lapping tool

The Easy Way to Piston Lapping

Most control line engines do not make use of a piston ring to maintain compression,

The work of the piston ring is done by a narrow sealing band close to the piston crown, sized to fit the liner at working temperature. 

The appearance on the piston wall of scratches, streaked or matt surfaces indicates friction that causes heat build up & poor performance. 

I will explain from my experience with simple tools how to lap the piston & achieve great results.

The piston/cylinder illustrations are exaggerated to clearly show the difference in shape between hot & cold.


From the illustrations you will see that the heat from combustion expands the upper cylinder, changing the shape from tapered to more or less parallel.

This is the reason a cold engine exhibits mechanical interference in the upper piston fit, more so in diesel engines than glows.


In order to achieve an optimum working fit of the piston it can be lapped to achieve this at the sealing band only.

The piston sides will need to be worked on above & below the sealing band with simply made tools from aluminium or celotex (fibreglass & epoxy)

All friction marks must be carefully removed using the tool with lapping paste & kerosene in a combined rotary & axial motion AVOIDING THE SEALING BAND.

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Pronounced high spots can be worked with 2000 - 2500 grit wet & dry paper & kerosene followed by the lapping tool.

Wet & dry is very good in some cases where small areas need to be worked & smoothing micro roughness after lapping.

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Piston Cleaning

After lapping the piston needs to be scrupulously cleaned with alcohol or thinners  and brushing, test for cleanliness by using kitchen paper,

It should remain clean after wiping over.

Carbon deposits should also be removed from the piston crown, the face of the contra piston can also be cleaned in situ with a wooden stick.

Testing For Correct Lapping

Reassemble the engine & run for 5 minutes at peak revs to achieve proper working temperatures.

Dismantle & inspect the piston, any signs of friction should be removed by further lapping, cleaning & testing, repeating the process as necessary.

Remember to pay particular attention to piston cleaning after lapping.

Additional Information

Engines with iron/steel piston/cylinder construction must be cleaned regularly, failure to do this will result in deterioration of performance.

The contra piston can be scraped clean with a wooden stick or chop stick, the piston crown again scraped clean & finished with a scotchbrite scourer.

After cleaning wash with alcohol or thinners until no dirt shows on the kitchen paper towel.

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Lapping Tool

Simple lapping tools can be made or bought with excellent results, a second tool can be fitted to the piston to protect the sealing band area during lapping.

T3 lapping tool

Necessary Tools

Lapping tool from celotex or aluminium ….   You can find this tools in this website

2000 - 2500 grit wet & dry paper

Kerosene, denatured alcohol or methylated spirit

Kitchen paper towel

Cotton cloth

Fine brush, toothbrush

© EXT2CL 2015