View single post by Esprit2
 Posted: 03-29-2015 10:11 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 311
Barthol wrote:
(Snip)... Using a dealer gauge shows me a clearance of 0,75 mm between the liner and the piston again with the piston in the middle of the stroke.
I insert the gauge from the top and guess that i can reach down to the top piston ring.
Is this the right way to measure the clearence between piston and liner?
Looking in the manual it looks like the clearance is supposed to be between 0.0965 to 0,1219 mm.

as previous I still measured compression of 190 psi which puzzles me , if the clearence is this big.

Any Ideas ( It all started with a lot of blue smoke under acceleration, - however the engine performed very well

The rings provide the compression seal, not the piston clearances with the liner. Excessive clearance will result in piston 'slap', which will result in excessive wear, and which can be heard.

The piston is not a straight-sided cylinder, rather it has a slight taper (smaller at the top, larger at the bottom). As a result, the diameter must be measured at a specified position:

Iron Liner ID -
Grade A = 95.275 - 95.288 mm (3.7510 - 3.7515") J-H
Grade B = 95.288 - 95.308 mm (3.7515 - 3.7520")
Bore diameter is measured using a bore gauge, or T-pins and a micrometer, across the thrust axis, at
50mm down from top edge.

Piston OD -
Grade A = 95.1662 - 95.1789 mm (3.7467 - 3.7472")
Grade B = 95.1789 - 95.1916 mm (3.7472 / 3.7477")
Piston Diameter is measured with a micrometer, 90° to wrist pin,
15.0876 mm (0.594") up from skirt's bottom edge.

Take both measurements above, then subtract to determine the clearance.

Piston/ Liner Clearance = 0.0965 - 0.1219 mm (0.0038 - 0.0048") J-H.
..................................... 0.1016 - 0.1270 mm (0.004 - 0.005") Lotus.
Same pistons, different corporate specs (Lotus rounded up the inch dimension).

The 'observed' clearance at the top of the piston will be larger, as you have noted.

Last edited on 03-30-2015 07:13 pm by Esprit2