Moly Coated Piston Installation

Moly Coated Pistons.

There is some advice for engine builders new to dry coated pistons who routinely mic the pistons and measure the bore size before assembly.

The recommended (nominal) piston clearance for a TZ350 is as follows:
0.040 - 0.045mm (0.0016" - 0.0018" in)

TZ 350cc bore nominal size 64.00mm will normally be fitted with a 63.96 piston, as we are all aware this is marked on the crown .96 which is .04mm (1.6Thou") clearance.

With the Meteor pistons we supply - the actual diameter of the bare aluminum piston is stamped on the piston crown.
Clearance is NOT adjusted to include the coating, so checking the size over the coating is not helpful - the sizing i.e. 96 is the only thing to look at with a new piston.

The bore should be measured with a bore gauge to find it's actual size & then deduct the piston size measurement from the bore actual size will give you the running clearance.

The coating on the Meteor piston will take up much of the clearance you would normally see with an uncoated piston - this is exactly as it should be - the coating acts as a "solid film" lubricant - and is in fact designed to fill the space.

The dry coating reduces clearance while preventing metal to metal contact (cylinder wall and Piston), and will, being sacrificial, "rub off" as it is "run in" and will automatically be self-fitting to the cylinder bore during this run in period - a "well fitting" piston will also not wear the cylinder as much - and provide better reliability and performance.

To state the obvious - if you have a worn barrel - check the "actual" size - if you have for example 64.01 - a 97 piston will be correct ( 64.01 less 63.97 = 0.04mm or 0.0016")

So, in summary, the coating is NOT part of the piston's mechanical dimension for fitting purposes.

If you need more information - just do a Gogle search for "Moly coated pistons" - there's plenty of information available explaining the technology and the benefits for our engines.

Meteor pistons are supplied with "chrome rings" - suitable only for Nicasil plated cylinders - these rings should NEVER be used in the original "Hard Chrome" Yamaha cylinders - a steel ring suitable for hard chrome can be supplied on request. 

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