Norton 850 Mk 1 1974 Service Notes Download Page 9

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GROUP 2:- Barrels, gaskets and head parts

CYLINDER BASE GASKET: This was deleted from Mk. I and II 850, plastic gasket such as Loctite,
Hermetite RTV or similar silicone compound being used. A gasket was again introduced, part no. 0638
12, on the Mk. III as people filled the oil drain with RTV, causing lots of trouble. Removing the gasket puts
the compression ratio up by approx 0.3 of a ratio, see table. The 850 gasket can be used on the 750 but
not vice versa as the cylinder apertures aren't big enough. 750 base gasket is part no. NM24249 or
Q67869.

The silver and black 750 barrels are identical dimensionally and incidentally, the studs all round the base
are exactly the same positions on the 850, so you can build a 750 using 850 crankcases, but not the
other way round, because the holes for the downward extensions of the barrel aren't big enough.

CYLINDER HEAD GASKET: This is where the complications start! There are plain composition,
aluminum, copper and eyeletted composition gaskets for the 750 (and copper-asbestos, if you go back far
enough!). The composition gaskets are about .028in thick in use, the copper gaskets are about 0.040in
and the aluminum 0.O80in thick. I will leave you to work out what these do to the compression ratio with
the various heads available. Whatever compression ratio I wanted, I would use an eyeletted gasket. The
others are all suspect.

(Mine has been perfectly satisfactory on standard compression i.e. 8.9:1. Anyway

the eyeletted one did cost 3 or 4 times as much).

The plain composition causes pre-ignition, because the

reinforcing wires glow at the end, and then blow. The aluminum gasket squeezes out almost as quickly as
-the custard in a vanilla slice-so you can hardly keep up with tightening the head bolts. They also spread
into the push-rod holes and rub them away (even if they don't rub them from new). Copper and aluminum
seem incapable of maintaining an oil-tight joint around the push-rod holes.

A smear of RTV in this area

can help.

J.H.:

I cannot agree that the plain composition gasket causes pre-ignition-it was used on 10,000 engines

(650 and 750) before the 1st Commando and gave no trouble; part no. 25324 on the 6.50 and 25318 on
the 750. It came in with the spigotless cylinder barrel on the 650 and 750 at engine no. 114870, and the
lst Commando was 126125.»

Cylinder heads exist in an even greater variety. There are as standard, 30mm, 32mm, and tapered 30-
32mm inlet ports, three different chamber depths, varying in stages of 0.020in from the gasket face, for
the 750 and at least two for the 850, and I'm not counting the 750 short-stroke head which only fits on the
850 barrel and is only around in small numbers. Someday, someone will issue a chart of all the heads,
gaskets and pistons you can use and what compression they will produce, but it's not going to be me! The
only real way to find what you've got is to use a burette in the traditional manner.

The threads in the heads are, in general, BSF and Whitworth, and the exhaust ports are 1 15/16th x 14
tpi.

CHANGE IN COMPRESSION RATIO

With reference to four different cylinder heads and three head gaskets that have been in use on
Commandos.

All numbers are compression ratios: l. Figures in brackets are compression ratios with base gasket
removed. Loctite plastic gasket being used instead.

HEADS

GASKETS

Part No.

Types

Port Dia.

Compo 30thou

1mm Cu 2mm AI

060988

STD 1968/72

30mm

8.9 (9.2)

8.7 (9.0) 8.1 (8.3)

063327