7 stud VB Barrel threads

[bevanclark]

Hi folks,
A previous owner has enlarged the barrel threads, presumably because the original studs had been stripped.
This has reduced the, already narrow, gasket sealing surface and is possibly the cause of two blown head gaskets, despite me lapping the head on a surface plate.

Given the lack of metal around the stud holes, how feasible might it be to use a helicoil or similar to bring the threads back to standard ? Or is that likely to scrap the barrel ?

Cheers,
Bevan

[nevhunter]

Helicoils need an even Larger diameter and don't provide a sealing face that's any good. Try stepped studs that end up flush with the Cylinder's top face. Have it all very flat and use a thin copper Gasket. You have extra studs on the Alloy Head Model but the expansion rates is different with the 2 Metals. Nev

[david.anderson]

The most likely cause of head gasket failure will be a distorted cylinder top face. Mine was very badly distorted and was milled flat, as was the alloy head which was not as bad. Given how close the studs are to the bore it is unlikely that a time sert or threaded bush could be used. It would however be possible to have stepped studs made, ie 3/8 into cylinder then stud reduced to 5/16. However I would first check and machine the cylinder top mating surface and try it.

David

[bevanclark]

Thanks Nev and David,
I'll pull the barrel and get it and the head flat and see about stepped studs as suggested.
I don't like that it currently has stainless bolts which, presumably, won't do the cast iron threads any favours.
Cheers,
Becan

[nevhunter]

Yes Stainless can GALL. Studs with NUTS, don't have to come out often. The Head will be a bit sloppy on the studs. Nev

[Roger Gwynn]

Stainless expands much more than iron or steel so not going to help a machine that tends to blow gaskets. Solid copper head gaskets are available with a bit more meat to them in the style of the later 9 stud gaskets.

[nevhunter]

The Alloy Head might tend to stretch bolts and PULL the threads. A properly designed bolt would have the stem relieved along most of it's length to a dia slightly less than the Minor dia of the thread. Depends a lot on the quality of the Alloy used in the Head. Nev

[bevanclark]

Curious that Draganfly list a stainless bolt for the 7 stud engines such as mine, not studs.

[Roger Gwynn]

Originally Ariel switched from studs to bolts for the alloy head, or is it the stainless that you are questioning? Stainless has a higher rate of expansion than steel so is better suited for use with alloy heads. The alloy expands and can stretch the steel studs or usually causes the washer to recess into the alloy, in any case it contributes to head gasket failure. Admittedly not a big cause of the failure on the VB but it is a large part of the problem on SQ4s where there is a greater thickness of alloy

[bevanclark]

My bike has a cast iron head, which is why I was concerned about the stainless bolts.