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MG Midget and Sprite Technical - Roll pins

How are roll pins specified?
I need one 19.79mm long (presume a 20mm one?) The hole just accepts a 4mm drill. I guess the roll pin for this needs to be fatter than 4mm when measured so that it compresses and fits tightly when tapped into place. But is it still described as 4mm? If not, then what do I look for as I don't want it working loose.

Following on, I recall a suggestion somewhere that after fitting a roll pin one idea being to tap a second thinner one inside for added security. Is that right?
GuyW

Hi Guy,
This link gives some details on roll pin sizes.
I read similar on using a second roll pin inside the first one for added strength. But that might be just when the correct solid pin for the particular application can’t be obtained?

http://www.spirol.co.uk/library/main_catalogs/SPIROL-Slotted-Spring-Pins-us.pdf

Simon
S Holt

Thanks Simon.
My AV software puts up a warming against that link. But browsing direct to the Spirol site seems ok.
GuyW

Guy - I have boxes of these things. Let me check on Monday morning. With luck I'll have the right size.
I've heard of people using a second, inner pin but never done it myself. It's supposed to compensate for a worn drilling which is either out of round or has opened up a bit. Personally I think there are better cures!
However I have shortened pins when necessary with a bench grinder. Not that I would ever advocate such bodgery, not me!
Greybeard

Grey,

You're not advocating bodgery, you're just saying what you have done. Two different animals. Not that most of us have done similar things ;-)
Martin

Guy. You use a roll pin of the same diameter as the drilled hole. Your 4mm drilled hole would use a 4mm roll pin which, being slightly oversized in diameter, will close in to fit the hole tightly. Over the years, some people have recommended the use of a cotter pin (I believe the proper term for them is a "split pin" in the UK) through the roll pin in case the roll pin were to, for some reason, come out of its hole. The rounded head, on one end, and bent over legs, on the other end, of the cotter pin would keep the roll pin from being able to move outwards. Interesting theory, but I have never found anyone who actually did any form of testing on said theory. In my own experience, I have never had a roll pin back out of a properly drilled hole in quite a number of examples.

To confirm: a 4mm diameter roll pin of somewhere between 18 and 20mm length with actual length to be determined by how it fits the hole it is used in. Can it be slightly short? If it is slightly long, will it catch on anything? Beyond that, no significant problems.

Les
Les Bengtson

So Les, just so I have this right. The hole accepts (very closely) a 4mm twist drill. So if I order a "4" mm roll pin it will, if measured be somewhat over the 4mm in diameter. Thus providing the required spring loading so that it will fit tightly and grip in place. I don't need to order something that is listed as 4.5mm or somesuch?

Thanks Grey, stage 2 of my "Cunning Plan", was to visit a local agricultural workshop armed with a caliper, in anticipation of being offered a box full of random sizes to select from. Of course if they are really organised they may have a box labelled "4mm roll pins" but I would need to know if they really are 4mm, which would be too small, or a nominal 4mm but actually that bit bigger and be Goldilocks ones. Just right!
GuyW

How is the cunning plan?
I checked this morning and found to my great surprise that I have no metric pins - they're all SAE.
The closest I have is 5/32" which is a smidge undersize, besides being 1.5" long.
Oops.
Greybeard

Thanks for looking anyway Greybeard.
Start button on Cunning Plan not activated yet! Later
GuyW

Roll pin campaign now complete. Local tractor shop offered a box full to sort through. Most were huge things but I found the Goldilocks one and it is now securely tapped into place.

GuyW

For added security I assume, I've seen them peened over both ends. Block at one end, bash over with a peen hammer I suppose. Guess you may need a slightly oversized pin initially.
Oggers

This thread was discussed between 06/02/2022 and 07/02/2022

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