Author Topic: Castings  (Read 392 times)

Offline a101960

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« on: 16.11. 2018 17:49 »
What if anything does anyone know about ally castings? I am now working on my third timing cover preparing it for polishing. I have been having problems with pin holes or porous cavities due to gassing. It is always in the same area. I have circled the spot in the attached picture. I have learned now if I see even a tiny pin prick that it cannot be polished out because if you try to do so it will get bigger and most likely will reveal even more of them. Were BSA castings generally of poor quality in this respect? I think that it is very odd that it always occurs in the same place. Until I set about polishing my timing cover I had never before encountered this problem on any other bike. Surely it cannot just be an A10/A7 thing, or can it?

Online RDfella

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Re: Castings
« Reply #1 on: 17.11. 2018 11:24 »
In the early 60’s I worked in the toolroom of a firm making dies for alloy pressure die castings. I’ve also been involved with sand casting, but that’s all irrelevant. What is important, is to understand that the idea that manufacturers seek perfection is wrong. Perfection – or near – costs money and so the best engineer will make things just good enough for the purpose and no more. Which is why the castings you have aren’t perfect – there was no need for them to be. It is not a case of 'poor quality' but rather a need to be good enough but no more.
The sand castings (LM25) I get cast in the UK to my patterns have always been good – no inclusions, imperfections or pin holes. Likewise those castings (LM6) that we did in the 60’s, but as the latter were highly polished and seen, we took trouble over metal control and the casting process. But in both cases cost consideration came below finish.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Slymo

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Re: Castings
« Reply #2 on: 19.11. 2018 04:09 »
Found a huge hole when fettling a early 90's RE Bullet timing cover. Took the aluminium filings that I had scrapped off and packed the hole with them mixed with super glue. Very slight grey spot that I could only see if I could be bothered to polish it now.