Long stroke piston / liner clearances?

bleudanube

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I have been hunting through the manuals and websites but cannot find any tolerance or clearance data between piston and liners for the long stroke DS/ID engines...

would anyone be able to advise? My machine shop is asking so he can machine the correct tolerances.

Appreciate any help. Maybe I am just blind... 😎

Sven
 
I doubt if it is in the shop manual. The piston clearance depends whether you have forged or cast pistons. For forged pistons, the rule of thumb is the clearance should be 0,075% to 0,1% of the cylinder bore. Cast hypereutectic pistons have a very low expansion rate, so all that is necessary is to allow room for lubrication. They can be installed with as little as 0.0005-inch piston-to-wall clearance on the major diameter.

I have assembled two long stroke engines recently. Both times the pistons came matched to a set of liners. The first set was unbranded and had about .001-.002 inch of clearance. The engine runs well although there is some noise. The second set, which was purported to be Mahle quality (made in Turkey) had only about 0.0007 inch of clearance according to my measurements. That engine has yet to be started, but will be (hopefully) started in the next month or so.
 
Thanks John! Also talked to Darrin from Citroen Classics in the UK and his shop runs off 2thou, so about 0.05mm, which is close to your percentage of 0.75% and matches the upper end of your first engine you described.

cast or forged? How can I tell? Here the piston photos: I assume they are cast as the top isn’t machined!?

1F9BCFA2-2EDE-4AC3-9DB8-9DB1D1F37535.jpeg
CBB8D6EA-3762-4954-9ABD-1EFA1F95F120.jpeg
 
Ringer has kindly pointed out that one thou per inch may be more appropriate! I was told a looooooong time ago by an engine reconditioner of the old school ( he may well have passed on by now ) and maybe I am not remembering correctly! In any case I think 'Ringer' may well be correct!
 
Thank you all for the information. Speaking to my machine shop as well we settled on 1.5 to 2 thou. He believes that is suitable for those older style engines. Which matches roughly the 0.5 thou x 3.3 inches diameter. I will go with that and see what comes of it.

Sven
 
Find another machine shop, they appear to have no clue if they are asking you.
 
Find another machine shop, they appear to have no clue if they are asking you.
Maybe a bit harsh.
They might be asking if these exotic and ancient vehicles have any special requirements.
They might be simply double checking, to avoid error.
Something that is not done enough these days.
 
Exotic and ancient vehicles can have special requirements.
Example 1968 BMW due to internal block temps have different clearances for each cylinder.
Machining cylinders shows how to finish to final bore and honing stones to use
Ask your machine shop what they do to get to final size. Then compare with doco above.
Old Citroen engine Piston to Liner Clearance 0.06 to 0.08 mm Spec Sheet of many
Enjoy the rebuild.
 
Well, got my cylinder head rebuild and the measurement for the barrel and pistons as well - the question now is whether I need new liners and pistons. 2 thou seemed to be the maximum clearance to go with. The attached table shows the measurements.

52F9A8BA-27DF-4926-97BB-1ABCE4333304.jpeg


anyone any thoughts? Too far gone or usable?

Sven
 
Plenty good enough for another 250,000 km.

I've reused piston/sleeve with clearances many times that, and for at least 100,000km.

I reckon they've been replaced not long ago, in engine hours that is. It might be many years, but very few hours.

But then, I'm a bushy with no money!
 
How noisy do you want the engine to be? With that much clearance it will work, but you will almost certainly hear the pistons slapping.
I don't think so.
The pistons are so long you need about 10 times that clearance to get any detectable noise.
 
Hi Sven, brand new Mahle piston/liner sets are available from CitroTech in Holland for about Euro 450 if you're thinking of replacing with new.
 
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