Options to Convert G80 to Head Mounted Decompressor

[Andy Whitaker]

Not sure why but my '48 rigid has no timing case or head decompressor. How much do I need to change on my iron head (7 stud rocker) system to convert to hairpin? I have been told rockers, head and barrel is this so?

Compression is now killer since I got the valve clearances sorted. If I can't fit a decompressor to the head (crankcase is too much work) are any of these options possibilities?

- Fit compression plate plus gaskets either side.
- Change Hepolite piston for original flat top.
- Fit two head gaskets (as well as compression plate).

All advice appreciated as always

Andy

[GOLDSTAR]

did you get any info on this question? kind regards

[clive]

I would not fit two head gaskets, they will either leak or need frequent bolting down. The early barrels were made to be fitted with a compression plate, the later ones had a deeper base, so yes it would be an idea to fit it.
I have fiddled around with various combinations of parts on a number of occasions and have the following suggestions, no guarantees though.
The barrel should allow either head to fit, although I note there was a change in part number between 48 and 49 when the hairpin head came in, so cannot say for certain.
For the head change you would need
The head, 2 hairpin valve sets, the trays they sit in and the collars for the collets. The valves should interchange still using the valve caps if you have them fitted.
The rocker box including rocker arms as I am not sure they are transferable. The valve lifter arm and return spring.
A head steady triangular bracket and the two studs in the centre that hold them down. The three studs that go over the rocker inspection cover side, the rest of the studs will fit I think.
You may need different pushrods or you may get away with it.

Possible problems. The earlier petrol tanks do not have a cut away large enough for the valve lifter arm and attached cable, they increased the size of the cutaway when the hairpin valves were introduced.
I think the angle of the exhaust as it left the head was different between the hairpin and coil heads. So you may need to replace the pipe. If not for this reason there is another which is that the hairpin head has a stub for the exhaust pipe, whilst the coil head is a push fit. Pattern pipes are often the wrong gauge and whilst a pipe might fit satisfactorily into the head it may have the wrong internal diameter to fit over the stud.

You may be better seeing if you can fit the crankcase valve lifter and leave everything else, your cases must have the cast for the fitting point, unmachined, as they remained part of the crankcase for many years.

[dave16mct]

Completely different idea....I had a Royal enfield Bullet back in the mists of time (or was it fog??) It had a decompression valve which was just that - a valve which screwed into the cylinder head like an extra sparkplug. It worked. (Hope my memory isn't playing tricks, it was nearly 50 years ago...) Might be easier than changing heads etc. I'm sure Hitchcocks would sell them as a kit.
Dave.

[clive]

Completely different idea....I had a Royal enfield Bullet back in the mists of time (or was it fog??) It had a decompression valve which was just that - a valve which screwed into the cylinder head like an extra sparkplug. It worked. (Hope my memory isn't playing tricks, it was nearly 50 years ago...) Might be easier than changing heads etc. I'm sure Hitchcocks would sell them as a kit.
Dave.

Where would you screw it into? There is not a lot of room on a coil head to drill the additional hole without risking a crack developing into the sparkplug hole.

[Andy Whitaker]

Many thanks indeed guys for some great feedback and advice. It may be engineering suicide but I have hopefully(?) designed a cunning plan to fit a valve lifter to the rocker box. Who knows, but early indication is that to lift on the end of the exhaust rocker arm left of the pushrod could work and needs much less effort than I had imagined. I'm converting a Derbi clutch actuation arm which will be mounted to the tappet inspection cover with a circlip and 'o' ring and drilled through into the rocker chest. Will have a 90 degree 'quadrant' machined out of the round shaft to lift the end of the tappet and be spring returned (on the cable/mounting) to get back out of the way once the engine starts. Only thing I have to watch is I don't lift the rocker arm clean out of the pushrod end?!

If it works I will post some pictures, if not I'll be looking to convert the crankcase.......

Andy

[clive]

sounds a better plan than replacing the head. The original shafts have a flat machined into them so there is no impact when the bike is running, cannot quite visualise how a 90 degree quadrant would work.

[dave16mct]

Sorry Clive what's a coil head? I was thinking it might be possible because I've got 2 comp heads with 2 spark plugs. Are they bigger?
Dave.

[Andy Whitaker]

Hello Clive. I'm not explaining very well basically I am machining a flat with my concept too. Regards.

Andy

[clive]

Sorry Clive what's a coil head? I was thinking it might be possible because I've got 2 comp heads with 2 spark plugs. Are they bigger?
Dave.

Sorry Dave by coil head I mean coil or ordinary springs rather than hairpin ones. Are your comp heads iron or alloy? not heard of such AMC heads before with two spark plugs, what model are they off?