Head gasket blowing

[1608]

I found that the boss in the head that the head bolt bares on next to the exh port had begun to squash or distort. This could lead to the bolt losing torque. I got it beefed up with some weld. Been good for years. Probable due to being over tightened in the past.

[Joker_Bones]

What do you fellas think about lubrication on the threads of bolts that are being torqued?
I imagine you must get an appreciably different tensile force in a bolt tightened to a given torque depending on whether the thread is lubricated or not.

[Expat]

An interesting issue Russell, with many varying opinions I’ve read about over time, never finding a definitive convincing answer. Maybe I’ve not looked hard enough or missed the answer, entirely possible. I agree with your assumption totally.
My only personal experience working on a car suspension years, ago was to wipe bolt threads with a cloth and slather them with anti seize paste then, using what I believed to be a good working torque wrench loaned to me, set correctly at something like 55 ft lbs, promptly sheared off the first two 3/8 inch dia bolts. I swore, then swore never to use one of the new fangled things again, never having any problems before just relying on my wrist and arm gauges.

Having said all that and read lots of posts here re torque figures, or the lack of them for our old bikes and now being in possession of a good quality new wrench, when I fitted the head on 1960 G3 the other day, the bolt threads were completely degreased, dried off, then given a light spray with WD40. Tightened the bolts in sequence up to 25ft lbs in three increments then tried to nip up further using a spanner by hand. They didn’t move further at what I decided was enough pull, so left them at that.

All the threads in the cylinder were cleaned out with a tap and degreased, blown out with compressed air etc etc and the used copper gasket annealed as best practice.

Well I seemed to have waffled on a lot about this sorry, it’s what comes of living an isolated life on a mountain plateau with very few people about and even less bikers!

Hope it might help a bit, Steve.

[EddieDay]

Steve, we don't get bored with this stuff because we are in the same boat. (or in the same garage.....?).
Russel is right about the difference between torque on dry threads and on lubricated threads. I used to work on refineries and when tightening (much bigger) nuts and studs the torque figures were always given on the assumption that the thread would be lubricated. Bolt tensioning gives far greater accuracy because the stretch of the stud is known accurately. It is not practicable for a bike or car engine though.
I have found that I cannot get a socket on some of the nuts, e.g. the cylinder base nuts, so I rely on "feel" a lot. In my youth I did strip a few threads until I gained enough experience.

[Expat]

Yeah likewise Eddie, and thanks for encouraging, though you may regret it. Older guys in the toolroom where I was apprenticed and often working at home, stripping threads and shearing off bolts on my Honda 50 step thru, suggested I should ‘cut my effing 10mm ring spanner in half’!

Hard but fair.

[Joker_Bones]

I had a Google and there are lots of charts, all much the same, converting torques into tensile bolt forces, usually qualified as being an estimation only.
Here's one...

BoltTorque.jpg

I was surprised how low the torque values were to reach 75% of the bolt yield stress.
In this chart if you take a 3/8" x 24 tpi grade 5 bolt (equivalent to a metric grade 8.8 bolt, 800N/mm2 ultimate, 640N/mm yield) the torque required for 75% of yield is only 19 lb.ft lubricated. Half the value of not lubricated!