Thread ID?

[Agroshield]

The OD of the male thread doesn't exist anymore, its been chewed off my some cackhanded spanner holder at some time in the past
There aren't 5 pitches to measure on the male thread - the last few are very flat.
There's no AF on the bolt as it just has a large head and recess to start it with a cranking handle.
The ID I have measured is unadulterated so is about the only precise thing to go off.

An OD measurement of what you have will nonetheless be useful: we can safely assume the correct one will be no less than what you measure. If, for example, it is bigger than 7/8", that would rule out the 7/8" UNF suggested above.

There are six clear and shiney (from flash reflection) thread crests in your second photo, which define five thread pitches.

ID does not tell you much, only tapping drill size and there is a range of possibilities for any particular thread.

A print of the female thread on some soft material is your best and most reliable chance of success.

 

[zzr1200]

Could it be 7/8, 15/16 or 1 inch SAE National Fine 14tpi?

It could even be an Perkins Inhouse purpose cut thread....

 

[pressbrake1]

Manufacturers like to use their own thread standards. I have boxes of odd bod taps
Beauty of cnc machining means you can Make non standard thread forms once produced to cause nuisance
The make of tractor would be useful

 

[pressbrake1]

A better average dim over more threads would be good incase it’s bordering 15tpi

 

[brightspark]

the perkins manual will give all the details

 

[KimB]

If you torque it to around 100 ugga duggas it’ll not come back out

 

[Pete.]

I agree with 7/8" -14 UNF the measurements work out.

1/14=0.071 (almost exactly your two-pitches measurement of qpprox. .140"

1/14 is also the double-depth figure so 0.816 +0.071 = 0.887" (or 7/8" plus 12 thou wear/clearance)

Run a tap in the hole and buy a new bolt.

 

[RichardM]

15/16 Admiralty

 

[Brad93]

Helicoil the female, get a new male bolt. Solved.

M22 or 7/8" Whitworth you could do.

Edit: 14TPI would be a fine pitch so M22x1.5 or BSF or UNF

 

[KimB]

Looks like 7/8UNF to me as well. Slightly down on OD but it looks very near.

 

[Welderpaul]

Thanks everyone!

 

[Welderpaul]

Slightly off topic, but the same bit of kit.

I want to check the clearance between the bearing shells and the crankshaft using plastigauge. So thats the clearance on the big ends of the con rods onto the crank, and the crank main bearings

Which are the clearance fgures i should be looking for?

 

[8ob]

They are just top and bottom limits Paul.

Bob

 

[KimB]

Red one is big end
White is little end tolerance
Black mains

 

[piman]

Hello Paul,

it is a strange choice but 5/8" BSP has a 14 tpi and a minor diameter near to what you have? so 5/8" BSPP?

Alec

 

[Hitch]

Virtualy all the threads on a Dexta are UNC/UNF aren't they...?

 

[brewdexta]

Tractor crank bolt

Hi Paul, If it's the Dexta crank bolt, I can see what mine looks like, if I can find it in the various boxes of bits. From memory it was tight, but not chewed.

 

[Agroshield]

Virtually all the threads on a Dexta are UNC/UNF aren't they...?

If they are, that would be a very strong argument in favour of the crank bolt being in the same thread system.

If all the other fasteners on the engine are UN form (standard A/F sizes, measure the tpi of a 1/2" coarse one and tpi of any of the fine threaded ones), then the crank bolt is likely to have that form.

If the engine fasteners happen to be Whit. form (A/F sizes, measure 1/2" coarse tpi, and tpi of any fine thread), then the crank bolt may have that form.

 

[Welderpaul]

Thanks everyone. I've bought a 7/8" x 14 TPI die to repair the bolt.
I have various sets of thread gauges but nothing fits the threads on UNC/UNF bolts. I need to find something.
Its it a whitworth form then? 55 degrees?

What do I need to turn the OD of the bolt to - 0.8750" / 22.22mm or a bit smaller?

 

[Brad93]

Should be in the Zeus book