Quote Originally Posted by look no hands View Post
So what you're saying is if the bore had been 25mm on the Theoben then it may have made power easier? I think the other thing is the weight of the piston (heavy) and the piston seals aren't the best and are pretty hard, so probably don't seal very well (so power is lost there as well), not sure what the TP sizes are or what the stroke is, if it was easier to put together and take apart then I'd muck about with it more but trying to get the spring in with my spring compressor and then getting the circlip in place is a pain, it's even worse trying to get it out, what makes me chuckle is that a lot of the shed tuners always seem to revert to O ring heads when doing reduced cylinder conversions, so the Airsporter/Merc heads can't be all that bad, I agree they run better with the Meteor O ring.

Pete
It should be making a lot more power than a 25mm job if it has a proportionally longer stroke as well as the bore. The 25mm ones are just in the sweet spot for 11-12 fpe. Scirrocos were making 20 fpe on FAC IIRC. Have you tried measuring the transfer port volume by using the shank of drill bit?

Are you saying the piston seals are hard when they are new? It's a shame it is such a b'stard to strip and reassemble, sounds like a nightmare. Would building a better spring compressor help, something with more control like straps or something?

The tuners do seem to like their 'O' rings, but then they have got the gear to true up their cylinders to perfect. I still think parachutes are the best for most guns and most home tunes. The BSA Mercury, Airsporter and Meteor did give excellent consistency when fitted with the 'O' rings, but I think they were robbing power through lost volume and friction.