Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA thread

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56G80S
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Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA thread

Post by 56G80S »

Latest difficulty, which I'd forgotten about.

Back when I was in my early twenties I snapped the top bolt nearest the crankcase. A local "engineer" managed to sort it out, after a fashion. For all these years I've been using an oversize bolt, of unknown thread, with an enlarged hole in the end plate.

The drilling isn't perfectly straight, the thread is weak and the bolt cut off shorter than it could be! I've got away with it thanks to the power of Stag Wellseal.

I see the following options and welcome advice:

a) Just carry on, it's worked for all this time - not keen as I'd forgotten about it before but won't now;

b) Splash out on a 3BA helicoil kit (Unithreads do them) at around £60 and get local chap to attempt it (proper engineer) - there's very little meat now between the threaded hole and the pump bore and it's stinking awkward;

c) find out the thread of the oversize bolt and get one in as long as practically possible - I'll have to find someone to do that, local engineer very busy and didn't have the necessary to hand;

d) turn it into a 3BA stud "glued" in and use the oversize hole as leeway to make it properly right angles to the face and centred in the cap (new cap to get) and use a nut to match the heads of those tiny bolts - I prefer this. It would be fiddly to get the get on and off in situ once the rebuild done but do-able. It would give me more confidence of airtight seal. Probably cheapest option too.

If I do (d) and the pump doesn't pump I'll not have wasted much money, which is starting to be a real consideration.

Sits back and waits for flak.

Johnny B
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clive
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by clive »

it would be a for me
clive
if it ain't broke don't fix
Mick D
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by Mick D »

Hi Johnny

I'd go for option 'C', get a suitable bolt of optimum dimension.

If the hole has been over drilled from 3BA there's no garantee that a 3BA 'Helicoil' will be possible.

Plodding on with a short bolt increases the risk of stripping during assembly.

Aligning the thread and fitting a stud will require removing more 'meat' risking breakthrough of the casting.

Identifying the thread is pretty easy, measure the diameter, crest to crest and count the threads per inch, then compare your readings to thread tables or post them on here.

Regards Mick
56G80S
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by 56G80S »

Hi both

Thanks a lot for your views.

I take Clive's point (not actually broke so don't fix it) but it seems a good opportunity to improve the position while the cases are split.

Mick, I was thinking of using high strength adhesive to simply fit a length of 3BA stud into the current (thoroughly cleaned) hole. I thought that if it was the full length of the (longer than normal) drilling the combined effect of the stud thread peaks and troughs and those in the case would provide a good physical key. I suppose, though, a longer version of the non standard bolt and some threadlock would have a beneficial effect?

See if there are any other views - not short of time!

Johnny B
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by 56G80S »

The oversize set screw (it's thread to the head) is 3/16" thread peak to peak. It's best match on my thread gauges (Whitworth/Metric) is marked 30G - what's that mean? The peaks are a perfect match to the gauge (cheap from Tracy tools a while ago).

Is this BSF (is Whitworth Fine the same thing) IKBA? I can't test against a die as the smallest BSF I have is 1/4".

Comments welcome.

I also reckon I could increase thread engagement by at least 25%. The screw has been cut providing insufficient allowance for the full thickness of the cap. As it is a standard hex head, it doesn't fit the the cap recess. I measured the available depth with a pin punch and a bit of masking tape - sophisticated, eh? Also screwed the screw in without the cap and it doesn't bottom.

It also must've been what the guy had lying around, I noticed now the head is drilled for a wire. A new, unused, longer screw may be just the trick.

Thanks

Johnny B
Last edited by 56G80S on Thu Mar 04, 2021 7:42 pm, edited 1 time in total.
Mick D
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by Mick D »

Hi

3/16" x 30 TPI is an unusual thread, but available - speak to Clay at Acme Stainless to see if he will replicate it.

Close options would be 3/16" x 32 UNF / BSF or 5mm x 0.8, (M5) - maybe get ones to trial fit, (check your eyes ;) )?

Regards Mick
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by ajsph »

2 BA is 31.4TPI
Poul
Stan Palmer
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by Stan Palmer »

Hi Johnny, 3/16 peak to peak ? Presume means the OD of the screw is 3/16, 0.1875. The OD of a 3BA insert is o.169 so that is excluded other than by welding the hole up. Lumiweld might claim it possible, but being in close proximity to the pump bore makes it unattractive to say the least.
If you want to remain close to the original I would suggest (the machinist) sets it up square with the old plate fitted, bore parallel to the best depth, tap and loctite in a stepped stud. A new plate and 3BA nut and small washer for that location.
One of mine stripped in the same location. I found the hole was already quite a bit deeper than the bolt, tapped it 3BA to the bottom and fitted a long stud. It was still there when I sold the bike.
I am sure I have a new plate, I'll look tomorrow.
Can't help with the 30 tpi I am afraid. That little tap could be expensive, due as well unless the machinist can cut it on his lathe.
The G on your thread gauge means Turns, in German I believe. For some reason only Whit gauges are marked like that
Stan
56G80S
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by 56G80S »

Well, Mick D is on the money again (damn). I need to check my eyes. I counted the threads for 1/4" and made it 8. 8x4=32 TPI. I was working from coarse to fine on the thread gauges and stopped at 30G which looked good. 32G is perfect.

So I guess the set screw is 3/16" x 32 TPI and that's BSF? The gauge is marked Whitworth. Is Whitworth Fine BSF as I (still)KBA. If that's true I'll order some screws and nuts a bit long so I can tailor them. If the thread is firm, fine. If not I'll settle for non-standard and turn a bolt into a stud and use the super strength Loctite adhesive I just bought. Await responses.

Stan - Apologies, it won't be standard but would work either way. Thanks a lot for the interpretation. Ironically I speak some German. enough to cope with the Rover 75 breaking it's front drivers suspension spring when in Leiwen in the Moselle Valley a couple of years ago; surprised myself. Thanks a lot for the thought about the blank end cap but either way, I won't need a new one as the oversize screw doesn't enter the recess in the cap. The other three do and so there shouldn't be an alignment problem despite a bit of off parallel thread. I might pop a small fibre washer under the screw head. If it's the stud approach the same applies.

Thanks for all the input, it's a big help.

Johnny B
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Re: Bolt, fixing rear end cap, oil pump, 5/16 in. by 3.BA th

Post by Mick D »

Hi Johnny

From your revised figures there are several possibilities:

3/16" BSF

10-32 UNF

2BA

M5

If the repair was carried out 40 odd years ago I think the options that would have come easily to hand would be the BSF or BA ones - maybe get examples of both and see which fits best. UNF and Metric were not that widely available at the time.

Regards Mick
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