crack in dynamo housing

[JonE]

re. importance of protecting the integrity of the original crankcase, I have a single stud (to block; right next to the area which James concerns himself with here) which I noticed sits at lower level than the others. It hasn't been widened - still same thread size. What would best practice be here? Is a bespoke longer stud less invasive than helicoil at this stage?

[Austin in the Shed]

Do you mean the first threads are pulled out Jon ? If so best to helicoil it.

[JonE]

lower thread still sound but has extended further 'down' so whole stud sits lower...

[Hedd_Jones]

Jon

Sounds to me that you dont actually have a problem. So long as your stud/bolt is a suitable length. 

Some comments on the advice here.

It seems the OP has oversize thread in good condition. 

It is not possible to 'helicoil' down a size. You have to make and fit an insert as Ian describes to do that.

But assuming your good oversize thread to probably be 5/16BSF or BSW, a quick look in the zeus book would show that you can't make an insert that has any sort of 1/4 female thread, with any sort of 5/16 male as really there isnt enough meat, given the pitch difference between the 1/4 and 5/16 it, so to fit an insert, your going to have to drill bigger again. 

Personally I prefer inserts to helicolis. But the disadvantage is you need a bigger hole to be successfull and it is not always possible. 

The real 'proper job' is to have the hole filled with weld and start again. But that involves a bare crankcase. 

The long bolt on the dynamo casing does locate some dynamos, but by no means all, 2 of the 3 cars here you can remove the dynamo without removing the bolt, indeed one car has a stud, rather than a bolt. I'm guessing a stepped one, but as I have no intention of fixing problems I do not currently have, I dont know.

With a stepped stud, and a dynamo that wouldn't come out with the stud in place you just take the whole casing off, so its hardly the end of the world. I generally take it.off before removing an engine out of a car anyway as I usually need to swap nosecone or whatever. 

Making a stepped stud at home with no machine tools is easy. You dont need to drill or tap your current crankcase (no further molestation). And you can use your replacement dynamo casting without modifying it

[Ian Williams]

In theory Hedd the pitches of the two threads might seem a problem, and if you have enough meat in the damaged area another oversize may be worth considering, but in practice it works just fine. Make your insert with a 1/4 tapping drill pilot hole in it, screw and loctite in place then cut your 1/4W internal thread, the insert is supported by the case so it basicity works like a bespoke helicoil.
If you want to plug a striped thread in an Aluminium crankcase with weld and its a clean fresh hole you could try Aluminium solder, it works quite well but must be free from contaminates. It is probably possible to solder, or weld the cracked casting together but all these other solutions are as much work as doing the job properly. Not something I have ever found the need to do Hedd but how long would it take you to file up a stepped stud long enough to fit this particular location, some considerable time I hazard.

[Hedd_Jones]

Ian. Using a pillar drill and a good file, I'd make you a stepped stud for that in less than 10mins using a long 5/16 bolt as a start. Clearly I'd have to used some dies too.