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MG TD TF 1500 - Spark plug gap

It's been more than 10 years since I changed sparke plugs and I have forgotten the proper gap.

I'm not sure that it matters but these are some of the engine specs.

Original engine for TD 1861 with a modified drivetrain.
12 year old Moss supercharger.
1 1-1/2" SU carburetor.
Cylinders are 0.060" overbored.
Roller lifters.
1979 Datsun 210 five speed transmission.
There is a name plate on top of the original valve cover with a gap dimension of 0.015" but as I recall that it is not correct.

I am just put in a set of new (10 year old) Autolite Platinum AP425 spark plugs. I set the gap at 0.019".

Thanks,
Mort
Mort Resnicoff

15 is the tappet clearance. Your spark plug gap should be 25 minimum (stock engine) and as high as 32 if you have a high powered ignition installed. If unsure, I would go with 25 to 28.
Steve Simmons

Mort:

The shop and owners manuals call for a gap of .20 -.22. This is for the TD,TF, and TF1500 engines. Unless you have installed a high energy ignition system, I'd suggest staying within the recommended gap range. The older ignition system likely are not that powerful, so a bigger gap may make it harder start and to get a good consistent spark. At .19, that's a bit shy of the recommended gap. I set mine at .20 anticipating erosion over time will expand the gap anyway. As a side note, there is a heat range consideration. The plug you are using has a heat range of 6. The original Champion NA8 has a heat range of 5. So your plugs are a bit hotter. I (and a number of others here) run the NGK BP6ES which also has a heat range of 6. My car (stock) runs just fine given we now use unleaded gas. Yours may run differently as it's supercharged and not sure if the .60 over raised the compression ratio or not. You may want to consider switching to a colder plug if you have any issues with the plugs you installed.

Interesting article on this topic from Motortrend:

https://www.motortrend.com/how-to/10-factors-that-affect-spark-plug-heat-range/

Jim
Jim Rice

As a follow up on Jim's comments about the heat range of the plug. When running a supercharger as a general rule you want to use a colder plug. Just because you had a new set of of old plugs on the shelf doesn't mean that is a good choice. The 3/4" plugs that come with Moss's other supercharger kits are NGK 8 series. With NGK the higher the number the colder the plug. A desirable 3/4 reach Champion plug would be a N3. Champions plugs are colder with a lower number. I would do some research for the 1/2 reach plugs you require and try and match up what originally came with the kit. A hotter plug can lead to detonation which isn't a good thing.
L E D LaVerne

The best spark plugs are Iridium and the best ones are NGK! They do produce a more complete combustion > more power.
The only upgrade you need is an improved ignition coil for a .032" gap.
The NGK Iridium for a TC TD with 1/2" thread reach is 4085.
https://www.ngkntk.in/benefits-of-investing-in-the-best-iridium-spark-plugs/
To complete an ignition system upgrade use a fully electronic distributor with 16 adjustable advance curves. I stock CSI ignition distributors for all vintage MG's.
Much better performance than a rebuilt / re curved original distributor.
Len


Len Fanelli

Whoops, excuse the brain fade, I was just working on my B. Plug gap is 20-22, not 25.

I've never seen any improvement whatsoever from the expensive fancy plugs like iridium, platinum, etc. They tend to last longer in the right environment, but that seems to apply only to more modern engines. Our cars run dirtier inside, and expensive plugs don't seem to last any longer than normal ones. That's my experience at least.

A distributor rebuild with the proper curve by someone who knows what they are doing (British Vacuum unit is my choice), and a new set of plugs. That's all you really need.
Steve Simmons

Based on advice gleaned from various sources, using my Sports Coil, I run a 30thou gap with a rebuilt dustributor.

Starts easy, runs well, NGK plugs, the one that was discontinued. Last 8 I bought for less than $20. Have 2 spare sets, should be enough to get to the final drive I make.

Peter
P G Gilvarry

What plug gap are you using with a Pertronics and high output coil?
M Hyde

Side question.
Once or twice a year I inspect the plugs by, cleaning them and checking the gap.
How can I tell during this effort when it’s time to replace vs reusing existing plugs?
How long should they last?
L Frisch

How long will they last has 1 simple, absolute, answer that is correct in every case…and that answer is:

“Depends”


Here is a link that you may find helpful

https://www.denso.com/global/en/products-and-services/automotive-service-parts-and-accessories/plug/basic/life/

Or this link:

https://pages.championpowersports.eu/assets/III-Technologies.pdf


Regards, Tom

tm peterson

I started using a Pertronix ignition about 20 years ago and a Sports Coil for 30 years, I have been installing a new set of NGK BP6ES spark plugs at every Spring tuneup. I've been setting the plug gap at .030" since installing the Pertronix ignition. Only variation was to try a set of BT5ES (hotter) plugs. Not worth while.

Expecting an Amazon delivery of a case of 6 Quarts of Valvoline 20W-50 VR-1 on Wednesday (~$40). Bought a new NAPA 1516 oil filter today. New set of BP6ES plugs will go in with the Spring tuning on Thursday.
Bud
Bud Krueger

Thanks Bud. I use the same plugs and I did set the gap at 0.030". All good.

Merv
M Hyde

This thread was discussed between 16/02/2023 and 21/02/2023

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