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MG TD TF 1500 - Obsolete French Metric vs ISO Fine Metric

I appologize for this question before I even ask it. I know that the subject has been covered ad nausium. But here I go anyway. Teach me, Master.

This question concerns threads only -- not heads and wrenches.

>> How do "French Metric" threads differ from the equivalent ISO metric threads?

Dave DuBois and others on this BBS and other T-series forums say, "The vast majority of the fasteners used in the engine are an old French standard Metric (as opposed to the ISO metric in use today)"

Dave's chart seems to show that M8-1 ISO is the same as French Metric 8mm, but 6mm and 10mm are different ??http://userwebs.donobi.net/sufuelpumps/Other_Subjects/Fasteners_Wrenches/TD/Standard_Threads.pdf

What problems will I generate if I use a modern day hardend M8-1 metric bolt from the corner hardware store in my TF gearbox or engine instead of the original French Metric bolt?

What about substituting an ISO 6mm or 10mm bolt or nut for the equivalent French Metric?

Do bolts (with Whitworth heads) sold by Moss, A/S, FTFU for T-series gearboxes and XPAG/XPEG engines have ISO metric threads or French Metric threads.

I've owned my TF for two years and I feel that I know less today than I did when I bought it! Thanks for your continued education.

Lonnie
TF7211
LM Cook

When Morris Motors acquired the French Hotchkiss equipment it came with French Metric Fine thread tooling. So they used it, but made sure the bolts had BSW / BSF heads to fit the available spanners. Those French Metric Fine threads are ISO Metric Fine threads today (but rarely used) and not ISO Metric Standard Threads (which are less tpi). That's the way I understand it is anyway.
If you order a bolt for your XPAG from Moss, B&G or any other bona fide supplier you will get the correct ISO Metric Fine Thread with a BSW head. A metric bolt from the corner store won't fit. You won't need any other size than 8mm Metric Fine.
Dave H
Dave Hill

Is M8-1 thread "ISO Standard Metric" or "ISO Fine Metric"? All hardware stores near me have M8-1 and other fine threads.

Lonnie
TF7211
LM Cook

I purchased a bag of 8 X 1 mm from McMasters and they fit fine.

Jim B.
JA Benjamin

Lonnie, M8-1 is an ISO Standard Metric, as is M8-1.25. The difference is that M8-1 is fine and M8-1.25 is course. Both are ISO standards. If you see just M8 it refers to the course thread. Bud
Bud Krueger

Lonnie the following shows the contents of a pretty standard metric tap & die set. Google metric tap & die sets.

17 Die Sizes:
3x0.5, 3x0.6, 4x0.7, 4x0.75, 5x0.8, 5x0.9, 6x1.0, 6x0.75, 7x1.0, 7x0.75, 8x1.25, 8x1.0, 10x1.25, 10x1.5, 12x1.75, 12x1.5.

17 Tap Sizes:
3x0.5, 3x0.6, 4x0.7, 4x0.75, 5x0.8, 5x0.9, 6x1.0, 6x0.75, 7x1.0, 7x0.75, 8x1.25, 8x1.0, 10x1.25, 10x1.5, 12x1.75, 12x1.5.

As Bud says there are both coarse & fine threads for each metric size. The relevant size for us has the fine thread. So when M.G. nuts talk about a "mad metric" bolt it will have a Whitworth (BSF/BSW) head & a fine metric thread. Jim purchased metric fine. Moss explain the history Dave describes in each of their catalogues. Cheers
Peter TD 5801
P Hehir

So if M8-1 is the same as an Obsolete French Metric thread, then why is it "Obsolete?"

Lonnie
TF7211
LM Cook

Not that long ago all of the locally available metric 6mm and up hardware was "coarse" thread, either 1.25 or 1.5 (?), depending. I'm surprised a local hardware has the "fine", or 1.0, but that is what you need. Guys above right on. I'm not sure that all of the new hardware from the usual suspects has Whitworth heads however. Some over the years has metric head sizes.
George Butz

You can still find quite a bit of useful M8x1.00 bolts, as they were used in some American cars. And, because it is a very strong thread pattern. It has never gone away.
D mckellar

A Metric Fine 8mm x 1mm bolt will work OK, though the head size will be wrong. If that doesn't matter to you then go ahead. Very surprised that you can find such a size in a corner store over there as you certainly couldn't here. I don't think that sort of bolt was used for anything much else in the UK.
Dave H
Dave Hill

This thread was discussed between 05/03/2016 and 06/03/2016

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