4020 Propane spark plug threda leaking.

Rapidrob · Dec 5, 2018

I had a dickens of a time starting my '68 4020 Propane today. Lots of fuel and a hot spark.
I did get it to run and plowed my road of snow with out an issue.
Since it was so hard to start,I thought I'd check the timing and it was right on for propane.
As I was turning the engine over with a remote starter and the coil wire off, I heard air escaping.
I mixed up some soapy water and sprayed the spark plug seats and sure enough 5 of 6 plugs were leaking!
I removed the plugs and all had thread damage,either due to cross-treading or the spark plug hole is bugged up from the wrong plug being used.( no idea of this tractors history)
I have an 18mm spark plug hole thread chaser and thought I might be attaching a strong rare earth magnet to the tip of the tap to catch any iron filings that may be cut loose when chasing the threads, removing the head right now is not an option.
Is this idea worth doing?
Without removing the head from the engine, what have you done that worked?

shaun wallace · Dec 5, 2018

I have heli-coiled them. Did one for neighbor year or so ago. Chased one out while back for him. Will be heli-coil it one of these days. Did with head on it, packed tap with grease. Real trickey to get tap started strait.

Lee B · Dec 5, 2018

Turn each cylinder in turn to TDC firing stroke, run the tap in, out and then insert long air wand and blow chips out by moving the wand around quite a bit. Standard practice for when you blow the plug out of a 5.4 triton and install inserts. Valves need to be closed so use distributor rotor to show you where it's ready to fire and do that cylinder. Rinse and repeat.

Mike M · Dec 6, 2018

If the threads are good enough to at least snug the plugs up and not blow out you may want to try some sort of sealant on the threads ?

Rapidrob · Dec 6, 2018

All are good ideas. I think I will try to chase the threads as seating the new plugs is tough.
A sealer may work, but it will have to take the high temp and pressures.

Rapidrob · Dec 10, 2018

The Thread Restorer showed up and really helped. I did one plug to see if it helped. I sealed the threads of the plug with Teflon tape and it seems to be working,no more leak in that one cylinder. I had to order a new set of plugs as the type is no longer common here where I live.

Rapidrob · Dec 12, 2018

I chased all the threaded holes using the grease trick and replaced all of the plugs with NGK AB-7's ( 3010) medium heat plugs). These plugs take the normal 13/16's plug socket and have a nice seating skirt on them. There is plenty of room to start the threads on the plugs without the chance of cross-threading.
I gaped them to .015 and turned on the gas. The engine fired up instantly and ran so smooth that it was if it was a new engine!
Thanks for the tips and help.

4020 Propane spark plug threda leaking.

Rapidrob

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shaun wallace

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Lee B

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Mike M

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Rapidrob

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