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
|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.
|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:
For any kind of feedback my e-mail address is
(sorry, this is not a clickable 'mailto:' hyperlink. If you want to write me, please type my address in your mailer. )
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