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Been having trouble starting my '41 120 lately.

Fuel system was gone through recently, so I was fairly certain that wasn't the problem.

So, after another failed start attempt this morning, pulled the plugs and found the gaps on all were between .012"->.015"! They are new plugs, Champion UY6's, and were gapped @ .030" on installation. For reference, pictured below are the recommended gaps from the '41 Owners Manual and a Motors Manual. I also recall there were recent comments on Site about the gap being in the .025->.030" range.

My questions are:
1) Generally, why do plug gaps close?
2) Specifically, any thoughts as why mine would close up so much after only about 100 miles of driving on them?
3) Car is currently running rich, would this contribute to the gap closing up?

NOTE. When I cleaned and gapped the plugs this morning, set them @ .027", started the Coupe and went for an easy 20 mile drive. On many previous occasions, for a range of 110's and 120's I've been associated with, have usually used .028" and this has seemed to work well.

Further note - Click on photos for a larger AND clearer image.

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There is nothing that can make the gap close up, nothing.
The answer is that you gapped them incorrectly. There is no functional difference between .025, .027 and .030, all will work well. As they wear and open up iof you started with .030 it will get to the unacceptable range faster.

Any changes to the cylinder head since just before you've noticed this? What I'm wondering is, if you ran out of clearance in the combustion chamber, would the exhaust valve be able to hit the lowest point of the spark plug (the fixed electrode) before it bottomed out on the top of the cylinder head?

I've encountered heads before that were surfaced excessively and when we tested them the valves did hit the top of the combustion chamber, but we never had spark plugs in them to see if they would have been contacted as well.

I'd also be curious if the new plugs were the same as the old ones as far as reach and did the old use a gasket while the new did not? Possibly a slightly different aspect of the plug is letting the electrode sit a minuscule amount closer to the valve.

HH, to my knowledge and looking at spark plug catalogs of the era, there were no alternative "reaches" offered for 10mm plugs, just the single "standard" reach.

I didn't think so but was just wondering if there could be differences in copper ring gasket thickness or tolerances between plug mfgs.

To have them all squeeze a more or less equal amount would make me suspect something in the head or head gasket thickness, ring gasket, or plug was different. I'd have to go back and read what happened or was done when the car was laid up and stranded in the driveway but do remember the mention of recent exhaust manifold work. Don't recall a mention of head work so if nothing was done there I was thinking plug related as the denominator.

Do you still have the previous plugs? If you do check the gap on them and I would look at them and see if you can see any difference between the new and old plugs. Plug gaps normally increase not decrease.
Wes

Good afternoon all...Now that Mal has been out for a ride, I would suggest pulling a couple of plugs and checking the gap. Then we will know if an error was made when the new plugs were installed or if something funky is happening between the exhaust valves and the plugs...Just my humble opinion. Ernie

Mea Culpa, just as Fred said: "The answer is that you gapped them incorrectly."

I sure did! But Your Honour, I'm pleading guilty with an explanation, here goes.

Over the years I've collected plenty of automotive stuff including 2 sets of feeler gauges. I'd actually never used one of the sets until I gapped the new plugs about a month a go. Set them to .030" or so I thought. Looking at that set of feeler gauges this morning found they are in mm sizes, so .30mm = .012"approx! When I regapped the plugs yesterday used my old set with .012"+.015" blades for a total of .027".

To answer your questions.

O_D, Any changes to the cylinder head since just before you've noticed this? - Head was surfaced 7 years ago when the engine was rebuilt and hasn't been off since then.

HH, I'd also be curious if the new plugs were the same as the old ones as far as reach and did the old use a gasket while the new did not? - New plugs are the same as old ones, UY6's, with one gasket on each.

Wes, Do you still have the previous plugs? If you do check the gap on them and I would look at them and see if you can see any difference between the new and old plugs. Plug gaps normally increase not decrease. - Old plugs seem to have opened up a bit.

Ernie, I would suggest pulling a couple of plugs and checking the gap. - Pulled 2 of the plugs this morning and they're still at .027". looking good!

Not an answer to a question, just something I want to say regarding my comment of, "On many previous occasions, for a range of 110's and 120's I've been associated with, have usually used .028" and this has seemed to work well." - The gauge I used when gapping those plugs was one of Wade's as all of that was done at the Workshop. The gauge was a wire gauge, being a ring with loops of wire around the circumference, one of which was .028", loved it. Didn't have to fiddle with blades to make up a thickness, very easy to use. If I ever see one at a swap meet I'll grab one for myself.

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Wire gauges will give an accurate gap on used plugs where a flat blade gauge cannot. Just visualize the erosive wear on the electrodes and the reason should become apparent.

PS - Image from Glenn's

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