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Cracked 'A' Crankcase - Large Idler Boss


Well Known and Active Forum Website User
VOC Member
Close examination of my stripped 'A' Comet Special cases reveals a crack along the top of the large idler boss disappearing into the oil-hole. :mad: It will be a real pain trying to get down inside there with any kind of welding torch, so I guess my other options are:

1. Just leave it alone on the basis it has probably stress-relieved itself - and hope it doesn't cause a blow-up in due course. :eek:

2. Bore the boss out and fit a thick bronze top-hat bush (but that might get tricky trying to maintain correct timing gear meshing...). :confused:

Has anyone else encountered this problem, if so, how did they fix it?



Well Known and Active Forum Website User
Non-VOC Member
why dont you take it to a good shop and zyglow it the old black light treatment , that might give you a base line , that you can progress from there , iv found that by doing that in a shop that does that kind of work , the views of the guys that do this on a day to day basis can save you a fortune ;)

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
Crack Repairs

Have you thought of having the crack 'stitched' ? In the past I have had cracked cylinder heads on earth movers and even a narrow gauge steam loco cylinder repaired with this method. It is quite suprising how far down a valve port they can get and the repair was even water-tight.

The company we used at the time was 'Granges Metalock' but they seem to have disapeared now. You can get an idea of the process at :-http://www.locknstitch.com/Metal_Stitching.htm These seem to be a US company but :- http://www.slinden.co.uk/ are in the UK. I have never dealt with either of these.

At the time that I visited the Granges works they were repairing some huge marine cylinder heads prior to Lloyds inspection so it seems to be a well recognised repair.



Well Known and Active Forum Website User
Non-VOC Member
this method can be a doubled edged sword , your major problem can be secondary , when other unkown things pop up , of course you can set up your own testing station quite cheap , but remember that a hole drilled at the end of a crack will stop it before you repair it ;)

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Crack stopping

In the presence of an alternating stress, what determines the rate of crack growth is the ratio "cracked surface divided by tip radius". The bigger, the worse. Tip radius is generally the distance between adjacent molecules: the ratio is therefore very high. Drilling an 'ole in the end of the crack increases the tip radius gazillion-fold, and the crack stops growing. I was delighted to discover this explanation: I wasn't bodging, I was applying state-of-the-art crack growth theory. Befor i koodnt spel enjunear, now i is wun!
However if your crack disappears into the oil hole, you can be sure it has stopped growing. That doesn't solve all your problems, but it is one less thing to worry about........


Well Known and Active Forum Website User
VOC Member
Hard to Get At!

Thanks so much for all the replies! (It was something of a surprise as I wrote the initial plea for help about 10 months ago!). But it remains relevant as I have done nothing about it thus far - too many other projects...

The trick with a Series A case is that there is no timing-cover! The crankcase folds around the timing chest and forms its own rigid box section for added bottom-end strength. Take a look at a photo sometime. So to repair the large idler boss by whatever means, it has to be done 3.5 inches down through a 1-1/4 wide slot at cylinder base level. Or through a similar sized hole behind the external oil-pump.

Alloy welding is probably out because of access issues; metal stitching sounds interesting and I have seen it done very effectively on a Series A cast iron head. Alternatively, I could bore out the boss and insert an interference fit bush to strengthen the area; "stop-drilling" might also be needed as the crack goes on through the oil-hole and out the other side for about 3/16ths inch. And dye penetrant might be needed to ascertain the true end-point of the crack. So all of your comments are relevant - thank you very much for the assistance.

Of course, any other thoughts would still be welcomed....

Peter Barker
Bristol, UK.
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