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PML Pistons and Rocker Box Covers for Matchless, AJS Motorcylces
    PML Pistons and Rocker Box Covers for Matchless, AJS Motorcylces
    Purchase PML Pistons and Rocker Box Covers for Matchless, AJS Motorcylces
    • PML Pistons and Rocker Box Covers for Matchless, AJS Motorcylces
      G80CS pictured

    • Call or email to order. We have very limited quantity.

    PML Pistons and Covers For Matchless, AJS Motorcycles

    For Matchless, AJS motorcycles PML makes pistons and rocker box covers. These are high quality parts designed by a Matchless G80CS owner.

    Parts are made in USA. Call or email to order. On line ordering not available.

    • If you have questions or wish to order, please contact us via email to or over the phone at 310-671-4345.
    • We ship internationally with the US Postal Service. It usually takes 6-10 business days. The shipping cost varies with the destination. Email us with your address for a quote. Some countries also impose taxes and duties upon receipt of purchases.


    Price: $325.00 US dollars for a set

    The piston fits 1960 to 1964 G80CS Matchless motorcycle.

    The piston will not fit 1956 to 1959 models.

    The piston is 2618 non-silicon alloy. The dome has a ceramic coating. We recommend .002 – .003 inches radial clearance at the bottom of the skirt, depending on if you are racing or street riding. These pistons fit much like the Hepolite wire wound pistons with .002 inch clearance. We have set up six of these pistons to date (October 2011) at the lower clearance and had no problems.

    The compression ratio is: 8.4:1

    These pistons are CNC machined from a forging and are lighter than the original piston by approximately 3 grams.

    Sizes available are:

    • 86mm (standard)
    • 86.50mm (+.020inch)
    • 87.00mm (+.040 inch)

    PML G80CS Matchless piston   Click on image to view larger.

    For example, the 86.5mm bore piston is made to fit a bore of 86.50mm or 3.4055 inch diameter after final cross hatch honing. A bore this diameter will give you .004 inch total clearance 1.100 inches below the bottom oil grove measured 90 degrees to the pin bore, and .002-.003 inch total clearance at the bottom of the piston skirt (90 degrees to the pin bore). This is shown in the installation instructions. In most application you will get the best performance and life from fitting to the 86.50mm diameter bore.

    Please note that piston clearance is always measured as total clearance (diameter – diameter) and not all around as a radial dimension. This sounds like very small clearance but these are forged pistons and not cast.

    PML G80CS Matchless piston, front view   PML G80CS Matchless piston, side view   PML G80CS Matchless piston, angle view   PML G80CS Matchless piston, top view   PML G80CS Matchless piston, view of inside
    Click on an image to view larger in a new tab or window.

    Matchless Piston Installation Information

    Usually it is easiest to install the rings on the piston and then put the piston into the cylinder (before installing the cylinder on the crankcase) with the cylinder base pointed up. Push the piston all the way into the cylinder. Be sure the piston is turned the correct way around and but the ring gaps at 180 deg. Then lower the cylinder down over the studs, but support it about 2-3 inches off the crankcase with a deep socket or a block of wood. Now lower the piston only to the point that you can install the piston pin. Heat the piston with a heat gun (hair dryer – it does not take much) and push in the pin. Put rags around the bottom of the piston and install the keepers (you can start with one keeper in place). This is usually easier than installing the piston on the rod and then trying to lower the cylinder down and get the rings in all at the same time.

    Matchless Piston Break-in Recommendations

    We recommend performing a very fast break-in with the first 1/2 hour of riding going up through the gears progressively harder, and a 2 – 4 minute coast or cool down (low throttle and mid range RPM) between hard running. Avoid more than 1-2 minnutes at more than 1/2 throttle without a several minuets for cool down for the first 100 miles. At this point you can consider the piston rings are broken in and seated. Do not be afraid to make power, or rpm, just not for long periods.


    Price: $100 for natural cast finish, $145 for polished finish (prices reflect discount for cosmetic blemishes in casting)

    The rocker box inspection cover for Matchless and AJS adds additional crankcase breathing to your machine and reduces crankcase pressure. This in return reduces oil leakage.

    This is a faithful reproduction of the Webco inspection cover made in the 1960’s. It is sand cast aluminum and heat treated 5051-T6 aluminum.

    PML supplies the rocker box cover with the brass 1/8 Pipe to 3/8 hose fitting, and sealing o-ring (#247) as shown below.

    The cover comes in natural cast finish or polished. For the polish finish, PML takes a cast part and the outer surfaces are polished. The area between the fins is left as the natural casting. For those who wish a fully polished piece, you can purchase the polish finish cover and polish between the fins or purchase the natural as-cast finish cover and polish (or have someone else polish) the whole piece.

    Fitment is heavyweight single rocker boxes (reference Matchless and AJS G3, G3C, G80, G80CS, 16M 18, 18S, and so on).

    The image on the left shows the cover in the natural cast finish. The image on the right shows the polished finish.

    PML Matchless, AJS cover, cast    PML Matchless, AJS cover, polished

    Images below show the fittings and o-ring and the inside of the cover. Even the WEBCO name is reproduced in the casting.

    O ring provided:
    PML Matchless, AJS rocker box cover o-ring and fittings    PML Matchless, AJS cover, inside

    This image below shows the cover after it has been completely custom polished and installed on a Matchless G80CS.

    PML Matchless, AJS cover, polished, on G80CS) motorcycle

    Click on images to view larger in a separate window or tab.

    If you have questions or wish to order, please contact us via email ( or over the phone (310-671-4345). On-line ordering is not available for these products.

    We ship internationally with the US Postal Service. It usually takes 6-10 business days.