Nut/ Thread size on Glow Plug Electrical Connection M18

Michael Davis · Nov 14, 2017

If the M-18 is anything like the M-12 it will NOT start without glow plugs. It's been about 7 years since I rebuilt my M-12 and replaced the plugs. They were only about $12 Cdn each and came with the nut. I think 4mm. but you should have a couple of spares anyway. From any diesel place or Kubota dealer..... Universal tends to overcharge I think.

jviss · Nov 14, 2017

I feel bad, I just threw out three burnt out glow plugs, with the nuts, which I had held on to for over 10 years! The nut is special, it's knurled, and there's a slot and an angle at the top into which you can insert a screwdriver.

I found inexpensive replacement glow plugs at clubplug.net. Currently $5.65 for NGK-Y103K, the glow plug in the Universal M25, which is a Kubota D-850. The part from Universal is part number 299685, priced at $64.90 each. These are exactly the same.

But, I digress. I suggest you buy a spare set, and use one of the new nuts. I have some spares, still, and will measure the thread on one a bit later.

jv

Maine Sail · Nov 15, 2017

The terminal nuts on many glow plugs, for Japanese engines, are metric. If the lower portion of the glow plug threads into a metric block the terminal nut on-top will also be metric.

The most common size terminal nut I see for Kubota blocks is M4 X 0.7 pitch. Most glow plugs are usually use a coarse thread pitch not a fine pitch.

I am not a fan of those older serrated thumb screws and much prefer a serrated flange nut as many glow plugs today will ship with, or a standard nut and lock washer if you can't find a serrated flange nut..

If you don't want to remove the other nut just hit the hardware store and grab two metric nuts in M4 & M5 sizes and one of them will fit. It will cost you less than $1.00 to try. If I recall correctly NGK only has two terminal nut sizes they use for all their glow plugs and those are M4 and M5 if my memory is correct.

jviss · Nov 15, 2017

I just measured it, I get M4x0.5, meaning, a 4mm diameter with 0.5mm thread pitch.

I like the nut that comes with them, as one can use a screwdriver on them; that's what that slot is for!

Maine Sail · Nov 16, 2017

Loose Ends said:
Thanks to all, I will buy a couple metric nuts (and a few spares) and get this taken care of.

Bill

Bill,

I happen to be in the shop early today and am waiting on something to cure. I grabbed about six glow plugs from Kubota blocks or Westerbeke blocks, & some M4 and M5 nuts I had kicking around to try them out. while waiting for the glass to kick off.. I have 3 NGK glow plugs, 2 Champion glow plugs and 1 Beru. All of them take an M4 X .7 pitch nut. You should be pretty safe trying an M4 X .7. I also have a brand new 10mm NGK I just found, a Y-114T, and it is also M4 X.7... If your plug takes an M4 X .5 those nuts are "non-standard" stock for your average home center or hardware store. If yours are M4 X .5 these nut can be found at McMaster Carr or other specialty fastener suppliers such as Fastenal.

jviss · Nov 16, 2017

Maine Sail said:
If your plug takes an M4 X .5 those nuts are "non-standard" stock for your average home center or hardware store.

It will be interesting to see. I measured the terminal thread on a NGK-Y103K. I didn't use my thread pitch gauge, but with a digital caliper across two threads I measured 1mm, as close as I could get with a magnifier. I'm not home now, but when I return I'll get out the metric pitch gauge and see.

jviss · Nov 17, 2017

Maine is right, it's an M4x0.7.

It turns out my Chinese metric thread pitch gauge is worthless! However, I measured again, by measuring the displacement of the nut over ten turns, and got 7mm. So, 1 thread = 0.7mm.

jv