What thread is this?

Post by **67-2door** » Mon Apr 09, 2018 8:48 am

Hi.
Does anyone know what the thread is where the vacuum pipe screws onto the distributor vacuum unit?
It appears to measure 5/16 x 25tpi, is that a standard thread type or something unique to Lucas?
Thanks.

Post by **pgp001** » Mon Apr 09, 2018 8:56 am

I dont know the actual answer to your question, but those dimensions are very close to an M8 x 1mm pitch thread which is a standard metric fine size.

Phil

Post by **taupe** » Mon Apr 09, 2018 11:25 am

Try this link

http://mgaguru.com/mgtech/carbs/cb125.htm

From the above info it is probably a 5/16 BSI thread

I use BSB threads fairly often and the 5/16" thread is 26 TPI so may also be suitable and taps are available

eg https://www.ebay.co.uk/i/253527316673?c ... 578&crdt=0

Taupe

Post by **67-2door** » Mon Apr 09, 2018 1:42 pm

Thankyou for the replies. It does look like M8 x 1 may be close enough.

Post by **simmitc** » Mon Apr 09, 2018 2:56 pm

When those units were designed I doubt that Lucas would have even considered using a metric thread

Post by **Sleeper** » Tue Apr 10, 2018 6:25 am

Isn't cycle thread 26 tpi ?

John

Post by **philthehill** » Tue Apr 10, 2018 7:41 am

In the main - cycle thread has 26 threads per inch.

If not already done so thread gauges applied to the threads to determine the threads per inch (TPI) for imperial threads or distance between threads for metric threads.

Post by **67-2door** » Tue Apr 10, 2018 8:00 am

Re-checking in daylight I think it could be 24tpi which I believe would make it 5/16 unf.

Post by **philthehill** » Tue Apr 10, 2018 8:10 am

A 5/16" UNF thread does have 24 TPI.

To confirm - screw a 5/16" UNF nut onto the threads - if it is right - rotating the nut should require no more than finger pressure.

A set of thread gauges is an absolute must when determining which thread is which.

https://www.ebay.co.uk/itm/52-Blade-Scr ... Sw1JVaMO38

https://www.ebay.co.uk/itm/UNC-UNF-Scre ... SwnHZYmIWZ

It is also helpful to have a copy of the Zeus engineers data-book-charts.

https://www.ebay.co.uk/itm/Zeus-Precisi ... SwUYNaE~rc

Post by **oliver90owner** » Tue Apr 10, 2018 4:12 pm

Agreed, Phil.

But even there can be a problem (for some) of differentiating between 55 and 60 degree thread forms....

Post by **67-2door** » Tue Apr 10, 2018 5:12 pm

Just tried & a 5/16 unf nut screws smoothly down the threads with no waggle so that must be it.
Thanks for your replies.

Post by **philthehill** » Tue Apr 10, 2018 5:33 pm

Well done.

Morris Minor Owners Club

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