Gearbox driving bolts size

[GBW]

There are five screws into tapped holes in the flywheel carrying the plate with a female spline centre to connect with the (Hurth) gearbox.

The originals had hex heads which made tightening difficult and I thought I would replace them with socket head screws.

 

26 tpi X 0.23".
I assumed it was 3/16" UNF and ordered more.
Too small, ordered 1/4" UNF too big.
Then I measured. I can't find a thread with these dimensions.
BSF is 1/4" X26 tpi is the nearest but doesn't fit.
Very unlikely to be metric but 26 tpi equals 0.97mm which could be 1mm. Diameter converts to 5.84 which could be 6mm. That doesn't fit either.
Weird.

26 tpi X 0.23".
 

 

Edited June 10, 2024 by GBW
hit the wrong key

[Chris G]

M6 pitch is 1mm. Why would you expect UNF or BSF threads on a German gearbox?

[David Mack]

15 minutes ago, Chris G said:

M6 pitch is 1mm. Why would you expect UNF or BSF threads on a German gearbox?

These are tapped holes in the flywheel, to which the drive plate for the German gearbox is attached. So those holes would have been drilled by the engine mariniser, and could be metric or imperial.

What engine and who marinised it?

And why is it difficult to tighten the original hex head bolts?

Edited June 10, 2024 by David Mack

[TheBiscuits]

13 minutes ago, David Mack said:

And why is it difficult to tighten the original hex head bolts?

 

Wrong thread on 'em ... 😁

[David Mack]

A 1/4" British Cycle thread is 26tpi, but long obsolete and unlikely to have been used on a boat I would have thought.

https://britishfasteners.com/threads-bsc

[Slim]

58 minutes ago, Chris G said:

M6 pitch is 1mm. Why would you expect UNF or BSF threads on a German gearbox?

Because the bolts go into a flywheel which

 

47 minutes ago, David Mack said:

These are tapped holes in the flywheel, to which the drive plate for the German gearbox is attached. So those holes would have been drilled by the engine mariniser, and could be metric or imperial.

What engine and who marinised it?

And why is it difficult to tighten the original hex head bolts?

Access?  I replaced my hex bolts with cap bolts, much easier.

[StephenA]

 

[john.k]

!/4BSF and 1/4 Cycle are more or less interchangeable ........the only cycle thread pitch that is interchangeable with anything else.........except British Standard Brass thread ,of course..............i fit were my boat ,I d remove the flywheel ,redrill and retap five new holes. in alternative locations.

Might add,1/4" or 6mm seems a very small fastener for a drive plate ......I d be much happier with 5/16 or even 8mm.

[Ronaldo47]

The diameter is consistent with 0BA,  pitch =1.0mm (25.4tpi), diameter = 6mm ( = 0.2362") according to my screw thread book.  The flank angle of BA threads is 47.5°

[john.k]

Much more likely to be plain 6mm metric std..............still too small for a flex plate IMHO.

[GBW]

I am driven to the conclusion they are M6.  I was diverted by thinking they belong to the flywheel, whereas, as someone has pointed out, they could equally belong to the drive plate which is probably German.  This is by Hurth (made in Italy!).

M6 does seems bit weedy although there are ten of them.

The old engine only had five.

The originals were hex head and the drive plate lives in a recess which the heads butt up against.  This makes it difficult to access an open-ended spanner and impossible to fit a socket.

[StephenA]

If the engine was marinised in the UK and its a BMC then I'd suspect that the drive plate isn't german. Our Hurth ZF12M uses a plate that we know was fitted in the UK ( I suspect Starline might have replaced it when they did a rebuild of the engine when we were at Upton upon Severn)

 

A photo of the fly wheel and the drive plate might help people work things out - For Mintball we bought two full sets of the most common sized bolts from fixaball (https://www.fixaball.co.uk/ ) and then used the ones that fit.  Fixaball have a contact email - they are in Cheltenham, so it might be worth reaching out to them directly.

Edited June 11, 2024 by StephenA

[agg221]

Photos of the tops of the bolt heads would also be informative. Without embarking on a rather lengthy explanation, if there is writing on there some of it may be a code which allows them them to be identified, potentially for thread and strength.

 

Alec

Edited June 11, 2024 by agg221

[Tony Brooks]

32 minutes ago, agg221 said:

Photos of the tops of the bolt heads would also be informative. Without embarking on a rather lengthy explanation, if there is writing on there some of it may be a code which allows them them to be identified, potentially for thread and strength.

 

Alec

 

or three little radial dashes, or circles stamped into one of the flats.

[john.k]

If you use M6 socket heads ,they will be minimum 8.8 grade ......Unbrako used to be 10.9 grade,but all Unbrako stuff is now Indian ,so who knows..........I would use a flat hardened washer under each bolt.,and possibly a smidgin of blue loctite..........or maybe not.