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 and 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.

1.- Fix the piston to one of the tool provided. Limiting the lapping área. The piston shoud not move.

2.- Put a small amount of polishing paste and lubricating fluid (kerosene and oil) to second tool.

3.- Make a movement to right-left and up-down. You must note the piston slightly adjusted.

      Now, you are starting to lap the piston.

Is not necesary clean  the tool after finish the work. The exces of  fluid can be activated in the next use. You only need put some drops of kerosene or fluid to actívate.

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

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Use as lubricant cutting fluid or mix of Kerosene +some  oil drops

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fix the piston with one tool to limit the lapping zone

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put inside tool a small amount of polishing paste

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Use the second tool for lapping operation. Quickly moving the tool right -left and up - down.

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Note how it has been the lower part after lapping process. 

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Now fix the piston with one tool for limit the upper lapping zone

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Use the second tool for polishing operation. Quickly moving the tool right -left and up - down.

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look the final result. Now is necesary clean piston scrupulously.

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work finished

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. Is very important repeat this procedure the necessary times until clean paper is without traces of dirt.

Testing For Correct Lapping

Reassemble the engine & run for 5 minutes at peak revolutions 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 until not more until the friction marks dissapear.

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.

Necessary Tools

1.- Lapping tool set …. you can find in this website   

2.- Paste for lapping… Be carefull not all are good for aluminium pistons. Avoid paste with diamond content . Lapping tool sets comes with a small amount of compound for use in aluminium and Steel pistons.

3.- 2000 - 2500 grit wet & dry paper

4.- Kerosene, denatured alcohol or methylated spirit

5.- Kitchen paper towel

6.- Cotton cloth

7.- Fine brush, toothbrush

Lapping Tool sets

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.


* 12,50 mm inside diameter for Parra 1,5 Wasp and other 1,5 engines.

* 14 mm inside diameter for Parra T3, Oliver Tiger 2,5 (.15), Rothwell 250, and most of Oliver replicas.

* 15 mm inside diameter for Parra 2,5 AAC -ABC- SSC, Fora 2,5 and most of 2,5 cc engines.

* 16 mm inside diameter for Parra T4, Oliver Tiger 3,2 (.19), Rothwell 320, and most of .19 engines.


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lapping tool


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