crack in dynamo housing

[jamesheath]

Hi,

Another quick question....

We've spotted that the casting that the dynamo mounts on has a crack - it's in the long bolt mount in the one on the right:
   

Closeup:
   

as you'll see from the first pic, luckily I have a perfect spare....
unluckily, there's a reason for the crack - clearly the thread in the crankcase stripped at some point and someone tapped this out to the next size up and drilled the casting, weakening it, and it then cracked.

I reckon I've got a few options....

1/ put the old one back on, with crack as it is (?repair with epoxy or something better?), and don't tighten too much
2/ fit some sort of insert (?HeliCoil) in to the existing tapped hole in the crankcase to take it back down from 1/4W to 3/16W
3/ source a long stud with 1/4W end and nice long 3/16W body (no, I don't have a lathe so can't make one)

It had been working OK for ages with the crack before, and it seems to me that the bolts at the other end take most of the force, given that the dynamo hangs out so far..... but if a HeliCoil would wind in to the existing hole then maybe that's the way to go. An issue there is that I've got the block attached to the crankcase and I really don't want to take it off again...

Thoughts / opinions?

[bob46320]

I think I would leave things as they are. The weight of the dynamo is downwards supported by the 2 bolts below the crack and one above, so the weight of the dynamo on that casting is in compression at that end and in tension at the "inspection cover" end. The 2 long bolts and the 2 short bolts holding the casting to the crankcase stop the dynamo from twisting in the horizontal plane.
Filling the crack with epoxy will not add strength, may stop oil leaking out.
For my money, the casting had suffered an accident some time in its life. Keep the good casting and should the engine come out at some point for a strip down, helical the "stripped" thread in the crankcase.

[Andy Bennett]

As they aren't that difficult to source might it be worth enlarging the hole on the good one and then going gently on the tightening. Whilst it will be weaker with the larger hole it will at least on day 1 be better than the cracked one?

[Hedd_Jones]

the stepped stud would be my choice. No further mods to anything else needed.

Original size would be 1/4

[Ian Williams]

Of course all the dodges suggested will work to some extent however the best option would be to plug the hole in the crankcase and go back to the original 1/4W thread, a stepped stud will not be a good option because that particular bolt also locates the dynamo into the housing. Do it once, do it right and never have a problem with it again that's my motto.

[Dave Mann]

Unfortunately the original 1/4" BSW thread is 20 Tpi and the tapped out hole is presumably 5/16" BSW which is 18 Tpi so it will not accept a 1/4" BSW helicoil. Suggest you follow Ian's advice.

[Barry Townsend]

Is getting it professionally welded not an option?

If it were it could be brought back to original

[Bob Culver]

It often not realised but the casting must be accurately located otherwise whenever removed the cross gears have to start the bedding process all over again. Once established dead square can drill and insert a couple of small locating dowells if it does not otherwise  stay in the right place.

If the aesthetics do not trouble file the outer cracked part down a few thou so it carries no weight, drill holes just beyond true end of cracks, fill with whatever and carry on.

[jamesheath]

Thanks all for your comments.
I think I'll progress with the original casting, filing down as suggested.
I'd hope that it has the best chance of aligning as it's been paired with the engine for a good while.
I can resort to other approaches if it fails.

Thanks again all!

[Ian Williams]

James I strongly suggest that you follow my and Dave's advice to repair the crankcase thread, its the only lasting method you have available to you. Just make an aluminium or brass threaded plug with a 1/4" threaded bore screw and Loctite it into the case, cut and file it off flush with the crankcase and never have to worry about it again. This is usually the best repair option for any thread which has been drilled oversize, as already stated Helicoils are great when a thread is freshly stripped but no use when someone has been there first with a backyard dodge.
Trying to bodge up the cracked casting is very poor advice in my opinion......added to which you have a perfect undamaged casting you can bolt on! Having taken it of already with no way of ensuring it goes back in exactly the same position any perceived alignment benefit will be of no consequence in your case. Even a different gasket will have an impact on meshing of the gears........ as any competent Austin seven engine builder will tell you, its more important to ensure a minimum of backlash without binding of the gears. Lash here will transfer through to the distributor and add to ignition timing scatter with the consequential reduction in overall engine performance.