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Long stroke piston / liner clearances?

bleudanube

Active member
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
 

citroenthusiast

Active member
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.
 

bleudanube

Active member
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
 

gerrypro

1000+ Posts
I was told the general rule of thumb for aluminium pistons was half a thou for every inch of bore size!
 

gerrypro

1000+ Posts
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!
 

bleudanube

Active member
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
 

driven

1000+ Posts
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.
 
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