Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Lucas L691 (and 692) Screws

There are actually two very similar but different thread forms, not interchangeable so make sure they match the brass fittings or run a small tap through. Can't quote the thread forms I just know they are different, I sourced some lamps from SVC in Tamworth and as mine had clear lenses and orange bulbs thought I would just swap the lenses onto the new bodies....not quite that simple, the lengths are also slightly different.

Ah! Two threads on the screws then not on the forum!

So it's not just MMC who mix things up

Colin is correct.

The brass inserts supplied with the clear lenses by SVC are metric, the original Lucas inserts are BA.

I had spare 'long' Lucas chrome BA screws and so ran a BA tap through the insert and used the better Lucas ones.

The clear lenses I purchased came with replacement threaded inserts.

For anyone mildly interested in the minutae of such matters.....

Cancel my previous comment about the original Lucas chrome screw being a BA thread, it is not.

The thread is UNC (course). I can't make out enough of the markings on the tap but it's UNC with a 2.8mm drill size.

What a sad life I lead!

You have checked, and I haven't, so you must be right, but I'm very surprised. 5BA is 2.7mm / 43tpi, Number 6 UNC is 2.8mm / 32 tpi, 3.5mm metric is 2.9mm / 42.3 tpi (.6mm pitch)

Quite a difference in pitch?

Ah someone at least as sad as me!!

Thank you Tim, you have clarified the markings on my tap. Big No 6 with a 32 in smaller letters; big and small are relative of course because the tap is rather small and a large magnifying glass is demanded to see the markings!

Personally, I don't think we're sad at all, David!

I have quite a large stock of M3.5mm stainless capscrews, so I retap to that size when I get a screw that's a bit chewed up.

I'm in complete agreement with you Tim. But then I am slightly biased!