How do you know if a bolt is imperial or metric? (trying to work out what size these bolts are).

[Chief]

Hi,
My mothers caravan needs some of its slotted screws replaced (well, 136 to be precise ) and we're trying to work out the size of the bolts used.

The caravan is from the '60s so should be imperial, however of course it is possible they were replaced at some point in the past with metric fixtures (though unlikely). The two nuts I managed to get undone without drilling/cutting did take a 10mm spanner, but it's possible that was due to rust wearing them down.

I've measured the two bolts that survived removal intact and have come up with this measurements image:

slotted_screw.jpg (55.63 KiB) Viewed 3732 times

While the majority of the bolts have nuts, some go into threaded metal plates within the caravan structure, so while it would be easy enough to replace those with nuts with anything that would fit, these kind really need the correct size.

Also to add to that challenge is that different sites appear to measure bolts differently, for example on one site we've seen they measure the length from below the countersunk head and on another site they measure the entire length.

Equally we don't know what thread size it is, but hopefully if we get the right looking size the thread size will be right as well.

Thanks

[les]

Could well be 1/4 OBA , looking at the sketch. Get a selection of 1/4 screws and maybe metrics around that size, worry about the countersink head when you know what thread works.

[philthehill]

Can you please quote the threads per inch (TPI). That is the number of thread tips per inch.
Have a look at and through the link below:-

https://en.wikipedia.org/wiki/Screw_thread

[Chief]

Thanks for the replies.

If I have understood TPI from the wikipedia page correctly, and counted correctly, then along the 23mm length I counted 22 pitches.

Extending this along an inch against a ruler I make the count 25 pitches.

[philthehill]

I have just spent some time with a set of metric taps, BA taps and various thread charts and have been unable to find anything of 5mm O.D. with a TPI of 25.
The nearest metric is 6mm with 25TPI. 2BA has around 31TPI (a 31TPI/32TPI thread gauge is not quite right.
All you can do is take a good specimen of bolt to your local hardware shop and see if you can match it.

[Sleeper]

Nearest I can find is whitworth with 24tpi , either 3/16 ( 4.76mm ) or 7/32 ( 5.5mm )...

John ;-)

[Chief]

Thanks for the response.

I suddenly remembered I had a box of metric bolts in the garage (cold, so cold out there) and although none of them turned out to be a match (the threads were too narrowly spaced) I did decide I may as well take a photo of the bolts (one of which is a broken one, but the thread is easier to see) while I was comparing them, so I've attached the image showing multiple viewing positions against a ruler to see.

slotted_screw_2.jpg (3.04 MiB) Viewed 3672 times

Enlarged View:
download/file.php?id=15048

Not the greatest of pictures but maybe it helps.

[pgp001]

Yes.........My best guess is 3/16" Whitworth.

Phil P

[les]

Forget my BA suggestion, your screw threads are a lot more coarse.

[philthehill]

As above - 3/16" Whitworth has a TPI of 24 and requires a clearance hole of 5mm.

[taupe]

These look right but Im sure you can get them cheaper for your quanity

https://www.ebay.co.uk/itm/3-16-BSW-X-1 ... SweR1eocVQ

yes here you go

https://www.margnor.co.uk/141w3-16x1.html

£29 plus pp for your 135 No.

Taupe

[Chief]

Thanks for all the replies, also after my own searching I had found margnor as the cheapest I could find so it's nice to have it confirmed by taupe.

Having re-assessed some of the wall depth I believe it's possible some of the bolts we've yet to extract (some of the ones taken out so far took an hour each !) may be shorter than the 1" so I'll just buy a test run of bolts for now which will allow things to move forward all being well.

[philthehill]

Before you try and fit the new bolts/screws run a 2nd cut tap through each hole/thread to clean out the crud. It will make the job so much easier.

[Chief]

Thanks for that tip, I wish I'd thought of that when I not so long ago put back in the drivers seat in my Minor - took an hour to get just one bolt in as it ground its way through the rust (also have some sheared passenger bolts to deal with when I get round to it).

[philthehill]

A tip if you do not have a tap but do have a bolt of the correct size.

Grind a flat on the side of the bolt to just below the root of the thread and use as a tap. The edge of the ground flat cleans out the thread and whilst not as good as a fluted tap it should work ok for the purpose of cleaning up the thread.

[Matt]

Some modern fasteners even have what Phil describes from the factory - normally those intended to form their own thread as they are forced in to an un-tapped hole