56 G80S blown head gasket

MatthewC · Post by **MatthewC** » Fri May 04, 2012 9:31 pm

During the time I've owned my G3 I've looked at more 350 heads with rotated inlet valve seats than ones without. By all accounts the 500s normally do the exhaust seats though. I've been told that when the head heats up under running conditions the differential expansion results in the (steel) seat coming loose and rotating which affects valve sealing as the seat is not concentric with the valve guide. The seat won't actually fall out though because it has a groove machined in its periphery before being cast in. The head on my bike had a rotated seat when purchased from an autojumble but I bought it because it was otherwise an excellent head (better than the existing one or the other spare ones!), so I had a new seat fitted.

Matthew

56G80S · Post by **56G80S** » Fri May 04, 2012 10:30 pm

Matthew

Thanks for that, it's a bit reassuring; I'll probably leave it as it is What kind of cost was it and how difficult to obtain a replacement seat (together with the unleaded confidence it brought?

Ironically, the inlet valve is seating well, judging by the colour of the seat and the valve before it was vapour blasted.

The exhaust valve was a mess! This was probably why I had a spare valve and guide in the new parts tin - I just hadn't fitted it when I should've for some reason.

Johnny B

MatthewC · Post by **MatthewC** » Sat May 05, 2012 11:10 pm

£85 for fitting a new inlet valve seat and two valve guides (I'd already removed the old guides, should really have taken the time to fit the new ones myself). Obviously being the inlet seat unleaded tolerance doesn't come into it. The seat is a bronze one which I find is a bit of a pain for road use, it loses clearance quicker and when it does so pits deeply quickly, so at the moment I'm experimenting with a little extra clearance for the inlet. On this basis, I would prefer a steel seat.

I took the bike to Scotland last year using plain unleaded and in 2000 miles (admittedly hard miles) the exhaust valve and seat ended up in a right state. There were bits of valve up to approx 0.1" across and probably 0.020" thick welded to the seat which had ripped off the valve leaving corresponding pits. Nasty! Since fitting the better head I now use Tetraboost in it and the valve and seat seem to fare very well. I suppose I could have had a new exhaust seat too but that would have been unnecessary extra work, and on this head the seat looked like it barely been used.

Matthew

56G80S · Post by **56G80S** » Sun May 06, 2012 12:33 am

Matthew

Cheers, think I'll stay as I am. The exhaust seat and valve are OK really, it's just that the stem and guide are very worn to the point where rocker box was full of carbon!

I've always ridden the poor old Matchless hard and reckon that the seat rotated a long time ago or left the factory wrong. Hope I'm right.

Ironically, the chap who did the work for me (and makes parts for and renovates Scott motorcycles) is waiting to fit new seats in an Ariel Square 4 head that his Dad (or was it father in law?) over worked and damaged. He thinks that the seat in the Matchless head has probably been as it is for a long time before now.

Johnny B

wilko · Post by **wilko** » Sun May 06, 2012 12:51 am

Give me an ironhead anyday!

56G80S · Post by **56G80S** » Thu May 17, 2012 9:21 am

Back again!

I got the old cylinder base studs out of the barrel and only wrecked one.

When I compared the old one with the new, the new are shorter (uploaded pic to Postwar Singles Parts - titled Cylinder base studs old and new).

There was never that much engagement between the nut and the stud in any case, probably to ensure that the one could get the nut on! There's not a lot of space, but the old studs still leave enough.

As for fitting the new studs, my plan was to cut one of the old nuts across, wind the halves onto the new studs and lock the "halves" together and use that to wind the studs into nice clean threads in the crankcases (adding a smear of Loctite).

Clearly, the depth of entry into the crankcases needs to be managed carefully.

Any comments?

Johnny B

Don Madden · Post by **Don Madden** » Thu May 17, 2012 5:44 pm

Why cut the old nuts? They are a British Standard thread, 3/8in BSC I believe & hex nuts are readily available. Cheers, Don.

56G80S · Post by **56G80S** » Thu May 17, 2012 8:53 pm

Don

Good point. One of the nuts has a bit of rounding on one "corner" and that's the one I'd intended to cut. As I'd planned on fitting two new studs thought that I'd match them up with two new nuts (which I have from JSL).

I was planning to keep the old matched sound nut and stud for possible future use as necessary. I suppose you have a point though.

If I feel the new studs are a bit short, I could just clean up the old ones really well and refit them, use the one new nut and the old sound nut!

In fact you've convinced me and that's what I'll do; just thought that I'd get a better fit in the crankcase with the new studs.

Cheers

Johnny B

56G80S · Post by **56G80S** » Sat May 26, 2012 4:38 pm

Post revised!

In prep for refitting the head, I checked the new copper gasket against the old ones that I had to hand.

New - 0.038"
Old - 0.028" - 0.030"

Also measured up the space available for the gasket by fitting the head to the barrel.

Smallest depth - 0.028"
Largest depth - 0.033"

So fitting a new gasket would mean only 5 thou of "pinch" on it before the barrel spigot would bottom on the head (at the point between the push rods).

Johnny B

Edited by - 56G80S on 26 May 2012 6:00:29 PMEdited by - 56G80S on 26 May 2012 6:01:49 PMEdited by - 56G80S on 02 Jun 2012 12:14:36 AM

wilko · Post by **wilko** » Sun May 27, 2012 1:00 am

Wouldn't hurt to take 10 thou off the spigot just to be safe with your bastard file and steady hand.