T-Series & Prewar Forum

after recommending to a contributor, that was in doubt regarding the use of long or short reach plugs, that a short reach plug would probably be safe regardless of which of the two heads was on their MGTD, i decided to put my money, a small amount of money ($2.00 apiece), where my mouth was and just see how an inexpensive 1/2" champion non-resistor plug (L78C) intended for use in a lawn mower and/or outboard motor stacked up against the often recommended, and the one i use, NGK BPR6ES. the head on my engine is cast with a 168422. and intended for use with a long reach plug, i think.

the short answer is that i couldn't detect any difference.

this morning to get a proper "before" performance level, the NGKs in the engine were cleaned and after a fresh rocker adjustment the car was taken out for a 70 mile drive. it was about 75F. the drive was run at about 45MPH on winding valley road with some stop and go mixed in. after the drive, the plug tips on removal and inspection were a medium grey and looked normal. i then popped the champions in and went on another, albeit shorter, drive.

i'm not done yet, as i haven't given the new plugs enough miles yet, or a cold start, so the jury's still out.

BTW, i'm not doing this to save money, i just want to satisfy my suspicion that spark plugs may not be a critical component where the XPAG is concerned, given sufficient clearance of a short reach plug.

we'll see what happens long-term...there could be over-heating, mileage, and heat-soak, etc., issues.



Edited 3 time(s). Last edit at 2024-04-21 08:14 PM by hueyhoolihan.

It will work, but the spark will be shrouded by the threads in the head, which is less efficient, and the threads (and probably the plug tip) will likely build up with carbon over time.

thanks, understood...

i'll be keeping an eye on it.

Hi Huey,


The major concern with spark plugs and cylinder heads is the risk of piston contact when a long-reach plug is used in the early head. As Steve mentions, the 1/2 plug in the 3/4 reach, later opening will be shrouded - with the efficiency losses mentioned.

---

Jeffrey W. Delk
Hartwell. Georgia USA
1950 MGTD #2301
1954 MGTF 1500 #7673
1957 MGA
1971 MGB Tourer



Edited 1 time(s). Last edit at 2024-04-22 04:25 AM by AbingdonNerd.

It will eventually be more difficult to screw in the correct 3/4" reach plug, as the last 1/4" of cylinder head threads will be filled with carbon, which, wityh the combustion gases, is hard enouigh to potentially cause damage to the threads.. My guess is that you would have a 10% reduction in performance with a short plug in a long head - measurable on a dyno, but barely noticeable.

Tom Lange
MGT Repair

yes, good point WRT the carbon buildup in the threads, hadn't thought of that. wouldn't want to have difficulty going back, if necessary.

seat of the pants shows no performance difference as yet, but i haven't given it my personal and definitive "performance test", a repeatable hill, speed, RPM and distance test that gives a very good indication of power.

and i really want to put a couple hundred miles on the plugs, pull them and take a look at the color of the tips. lately, the tips don't turn tan anymore on the NGKs, but rather a dull medium grey.

When doing a spark plug cut, remember to push in the clutch and turn off the key when you are driving at the speed you want to test, rolling into a planned-safe area. So many people get home, turn off the engine in the driveway, and expect the plugs to display a color that indicates the engine's tune. I confess I struggle to make any sense of plug color with modern gas, anyway. It used to be so predictable...

My guess is that it will take a while - 5-8,000 miles - to display any loss of power with a short plug in a long head; the plug tip would take a while to get fouled, shrouded as it is.

Oh, well. Tom Lange
MGT Repair

I could run and dart in and out of traffic on the road and that may work for a while. But still ill-advised. I'm just saying....wrong plugs may work but not a good idea whatsoever. Tom is correct that plugs can't be read very well with modern fuels. Obviously if black/sooty, oily, super white, or damaged there is a problem. But forget about reading for mixture for the most part. George