Piston Height

[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.

[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]

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.

[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

[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]

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.

[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.