MTFCA Forum

It should take a lot for that to happen to any extent...like hours of driving in 100+ degree heat, and even that shouldn't cause the amount of leaking you're experiencing. What you're describing is not normal and may indicate maladjustment (excess current draw) or they have been rebuilt and repotted with the wrong tar.

I would start by having them tested for proper adjustment.

I have never had a coil leak in 24 years of driving Model T’s and it gets really hot here in S.E. Texas. I wonder if the coils have been rebuilt in the past and re- potted with something besides the original tar formulation.

Question: Do the coils get really hot while driving? If not, do you know who last worked on them and what they used to re-pot them?

After rebuilding a set of coils, I put in some tar I got from a roofer. Apparently this tar has a low melting point, because the tar would melt out of the coil box when the car warmed up. I have a '26 and the coils are on top of the engine. Roofing tar must have different melting points. Could it be you have some soft tar? Mike

a GOOGLE search "MTFCA coil tar" will tell you all you need to know about tar for coils.

The coils have most likely been refilled with incorrect tar - eventually woodtar (tar made from wood) and not tar from oil or coal with higher melting point.
I believe it is possible to get the right tar in the US from the roofing business, but it is definately a problem in Europe. The EU have put restrictions on retail sale of coal or oil tar but due to local traditions (sealing of wood boats) allowed wood tar.

I believe the recommended is Type 3 Step Roofing Asphalt...

I’ve worked on lots of T’s and occasionally see this dark, somewhat oily substance “leaking”. The stuff that generally leaks out is not really tar, but somewhat oily. It can happen on original coils or rebuilt coils. It seems that the original tar maybe degrades over time and there is sometimes a sort of separating of the “tar” component parts and a slightly more liquid product that tends to leak out the bottom of the coil. Some coils seem to have tar that is a bit more stable, some coils maybe don’t leak because the finger joints at the bottom of the boxes are absolutely tight. If you have rebuilt a ton of coils, you have seen the internal tar in many different conditions; rock hard but fractures, stiff & rubbery, sticky & rubbery, crumbly, crumbly & slightly oily. I would like to think that the potting compound used in the Ford script coils all started out about the same and different climate, temp, humidity, storage conditions, usage, exposure to various substances, etc over their life has changed the potting compound and caused it to break down in some coils. After ‘18 or so the tar had a higher melting point. ‘18 or earlier coils can leak/separate if used on Fordsons or mounted on engine like ‘26-‘27’s.
This is one of the reasons that when rebuilding a coil, I like to remove absolutely as much of the original tar as possible. If a coil has the “crumbly and slightly oily” tar in it I will throw it away. Everything else I will mix with at least 50% new tar to help eliminate the potential of this issue, but it still can occasionally happen (but usually only on boxes that have less than perfect finger joints at the bottom).
I also have thought that re-finishing, especially staining coils, might have an effect on breaking down the tar, especially at the very corners. The wooden boxes were originally treated with paraffin which was likely more of a waterproofing than anything else, but probably also made the corners of the boxes leak-proof to the tar for quite a long while.

I’m not 100% in agreement that the coil leakage is totally due to the quality and/or type of tar used.

This photo is a coil the failed on Dec 24, 2016. You can easily see the leakage. Now, look at the blue tape that I placed on the bottom with the date of 5/12, which is the date of the re-build. It was rebuilt by a highly respected member coil-guy type re-builder. He has re-built several for me over the years.

Here’s my question- If the failure was due to poor quality TAR, then why did the tar in this one wait 4 ½ years of touring in hot summers before it decided to melt on a cold December day? Just wondering.

Good question. Most of the “leaking” I’ve seen is small amounts of a thick sticky/oily substance that seems as though it precipitates out of the tar and leaks out of the box. I have only seen small spots of regular tar outside the box a couple times, but nothing like you show in your photo. I’d be curious to know whats really going on there. I do have a couple ideas, but not proven, so I’m not going to mention it here.

I had a coil over heat on my '27. Apparently the cover was on crooked and one of the clips on the cover that hold the coils down caused a short. With the cover on properly I had no further problems. I replaced the coil though.

John- that lid cover being mis-aligned makes sense.