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MG MGB Technical - Crankshaft pulley bolt

Hi, does anyone know the thread size of the B series (same as an A series) front pulley bolt? I've been through a tool catalogue and the only thing similar is a 5/8" 16 TPI cycle thread which seems a strange thread to put on an engine. Any ideas?


Terry
Terry Drinkwater

Terry, 5/8" 16 TPI is correct.

Clifton
Clifton Gordon

Clifton - are you sure that it's 5/8 - 16? that would be a BSF thread and I believe that this one is a UNF, which is 18 TPI.

Terry - It would be strange to have a BSC thread count of 16, they are normally 26 TPI regardles of the bolt size.
Cheers - Dave
David DuBois

Dave, I have a front pulley bolt and before answering I checked the diameter in a plastic gage sizing gage I p[urchased from Automotive Fasteners. It fits the 5/8" hole. I also checked the thread pitch with a pitch gage, the only pitch that fits is a 16 tpi. I'm posting a photo, noy sure if the thread pitch # will show. There is no shank on the bolt but the outer thread diameter is .620". My LS Standard tool decimal equivalents chart reads .6250" for a 5/8" bolt, .005" off the chart but other bolts check the same. My cheap dial caliper zeros OK. As I recall the bolt I have came from an 18V engine

Clifton Gordon

Dear Terry,

From the archives. This was for tap, and the help came from Les. The tap worked beautifully thereby confirming the 5/8"-16 thread.

I have used Rutland Tool as a source of machine tool accessories for many years. Their stock number for the 5/8"-16 tap is 2144 0292 at a cost of $15.60 per tap. They also have the die available in that size if the crankshaft pulley nut is buggered up. The die is stock number 2108 0313 at a cost of $17.70. Rutland's telephone number is 800-289-4787 and their website is www.rutlandtool.com if you wish to find to check it out. Their catalog is worth having just to see what is available.

Another source for tooling is J&L Industrial Supply at 800-521-9520 or www.jlindustrial.com on the web.


Shareef
Shareef Hassan

Well, here's some truly arcane information.

In the 1100/1300 (transverse) shop manual, it says:
"The flywheel retaining screw in the end of the crankshaft is not standard Whitworth, but is of Whitworth form:
Diameter 5/8" 16 TPI 1 1/16 full thread
If it is found necessary to clean up the thread, the operation must be confined to CLEANING UP. This thread is highly stressed and must always be up to full size."

I just tried one of those bolts in a B crank and it is the same. The front end of the A series cranks (both transverse and inline) is the same as well.

Thus, it is not correct to use an American - SAE - UNF form tap to clean these up, though might get by on the crank snout, but NOT on the transverse flywheel bolt. Naturally, it is the tranverse cranks that always have damaged threads, a result of the forces involved!

AFAIK, this is the one and only British form threaded fastener on any of these engines.

Note that BSF is 5/8-14, NOT 16TPI. UNF is 5/8-18. CEI starts with 20 TPI at 1/2" and up.

FRM
FR Millmore

Clifton - "I have a front pulley bolt and before answering I checked the diameter in a plastic gage sizing gage I p[urchased from Automotive Fasteners. It fits the 5/8" hole. I also checked the thread pitch with a pitch gage, the only pitch that fits is a 16 tpi." Can't arge with that. I suspect that this is a Special British thread, but one would have to measure the included angle and profile of the threads to determine that. Rutland Tool calls the 5/8 - 16 tap 'Special', but doesn't say American or British. The main thing is that Terry has got his answer. and yes, it doesn't make sense to have a cycle thread on a MGB engine, there are other british threads on the B series engines (mostly BSP). Finally, I read my chart wrong when writing my first post. 5/8 - 16 is not BSF, rechecking the chart, 5/8 BSF has a 14 TPI thread count. Cheers - Dave
David DuBois

Clifton and Shareef are correct. The bolt is a 5/8" diameter having 16 threads per inch. This I have verified and have posted on my website.

Do not know if Fletcher is correct. My thread pitch gauge is for the standard, 60 degree, thread and, even under magnification, I do not see any light showing between the threads being measured and the leaf of the pitch gauge. This, it would seem, indicates that the thread is of the 60 degree, included angle, pattern.

The "included angle" defines the shape of the thread. Take your hands and touch the middle fingers together. The angle between the palms is the included angle. Move the palms outwards, making a 60 degree angle, the same as is found on an equilateral triangle, and you have the shape of the cutting portion of the tool bit use to cut a standard UNC/UNF thread. Narrow the palms a little, to a 55 degree angle, and you have the shape of the tool used to cut a Whitworth style thread. So, what Fletcher is telling us is that the angle of the threads formed are 55 degrees rather than the now standard 60 degrees. That may be true, but I cannot prove it with the equipment I have to hand.

As to whether it makes any difference on the crankshaft nose of an MGB, I doubt it. I have had to clean up several over the years and none have shown any problems after having the threads cleaned up using a 5/8"-16 plug tap.

Les
Les Bengtson

Les-
While it is difficult to see light on the thread flanks, it is very obvious that the threads are deeper than the 60 gauge, and very apparent that this bolt has the nicely rounded root of the Whitworth form. This form does not occur on any of the 60 degree American base threads, except in the UNJ aerospace high stress standard, proof again that old Whitworth had his act together.
FRM
FR Millmore

Hi Guys

Thank for the info, it makes sense that good old Morris Motors would make it difficult and then continue the same for 40 odd years!

Terry
Terry Drinkwater

Whitworth and BSF threads have a 55° included angle, but the CEI/BSC and some of the special threads are 60°, only the profile of the peak and root are rounded as in the Whitworth form. An interesting find Fletcher, your post must have come in while I was composing my previous post, so I didn't see it until today. Cheers - Dave
David DuBois

Terry,
Spanner size is often quoted as 3/4" W/W (rather than 1/5/16" A/F, which is more useful). I've got a bolt in the garage (Port-zed) if you want to measure it. E-mail via MGCC SW site.
Neil
Neil22

This thread was discussed between 15/06/2007 and 17/06/2007

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