stamped size. 71mm + .040" equates to virtually 72mm, with 72mm being only half a thou. more, so that would work well. I don't know if you are aware, but the 1930s CJ, Inter 40 and Model 50 all use the same piston in standard form.

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I wouldn't consider .001" extra as being fairly worn myself, having had many bikes which have run perfectly with much much bigger clearances than specified.

I too have never heard this before but what bothers me is that modern piston manufacturers may be specifying clearances suitable for close tolerance liquid cooled engines.

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I think Richard is saying that, when a piston is marked as 'standard' (i.e. to be 71mm for the 350), it is really machined to 71mm less the specified clearance (.007" below the bottom ring in this case).  So if you bore to 71mm plus .007", you would have .014" clearance - the same as .007" wear, which is the upper limit.

Because the alloy pistons expand more than the iron barrel, and also probably get hotter, the clearance required depends on the different expansion rates.  I calculate that, with .007", the piston clearance will fall to nothing at a piston and barrel temperature change of about 200 deg C. 

But that makes me think - with a water cooled iron barrel, there will be less barrel expansion than with an air cooled iron barrel, so the clearance should be bigger!  Unless the colder barrel also leads to a similarly colder piston (which it probably does...)

Low expansion alloys are usually used, but even then have a rate a bit less than twice that of iron, instead of a bit more than twice.  Only the piston manufacturer will know what alloy he uses...

All modern vehicles use alloy for block and barrel, so if the temperatures are closely similar they need next to no clearance.  I think I read somewhere that some F1 cars have interference fit pistons when cold, so have to be preheated to start!  The pistons must expand less than the barrels!

 

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Ian,

   Sorry If I didn't explain that very well, but It was nearly mid-night when I posted, so probably nodding off ! David has put what I meant, so that's good, but I didn't want Philip to add 7 thou on top of the stamped size as that would be worth about 15,000 plus miles of wear. Yes a couple of thou extra is O.K. and is what I would do myself.

Re.- Piston sizes, if you go to an auto-jumble any time, take a micrometer or a dial gauge and measure the front to back of a new piston skirt and compare it to the stamped size and you will see the difference which is the manufacturers clearance.

David,

   Thanks for clarifying my post. Yes F1 engines do have to be pre-heated with coolant and oil which is at operating temperature They are probably magnesium alloy blocks with ceramic coated bores and designed for about a 2,000 miles life-span.

 

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