Engine fixings

[Columbo John]

See what you mean. Very clearly.

I've just blown up a hi res pic of a drive side case I have here (too big to attach as a file) but those 2 holes on mine also have 'threads', rather vestigial on the one below the oil filter tunnel, deeper on the other one at the rear. Yours are both more pronounced than mine.

I then looked closely at a spare engine, a 500, to see what that was like - and lo and behold, a vestigial thread in the hole under the tunnel, and more pronounced on the one at the rear. A 3/8th BSF bolt would go in just a turn or so - but the majority of the depth of the holes is 5/16th like you say and no sign of any 'thread' other than towards the drive side ends.
Just the same on another spare set of cases lying around the place, which also has 'thread' on the same two holes, and on the lowermost one at 6 o'clock as well.

Weird, but they are just 'holes'. Quite how they have ended up looking almost like they have a proper thread I don't know - can only think it's because fasteners have been wound in against the weight of a loosely attached engine, maybe very slightly bent ones even, and have done it. But if so, they've done it quite neatly in your case, a bit roughly in mine!

Or - edit - maybe someone thought they should be there having seen the tell-tales, and ran a tap in??? Stranger things have happened I guess!

They are 'proper' threads. I get what you say about the thread being cut whilst assembling the engine in the frame but these are perfect 3/8" BSW threads for the first 3/4" or so and then a good 5/16" mounting hole

[Columbo John]

The 2 tapped holes were they used to bolt the case to a fixture during machining ??

That seems to be the most likely explanation I think.

[Columbo John]

there is 3 holes with 3/8 BSF thread, the 3 holes are very practical, see picture
Poul
20220131_173921_1.jpg

hmmmmm, mine are definitely 3/8" BSW

[ajsph]

I have rebuild more than 10 twin engines,from 500 to 650 models, and I have never seen one without the 3 threaded holes, very often are the thread damaged , but can easily be cleaned up.
Poul

[Columbo John]

I have rebuild more than 10 twin engines,from 500 to 650 models, and I have never seen one without the 3 threaded holes, very often are the thread damaged , but can easily be cleaned up.
Poul

Any idea why they are there?