Piston Height

[Nigel Barton]

Thats all good information Stan I wish Bob Mills was still alive he would be able to give me a piston height for the TFS its a shame all of this data is not documented very well.

[Dick F]

Nigel, I understand that you are running the TFS with an S25 carb? correct, Rob and myself are using the Amal concentrics on our Anglians, and Robs is the newer version as well i have forgot what it is called, but my engine has had the head filled and like Rob it is better but still a bit of a problem with mid engine revs and still will not rev out on full throttle! i think that Terry Sewell uses an S25 on his Anglian and that is very smooth, but what else he has done i dont know, I or Rob dont have head gaskets either, so we could be wrong ther maybee, it needs a lot more info from those who know as we all seem to be stabbing in the dark a bit, exhaust systems need to be better than the ones fitted to Anglians that left the factory years ago but you can go on forever changing bits and pieces in the pursuit of perfection . lets hear some more from those ace tuners on hear. grinnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnnnnnnnnnnnnnn

[Nigel Barton]

Nigel, I understand that you are running the TFS with an S25 carb? correct, Rob and myself are using the Amal concentrics on our Anglians, and Robs is the newer version as well i have forgot what it is called, but my engine has had the head filled and like Rob it is better but still a bit of a problem with mid engine revs and still will not rev out on full throttle! i think that Terry Sewell uses an S25 on his Anglian and that is very smooth, but what else he has done i dont know, I or Rob dont have head gaskets either, so we could be wrong ther maybee, it needs a lot more info from those who know as we all seem to be stabbing in the dark a bit, exhaust systems need to be better than the ones fitted to Anglians that left the factory years ago but you can go on forever changing bits and pieces in the pursuit of perfection . lets hear some more from those ace tuners on hear. grinnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnnnnnnnnnnnnnn

Hi Dick I am using the new light 626 Amal engine revs out, electrex ignition, engine nice n soft runs very well except for the detination... I am getting there with it and will do write up when its sorted out.

[John Wakefield]

Not sure about the Trials/Scrambles Greeves alloy cylinders, but on my RDS Silverstone the instructions are to shim the base of the cylinder to give .065" - .075" from the top of the cylinder sleeve to the top of the piston at TDC. The cylinder has to be temporarily clamped down to the crank cases to set this up. This is supposed to give the correct port timing. On the RDS cylinder this still leaves about 3/32" of piston above the bottom of the exhaust port at BDC but the piston is more or less level with the bottom of the transfer ports. On the RES cylinder this is worse as, although the port map (dimensions of ports and distance between them) is the same as the RDS, the whole port map is moved down the cylinder by 1.75mm so the exhaust port is even more covered by the piston at BDC and the bottom of the transfers are covered as well.

As for the piston skirt being level with the top of the inlet port at TDC, on the RDS cylinder the skirt it is about 3/8" above the top of the port when set to the .065" - .075" dimension.

So, in my experience, whether the piston partially covers the ports at BDC is not an indication that the cylinder is shimmed correctly to give the original factory port timing.

Trials/Scrambles types please weigh in if this only applies to the Silverstone set-up.

Stan

This would appear to confirm what Villiers Services have said ie that the top of piston should align with the bottom of the transfer ports at the bottom of its stroke, that should set the position for the other ports. Obviously its more critical on a road racing engine that a trials or road bike engine. The best idea would be to refer to the relevant workshop manual for said engine.

[John Wakefield]

I have found this book Two Stroke Tuners Manual as a pdf on line http://www.amrca.com/tech/tuners.pdf Lots of technical stuff there which may help

[druid]

In answer to Stan's post all longstrokes used this figure to determine the cylinder height and it was the only this measurement that the engine shop was concerned with. Squish bands and the like were determined by the as cast cylinder head.

Druid

[Stan Nicholson]

Thanks John, it's great to have this confirmed. I had another look to see where I found this dimension for setting up the cylinder but the only place I found it was a reprint, in Derek Pickard's book, of a 5 page piece from Motor Cycle written by Vic Willoughby in the 20 August, 1964 edition. The article was based on an interview with Bert Greeves about the, then new, motor used in the RBS. I don't know if this has been posted on the forum but I guess a copy could be obtained through Morton archive service. There is a lot of useful information in the article.
I am surprised that there is no mention of this set up dimension in the RDS service notes I have.
I have always wondered how people have supposedly set up squish clearances of .030" or so when the piston stops .065" from the top of the cylinder sleeve. It's not as if the flange on the top of the sleeve can be machined down as it doesn't protrude that far past the top of the cylinder casting but I guess the casting could be machined back to suit. Then there is the compression plate on the road racers but this could be eliminated if the head was machined to give the 9.15 to 9.35 to 1 compression ratio required for the road racers.

Stan