Piston Height

[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

[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

[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

[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.

[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.

[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]

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.

[Stan Nicholson]

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

[Nigel Barton]

I have today spoken with Villiers Services and as I suspected there are thick gaskets for setting piston height / port timing they come in 1.5mm, 1mm, 0.030" and the usual thin ones.
Barrel should be shimmed up using gaskets to get transfer ports level with top of piston at BDC.

I shall keep you informed of outcome once said gaskets arrive.

[Nigel Barton]

Thanks for your input John, I will whip out the intake manifold and check...

[John Wakefield]

Re Compression ratio, not sure about adding in the head compression space I worked it out as 246 divided by 9 which would equal 27.3 as the head volume. This of course is a theoretic calculation as compression does not actually start till after exhaust port has closed.
With regards the base gasket thickness, This needs to ensure that the piston shirt aligns with the top of the inlet port at top dead centre and at the same time is a few thou short of the piston crown reaching the top of barrel. So your base gasket needs to ensure these dimensions are achieved.
the 8.45 comp ratio should be fine. Thing is the large alloy fins on the barrel do tend to 'ring' a common problem with alloy barreled 2 strokes, many continental and Japanese engine manufacturers fitted rubber bungs into fins to reduce the noise. So it maybe there is nothing wrong with your engine.

[Nigel Barton]

Am i right i saying compresion ratio is the complete volume of the cylinder including combustion space devided by combustion space? This would be 246+27.3÷27.3= 10 so a ratio of 10:1
That aside currently the bottom edge of transfer ports is below top of piston at bottom of stroke is this correct i reckon i need a 2mm gasket to flush them with top of piston, currently no gasket on base.
I am using no 8 plug 140 main 126 needle and beedle in middle slot. Two 2.1 mm compresion plates under head, i have calculated as above comp ratio at 8:45 and still knocking on needle jet fine on main. Have tried main jets from 170 down the bigger the main the less knocking on needle so fuel is my freind but when i fit 140 to not 4 stroke on main it knocks on needle.

Increase needle to 127 or down a slot?

Any help apreciated starting to loose patience

[John Wakefield]

Might that be millilitres, John (i.e. thousandths of a litre)? Micro litres are probably millionths of a litre, rather difficult to measure outside of a specialist laboratory.

Yes millilitres John my typo, duly corrected
Do not fit a thicker base gasket as that will alter port timing, better to make up a thicker head gasket. But you really need to measure the capacity of the combustion space to determine the actual compression ratio. To do this (with head in place) move piston to top dead center, remove the spark plug and using a calibrated syringe and some engine oil, measure the amount it takes to fill up to the plug hole. For a compression of 9:1 the capacity needs to be 27,3 milliltres litres.

Note I have had to re post this comment due to this stupid website thinking my correction was spam

[johnrunnacles]

Might that be millilitres, John (i.e. thousandths of a litre)? Micro litres are probably millionths of a litre, rather difficult to measure outside of a specialist laboratory.

[John Wakefield]

Following on with the issue of detonation I am getting there running with two 1.2mm head gaskets has almost done the trick. Two head gaskets is not ideal so I intend to cut a thick base gasket.
Has any one got a base gasket they could scan and email the image? I can then import to autocad scale up and cut a gasket with my cutter.

Do not fit a thicker base gasket as that will alter port timing, better to make up a thicker head gasket. But you really need to measure the capacity of the combustion space to determine the actual compression ratio. To do this (with head in place) move piston to top dead center, remove the spark plug and using a calibrated syringe and some engine oil, measure the amount it takes to fill up to the plug hole. For a compression of 9:1 the capacity needs to be 27,3 milliltres litres.