Alloy or Cast Head G80

[Peter Lecompte]

Hi, I am wondering what the thoughts are of fitting an 1952 alloy head onto a 1950 G80 cast barrel instead of a 1950 cast head? Is there any real benifit apart from the weight? Or am I better off sticking with what was made for that year? Thanks Peter

[Don Madden]

The obvious advantage is better cooling. They used lower compression ratios with iron heads to prevent overheating. The high compression race kits of that era used first silver, then bronze for high heat flow. The silver ones were melted down to recover the metal & the bronze are very scarce. Standard engines with alloy head feature increased CR. I do not know how well the alloy heads match the early iron barrel but matching barrels are not the hard to obtain. Go for it! Cheers, Don.

[1608]

Alloy heads are a general improvement all round on all engines I believe. As said, better cooling, and I remember that there was a 2 HP difference on the BSA M21 600cc,
13HP raised to 15HP. I've no idea about the measurement differences btn alloy and iron though which may affect push rod lengths etc.

[MikeM.]

My current mix and match on my Twingle has a '58 alloy head on an up to '52 Matchless G80 barrel (flat fins at rear to clear mag). Not only do you need care with pushrod lengths but also pushrod tubes, also the top rubber sleeves on the pushrod tubes may need to be shortened otherwise the top of the tubes may collapse when head is tightened down, I speak from experience.

[clive]

I would second Mike's experience but I think the problem with pushrod tubes and pushrods is caused by the length of the barrel. Some early 50s barrels had a reduced depth flange at the bottom and were intended to be fitted with a metal gasket between barrel and the crankcase. Miss it out and the piston may make contact with one of the valves. I speak from experience unfortunately.

[MikeM.]

Hmm, there's a point Clive. I have had a problem with a rattle since putting in a new plus 60 piston after burning a hole in the original after 60K miles, it was the same Hepolite number but the valve cutaways were differant, I had to have them machined out to suit. Maybe the new piston is differant I'd best have a look. Oh I haven't ridden it like it is though.

[clive]

mike if you have another later barrel measure the distance from the bottom of the flange to the spigot at the top of the barrel. With the earlier barrel this is shorter than with the later barrel. Just to make it difficult if you put both barrels next to each other they are the same height. the older barrel has a longer throat allowing a metal gasket to be fitted. notice how all the decoke sets had an unnecessary extra gasket for the cylinder base? When I fitted mine without the spacing metal gasket I bent two pairs of knackered pushrod tubes. Put it down to them being wrong size when a new set survived. However on running the engine the piston made weird noises which when finally investigated indicated a very slight touch on a valve causing the piston to rock into the barrel at the top of the stroke. Mind you the valve may have been a slightly thicker non standard one. later barrel and piston and it all went without problems, although were it not for the damage on the barrel I imagine refitting with the metal gasket would also have worked. Yes I had one to go with the barrel and couldn;t remember why!