M33 engine case screws

[Rog1]

I want to replace the primary drive cover and timing cover screws on my M33. I'm after Allen screws which is what I've already got, but some of the sockets in the heads have worn round, and the screws in the timing cover are too short. I've seen several places where a kit of these screws is for sale, but they refer to them as Whitworth screws. This doesn't seem right to me; surely at 26 tpi (I checked with a thread gauge) they should be cycle thread. So my questions are:
1)
Where can I get a set of screws that will fit?
2)
What nominal diameter are the screws in the timing case? I measured one of the screws that was in there, and it's between 7/32 and 1/4"; I want to buy a tap to clean the threads out, but I'm not sure of the size.

[Ian C]

AFAIK Rog, the M33 primary chaincase screws should be 2BA x 1/2" long (16 req'd) and the timing cover screws should be 1/4" x 20 (whitworth) x 1 1/4" long (9 req'd) HTH.

[Rog1]

AFAIK Rog, the M33 primary chaincase screws should be 2BA x 1/2" long (16 req'd) and the timing cover screws should be 1/4" x 20 (whitworth) x 1 1/4" long (9 req'd) HTH.

Ian, I've just come to the same conclusion about the primary case screws, but the timing cover screws are not 1/4 Whitworth; perhaps BSA changed the screw type in later models. I should have said; it's a 1953 machine.

[idie]

Both my M33 and B33 have Whitworth threads in the timing cover.

[Ian C]

AFAIK Rog, the M33 primary chaincase screws should be 2BA x 1/2" long (16 req'd) and the timing cover screws should be 1/4" x 20 (whitworth) x 1 1/4" long (9 req'd) HTH.

Ian, I've just come to the same conclusion about the primary case screws, but the timing cover screws are not 1/4 Whitworth; perhaps BSA changed the screw type in later models. I should have said; it's a 1953 machine.

The problem for us Rog is we are fighting PO bodges.. BSA used whitworth all the way through on the timing cover as the thread is course.  26 TPI is really too fine for screwing into aluminium.. May I suggest you try running a whitworth tap into one of the holes then inspecting the internal thread. When I rebuild engines a use a drill bit slightly smaller than the thread to clean out the threaded hole first then run a tap then clean out with some aerosol cleaner and dry with air.
I have had another thought.. Has the case been helicoiled for 26TPI?

[Ian C]

I've just realised that the later unit engines used a finer thread (BSF or Cycle perhaps?), but my comment still stands that the pre-unit singles used whitworth screws for timing cover. I own a 1952 M33 and the timing cover threads are whitworth on that bike. I've also checked the parts list for the 1958-59 B31 and 33, which basically used the same cases and they are still listed to be whitworth.

[Rog1]

The problem for us Rog is we are fighting PO bodges.. BSA used whitworth all the way through on the timing cover as the thread is course.  26 TPI is really too fine for screwing into aluminium.. May I suggest you try running a whitworth tap into one of the holes then inspecting the internal thread. When I rebuild engines a use a drill bit slightly smaller than the thread to clean out the threaded hole first then run a tap then clean out with some aerosol cleaner and dry with air.
I have had another thought.. Has the case been helicoiled for 26TPI?

The mystery is solved. It isn't Whitworth, but it hasn't been helicoiled to 26 tpi either. It is.... (roll of drums) M6 X 1mm!

Ian, it never occurred to me until I read your comment (quoted here) that the thread might have been changed. That got me thinking, so I went out to the shed just now to confirm my suspicions and I've just (carefully) fitted an Allen screw from the metric box. This accounts for why the diameter is too big for 7/32" but too small for 1/4", and why the 3/16" Allen key is a loose fit; it should be a 5mm key!

Oh well. I want to put it back to the original thread form at some point, but as long as the joint is keeping the oil in it'll do for now. At least it shouldn't be too hard to find some case screws that are the right length!

Many thanks Ian, and thanks to idie too.

[Ian C]

Good to know you've reached a conclusion Rog.. I do have a confession to make re:use of metric fasteners myself.. I tapped out the 2BA captive nuts on the M33 primary inner cover to M5 as I had access to M5 cheesehead screws at the time of restoration.. 

[Mike Farmer]

 

M6 x 1mm is realistically M6 x 25.54tpi. but I don't know the thread angle, so in theory your threads should not have been seriously damaged. So if you just use a 1/4 x 26tpi tap it should be straightforward. But whether 26CEI(Cycle thread) or BSFx26 I don't recall.

However if it's 1/4BSW then a bit more care is required.

Mike

[Rog1]

It's your bike, Ian, you do what you want. I'm staying with 2BA for the primary cover, but I'm going for non standard Allen screws instead of cheese heads.

Meanwhile, me being of a suspicious frame of mind, I'm wondering if my 6mm metric Allen screws are screwed into 1/4" x 26 tpi threads, and they've got away with it because the screws are a loose fit in the holes. We shall see...

(Edited)
Mike Farmer posted just as I wrote the above, and he is thinking the same thing. Considering it now, I don't have a 1/4" x 26 tpi tap, but I did the old trick of hacksawing a couple of slots in an old 1/4" cycle thread bolt and it cleaned out the thread a treat. Had it been a 6mm thread it wouldn't have gone in, being 0.35mm too big. Right! I shall be ordering a tap or two and some suitable Allen screws. Thanks to all.

[Mike Farmer]

Must say what I've found with allen screws is to buy a few of the max length then cut to size

[Rog1]

Must say what I've found with allen screws is to buy a few of the max length then cut to size

Funny you should say that, because yesterday morning I ordered some slightly too long ones.

1/4" Cycle thread Allen screws are hard to find. I did much internet searching and couldn't find any in bright finish or stainless unless they were about £3 a pop. I gave up and tried BSF, which if not quite the same at least are the right diameter. That's when for some reason this set for a C15/B40 primary cover, 1964-68 appeared. They are cycle thread and I even get a spare one. They were £6.40 with postage:

https://www.ebay.co.uk/itm/BSA-C15-B40-etc-64-to-68-Allen-Primary-Cover-Screw-Set-Bright-Zinc-Plated/161001905906?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

The seller is better known as Nooky's Nuts. To the Tracy Tools site next, to get some taps and dies.

[Rog1]

Here's an update.

I finally got round to removing the timing cover and cleaning out the threads. They turned out to be ¼" Whitworth after all, as they should be. That's a bit of a surprise, considering that the 6mm screws were going in and (mostly) tightening up, and that my home made tap (a cycle thread bolt with a couple of slots in it) previously went in one of the holes and (as I said above) cleaned out the thread a treat.

I used a Whitworth bottoming tap and cleaned out all the threads, taking four or five goes for each hole. I also cut the thread one turn deeper to make sure it was clean all the way down. I found that the holes were more than deep enough to do that once I cleaned them out first with a drill bit as Ian C suggested; each one was about half full of various colours of gasket sealant. A pack of ten stainless 1½" long Allen screws from Ebay gave me a complete set plus a spare, and the screws all go in finger tight when there isn't a gasket, so I'm happy. The next problem is the tiny crack in the cover where the oil valve spring screw has been overtightened. I might try Lumiweld.

I found some stainless ½" 2BA Allen screws for the primary cover on a website that sells stuff for Nortons:
https://www.rgmnorton.co.uk/buy/2ba-x-1-2-under-head-stainless-allen-screw-10_4402.htm#!prettyPhoto
I bought a couple of packs of ten for £6 each and 16 of them now hold the cover on.