Frequently asked piston questions:

What is the recommended cylinder to piston clearance? 

For cast aluminium pistons we recommend .0035 – .005 (three and a half to five thousandths). This depends on a few things: the intended application, climate, and rider empathy. In all three of these variable the issue is heat. If you live in a warm climate, plan to race the bike, or ride it hard a few extra thousandths of clearance would be helpful. Read our article on cylinder boring for further info: Cylinder Boring.

Forged pistons will require larger. Since we don’t sell any it is best to contact the manufacture for their recommendations. To read about breaking in new piston rings read our article: Ring Seating Problems.


Why do modern rings have a larger end gap?

Modern materials will require changes to fitting practices. Triumph in their 1969 Service Bulletin amended the figures which still appears in most workshop manuals. The original figures called for .010 – .014 thousandths end gap (measured with rings inside cylinder bore). The revised figure calls for .015 – .020 thousandths. It is not uncommon to see up to .025 on a few makers. This should not be cause for concern.

What do the paint marks inside JCC pistons indicate?

The green and pinkish red paint marks are factory weight indicators. Green gets packed together and pink gets pack together at the factory. We don’t have specific data on actual weights. Though JCC pistons and rings have been excellent when it comes to sizing.

Does it matter which way you install the piston? Or are pistons directional?

For at least the popular Triumph, BSA , and Norton models very few are sided or directional. Norton 750 Commando uses a left and right hand piston. The valve reliefs give you a good idea which is which. They are also stamped L and R. All the other pistons should not be directional. We know that the inlet valve is usually larger than the exhaust. So if you encounter a piston with unequal valve reliefs the large cutaway will be the inlet side.

What does the stamping on the piston indicate?

All pistons we sell are stamped with the size. British pistons are metric sizes but expressed in imperial over sizes. IE: 650 Triumph pistons are 76mm and stamped .020 or .5mm larger. Other brands may be stamped with metric equivalent size markings.

Why is my bike smoking after fitting new rings? Which way do I install these piston rings?

Check out our tech article Ring Seating Problems.


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