Piston Height

[Nigel Barton]

Hi does anyone know the correct piston height TFS Viliers with Greeves chalenger trials barrel?
I am geting detonaton, knock when off load mid revs. Elextrex ignition timed on z mark checked with strobe against marks for tdc and 3.5mm btdc and strobe shows just retarded from 3.5mm at fast idle, advances to advanced of 3.5mm. When timed on T mark detonation or knock was much worse. Engine just had full rebuild.

[John Wakefield]

It wont be the piston height but compression ratio which may be too high if you have a scrambles head. Chairman Rob has done a lot of work on his bike to solve this problem and it was written up in LL a year or so ago. No doubt Rob will come on board and explain. I think this is a common problem with the Challenger set up on the TFS and Anglian.

[teamferret]

I've had similar. I've had the head in my lathe and taken off quite a lot of material and widened the bowl such that it extends out to the bore diameter. It seems good but I won't know for sure until the trial on new years day. If it runs well, I'm going to remove the rather thick head gasket and put the same thickness under the barrel as I'm running a short piston from a starmaker.

[John Wakefield]

Reducing the compression ratio is the answer, thats what Rob did, Maybe someone can find the relevant article in LL and scan it.
You really need to check the volume of the cylinder head, as the basic casting was used for both trials and scrambles bikes. The Compression ratio on the TFS TGS THS & TJS being 8.86:1 The scrambles version was 10:1 so it could well be that you have a 10:1 head on a trials bike. One easy way to correct this rather than machine the head is to make of a thicker head gasket 1/16 or 2 mm should do the trick

[Nigel Barton]

Thanks for you input Simon (nametab) has confirmed TFS should have spacer between head and barrel, also found thread on here with greeves original drawing of said spacer or compression plate as they called it.

[Rob]

The first thing is to check the compression ratio. My Anglian (which knocked and clanged badly) was 13:1, the problem when putting together a basket case with parts as supplied! I filled the head with weld and then machined a 10 mm squish band with about 35 thou clearance and machined out the middle until I arrived at a 9:1 CR. The result was amazing and it was obvious just from starting it how different it was. It whizzed along like a Bultaco and with no banging and clanging! In fact, it was a different bike and I had previously thought the harsh engine was to do with the Electrex World ignition but it wasn't. Incidently, I found that by timing on tdc, using the appropriate mark, gives the best results! Yes, I know it is bad practice but for some reason, it works! I use no head gasket or any oher spacers.

[Nigel Barton]

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.

[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.

[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]

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

[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]

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]

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

[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.

[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