Here's a question, just out of curiosity.

In the 325 Sherpa barrel there are two slots at the rear, either side of the inlet port, that run vertically. There are two corresponding holes in the top of the piston. I've also seen these on 349 Montesa barrels and on the Aprilia Hiro barrel, although the Armstrong Hiro barrel doesn't have them.

As they aren't higher than the top of the piston, any fuel mixture that gets pushed up inside the piston on its downward stroke and goes through the holes in the top of the piston and into the slots, doesn't actually have anywhere else to go. The slots remain covered by the piston.

So, anyone know what purpopse they serve? Just curious as I can't work it out. I can't see how they can be performance related and can't see that they help with lubricating or cooling the piston.

Lubrication?

Surely intended as little puffer ports which also help to circulate gases under the piston crown to keep it cooler than it otherwise would be? Its a long time ago, but I'm certain that these ports on my 76 325 were in the right place to give a little extra 'puff' at BDC.

I can't speak about the Bultaco barrel but on the Hiro the slots you're refering to are additional transfer ports. The length of the slots corresponds more or less to the distance from the piston skirt to the bottom of the holes in the piston, both of the slots terminate in the barrel at the same level as the top of the transfer ports. Regarding the fuel having nowhere to go, the slots and holes are postitioned to operate at the same time as the normal transfer ports but with the bottom or top of the slots timed to be sealed by the piston skirt or piston top so I'm sure it does go somewhere although maybe not that efficiently. With respect to Hiro engines, the late Aprilias and Mk3 Armstrongs are ported in this way to give smoother power (according to the sales speak of the day!). I have always suspected that Bultaco & Hiro/Sammy Miller/Armstrong etc. have more than a little in common.