Hi Rusty,
  I will take a stab at this. I believe that very early LA40s didn't have a boost port. I have never seen one myself, but it has been reported on the forum, I think. The piston problem puzzles me a little. I have never given it much thought, but I was under the impression the LA pistons were cast and then machine finished. More than likely wrong on this and I am sure someone will come up with the correct answer.

Regards,

Andrew.

the one with the boost port is a very early one.  OS removed the boost port because it does nothing at the speed an LA 40 normally runs for RC  ~ 13,000rpm.  It also does nothing for a stunt run.

Somewhere lost on the web is a report from a guy in England. He had a two LA40's, one with and one without the boost port.  He ran both engines with both liners and found they ran exactly the same on the same prop.  I believe he also tried blocking the boost port in the case and it made no difference.

The machined piston is the newer version. 

I have 2 OS LA 40s.   They are constructed differently.  One has a cast piston and the other has a machined piston.  One has a transfer port on the side of the liner opposite the exhaust and the other doesn't.  Everything else appears to be identical.

What do you suppose the reason for this is?  Do you think one is an older version or what?

Rusty

I believe that the later model LA series (without the boost port) was to distance their plain bearing sports engine from the ball bearing AX series and polarize them a bit better into a market niche.

I have read that the later version actually performs better at the lower part of its power band, and its slightly cheaper to make.

Cheers.