22 replies to this topic

[Cooperman]

8 mm is 0.0025" (2,5 thou) larger than 5/16".

An 8 mm dia shaft will normally fit through a drilled 5/16" hole,

A 5/16" pin would normally fit into a hole drilled to a 5/16" clearance to give a running fit, so an 8 mm hole for a 5/16" moving pin will be just fine, but an 8 mm pin would fit OK under normal workshop tolerances.

[tiger99]

It is not guaranteed to be a PROPER fit. Why bodge a job when the correct implementation takes no longer? Maybe 9 times out of 10 it will be fine, but if the 10th is too tight or too slack, time and effort are wasted doing it again.

But I am mostly trying to counteract the constant errors on this forum relating to spanner sizes, where the discrepancies are potentially very damaging.

Edited by tiger99, 23 December 2016 - 08:55 PM.

[sonikk4]

Why are you being so pedantic about this tiger99
 
Why bother mentioning brake pedals.
 
Is the bolt in question a close tolerance bolt.
 
Is the hole its going into perfect??
 
Please stop nit picking on every thread you participate in. 
 
I have mentioned this before, not once in 7,364 posts have we seen any pictures of work you have done, no car, nothing at all. You spend a lot of time berating members on here but have nothing to show for it so we can see just how good you pertain to be.
 
By all means back up your comments by physical proof of the work you have alleged to have done on classic minis or anything technical.
 
As the old saying goes, "You talk the talk, but can you walk the walk" 

[Cooperman]

Oh dear - and it's Christmas too 😃.
If a 0.0025" running clearance, which is about the same as a correct big-end to crankshaft running clearance is not good enough for a 60 year old design then I don't know what is.
You should not put a 5/16" pin through a 5/16" hole if we are being pedantic, it should have a couple of thou clearance which is exactly what an 8mm hole gives.

Just checked my data sheets and the correct drill size for a close fitting 5/16" bolt is 21/64" which is fractionally larger than 8 mm, so an 8 mm hole will be perfect.

Edited by Cooperman, 23 December 2016 - 10:28 PM.

[Spider]

Merry Christmas everyone

[tiger99]

Cooperman, I apologise. I have double checked and you are correct. My calculator shows 2.46 thou clearance for an exact 5/16" pin in an 8mm hole. For some reason my mind had it the other way round. Must have been thinking of spanner sizes, where the 12 thou difference between 1/2" and 13mm plus the standard clearances and tolerances is going to lead to damaged heads and skinned knuckles if the bolt in question is very tight.


Merry Christmas.

[Cooperman]

I think you meant that you must not put an 8mm (or M8 bolt) through a 5/16" hole as even if you can 'persuade' it to go through it is incorrectly fitted.

[tiger99]

Maybe. It has been a very tiring year at work and I don't seem to be thinking clearly. I really need a rest. Let me double check to convince myself...

Just checking with the calculator. 8/25.4=0.31496

5/16=0.3125

0.31496-0.3125=0.00246

If I am not completely senile, the 5/16 bolt is marginally sloppy in the 8mm hole.

So yes, I agree with what you are saying. Some people will force the 8mm bolt into the 5/16" hole but it will be horribly tight. I think my brain is just about back in gear now.

Funnily enough, in the very bad days Rover did in fact put 8mm bolts in holes intended for 5/16" bolts. The normal amount of clearance in sheet metal bolting is a lot more than 2.46 thou so it would have been ok. But what I fear they forgot to check was the significantly reduced strength and rigidity. The manual allows the same torque on both bolts, but the UNF bolt generates a higher clamping force due to the finer pitch, and has more tensile strength due to the larger core diameter resulting from the finer thread, so the safety factor for the UNF bolt is also better.

There is probably sufficient margin on the engine mount and front subframe lower mounts, where metric bolts are sometimes found, but I would still feel a lot happier with UNF bolts as Alec designed in those places. No need to make things weaker than they were. But when you check the price of both types of bolt it becomes clear why it happened. A maintenance nightmare, because you need to check the bolt markings every time to select the correct spanner, and that will add real confusion and misery to newcomers whom we advise to get "Imperial" A/F tools.

I see now why the USA has so persistently resisted metrication!