The Classic Machinery Network

Does anyone know the thread size of the spark plug hole on a 1935-38 Fordson Model N Standard?

My friend needs to retap the head. He knows it's a 7/8" but not sure what type of thread - (It's not UNF as we've just tried that and it's too coarse).

Any ideas?

Cheers!

Would that not have been tapped for an 18mm plug?

Think we've sorted it.

We believe it to be 7/8" SAE 18tpi thread.

Hi,
I have a Fordson N in the garage and I just checked out and measured a spare plug.
It 's 22mm metric.( Not imperial 7/8 SAE or UNF.)

It's a 22 MM fine metric thread with a 1.5 mm pitch. That works back to roughly 17+ threads per inch.
There should be no trouble finding a suitable tap.

I hope this helps.
Good luck.
Fred

Forgive me for sticking my nose in Fred but is that a little unexpected? That is a metric thread on a pre 1945 Fordson?

Just read a post on the Yesterday's Tractors forum that says 1/2"BSP - really??

Then this on another post

I did a little research (a while back) on spark plugs for the Fordson Model E27N.
I found that the C4 & C5 Champion spark plugs are no longer made.
The C4 is now a Champion W16Y with absolutely no crossovers available.
The C5 is now a Champion W14 with three crossovers:
AC Delco numbers C75 or C77
Autolite numbers 3075 or 3076
Prestolite numbers 78-8 or 78-10
The thread used for these plugs is 7/8” – 18
Reach is 5/8”
Hex is 15/16”
Heat range is 6
Spark gap is 0.025” for all plugs.

essexpete wrote:Forgive me for sticking my nose in Fred but is that a little unexpected? That is a metric thread on a pre 1945 Fordson?

Yes I thought that was rather unusual? Perhaps a non-standard repair? Fred?

Jeremy

Has it been helicoiled?

essexpete wrote:Forgive me for sticking my nose in Fred but is that a little unexpected? That is a metric thread on a pre 1945 Fordson?

Hi,
I'm sure every other thread in the Fordson is SAE, but most plugs had gone metric pre-war.
I checked my plug with a Micrometer and Thread Guage.

Here is a Fordson N plug from the Old 20 Parts List.

FOWLER MAN wrote:

essexpete wrote:Forgive me for sticking my nose in Fred but is that a little unexpected? That is a metric thread on a pre 1945 Fordson?

Hi,
I'm sure every other thread in the Fordson is SAE, but most plugs had gone metric pre-war.
I checked my plug with a Micrometer and Thread Guage.

Here is a Fordson N plug from the Old 20 Parts List.

How very interesting Fred I never realised that metric plugs were around so early; coincidently that is how I check unknown threads at work with a micrometer and a thread gauge plus a Zeus book that lists all the threads and what TPI or pitch they should be.

Jeremy