Vintage Workshop
Services for Brough Superior motorcycles and their contemporaries

Throughbolt conversion for 680 ohv engine
last update 6/2003

One of the problems with the JAP 680 ohv engines is that the cylinder barrels are only held by 4 bolts on a relatively light bottom flange which has a tendency to crack. This is made worse by the very thin cylinder wall above it, especially if you are running on the 3rd re-bore. The wall can be as thin as 2.5 mm by then!

I am not sure if the reason is the shaking of the tall cylinders, the combustion pressure or an occasional seizing piston, but I have heard of quite a few failures of the lower barrel end. The result is invariably a BIG DISASTER:
Usually the barrel brakes of slightly above the base flange and is lifted a little, on the downward stroke the piston gets out of it and on the next upward stroke it does stove the barrel and head into the tank. Inlet manifold and exhaust pipe are wrecked as well in the deal and, what is worse, the connecting rod usually gets very much out of shape.
A fellow Club member from the States even reported that his barrel and head assembly flew onto the other side of the road, where a lorry went over it! I hope he has found a new cylinder head and rocker gear in the meantime...


In Bill Gibbard's "bible" there is a good description by Gerry Fitzpatrick of doing a throughbolt conversion which will prevent the above from happening.
The procedure has one shortcoming, however: Putting long stationary studs into the crankcase means you cannot lift the rear head off without removing the engine from the frame.
I have thus modified this a little. My thoughts have been given in an article in the BSC newsletter two years ago, but you can download the article here.


(click the pic pic
to enlarge drawing)
Now, this is how it looks.

I have left the old studs as locating dowels for the bys flange. If you look very closely, you can just about see the M8 helicoil inserts in the crankcase.

Ah yes, and the nice FIAT piston, just in case you are interested. Details are also given in the article above.

The picture is from a recent dismantling session, which was due to a set of poor valve springs I had bought, incidentally...

This is how it looks with heads off. You can see the narrowed-down head gaskets which I consider to be essential..

This is how it looks with the heads on. You cannot see a lot of the conversion, but that is not a thing to complain about, is it?

I can only say it not given any problems during 1250 miles of partially very hard use, so I think the procedure can be recommended.

And I have a few comments on Gerry Fitzpatrick's performance issues:
I use a 1:6.8 compression ratio and a 1" carburettor, but still the original (dare I say flimsy?) 680 connecting rods. The engine is revving lustfully but, knowing the rods are a bit delicate, I try not to overdo it.
The bike will do a true 85 mph on the level, but I do not maintain that sort of speed for a long time. However, I do cruise happily around 70 mph as long as roads (and brakes) permit.

 

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