Leyland 0.500 series

gingerfold:
Good post Roy and it makes your point very well. The AHU / AVU 470s with unified thread studs cured most of the head gasket failures of the earlier engines. Why would that be?

The AVU470 engine I had in my restored Seddon was rated at 140 bhp and in the 9 years I had it never gave any problems with its head gaskets, which I had renewed at the renovation stage. Granted it was not running at anything like the stresses imposed on an engine in revenue earning service. But I know of two preserved AEC Mercurys with the non-unified head threads having head gasket failures on road runs in which I was taking part. Both Mercurys were owned by people who are meticulous in their preparations and they are in fact engineers. Going back to the engine in my Seddon the only signs I ever saw of any water being where it shouldn’t have been was when I changed the engine oil at the end of every rally season. I always used Bluecol anti-freeze all the year round and when I removed the sump plug no more than an egg cup full of Bluecol would come out first. It never went any worse in 9 years. Being a wet liner engine there must have been an ever so slight weepage from one of the liner O rings.

Less of a pitch on the thread,more thread per inch and less chance to back off underload due to less of a ramp.Superb post,goes to show that with single heads how busier the deck of the block would be.As shown in them excellent pictures the clamping devise of one cylinder is also the clamping devise of the one next to as against singles were they all require their own clamping devises.But to be fair one area were single heads excel is to compensate for a low liner if you can call that correct in the first place.