Right, regarding the cracked piston, it’s bad news so far unfortunately…

To start with, I’ll actually show you the problem…

As you can see, on the top edge of the piston there is a crack running vertically from the top of the piston edge, down through the top two ridges. This is what makes this piston unfortunately useless.

So anyway… the brand new piston that was kindly posted to me from the chap in Derbyshire was sadly the wrong diameter – I need a 72mm diam. piston, and this was 74mm. That piston was therefore sadly posted back.

I then started making phone calls again and found that there a 72mm piston at AMC Classic Spares. I drove over there Saturday morning to compare the piston with my original one, and to hopefully make a purchase.

When I arrived, the piston was the correct diameter (72mm), it had the correct gudgeon pin diameter and distance from the crown, and everything about it was correct apart from the grooves cut out of the crown for the valves to drop into as they open.

Nevertheless, I purchased the piston and set about assembling the bike carefully, minus the gaskets, to see if the engine would turn over, and sadly it doesn’t. The inlet valve touches the top of the piston, meaning I would have to have the piston milled out to the correct size. The problem with this approach, however, is that there doesn’t seem to be enough metal on the underside of the piston to mill into and the strengthening ridges on the underside of the old piston to account for the deeper grooves don’t exist on the new piston so unfortunately, I’m back to square one again!

Just in case there are any fellow motorcyclists reading this blog, please get in touch if you think you can help! You can contact me through my 1966 Matchless G3 site!

Also, here are a few pics of the original piston so you know what i’m after…

1966 Matchless G3 "short-stroke" piston - 72mm, 85.5 stroke

Top of the old piston

The inlet valve groove is much larger than the the exhaust groove, as you can see. In the above photo, the inlet groove it at the bottom of the photo.

So… there we go - I still have a cracked piston, I still don’t have a replacement, but I’m ever hopeful that I might stumble across one soon enough. Let’s hope so, anyway!

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