Bingster Posted October 17, 2023 Report Share Posted October 17, 2023 I removed the pipe plug above #6 cylinder to check TDC on my flathead and it wouldn't thread back in without stopping half way down the hole. A look at the brass plug and a thread was slightly smashed which prevented it from going down all the way. Will it re-thread if I wrench it all the way down into the hole or get a new plug? Why is the thread in the hole doing this to the plug? Must be some dirt got into the threads in the hole, although I was very careful not to do that because of the cylinder. Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted October 17, 2023 Report Share Posted October 17, 2023 This is pipe thread. The threaded hole gets narrower the deeper it gets, and the end ot the threaded plug is narrower. This produces an interference fit as the plug (or pipe, or fitting) is screwed tight. You could get a pipe thread tap to chase the threads in the block. Do not use an SAE thread tap or it won't get narrower and seal right. (I remember old plumbing specifications requiring that the pipe joints be sealed tight without the use of pipe dope. I tighten the joints but still use some pipe dope.) 1 Quote Link to comment Share on other sites More sharing options...
Los_Control Posted October 17, 2023 Report Share Posted October 17, 2023 Just curious if the hole was open? Just common to get carbon build up on the plug hole from inside the cylinder ..... I suspect you may have some carbon that caught in the threads? In theory it probably should thread in with force and clean itself. .... Not my first choice though. I have a couple sets of Taps & dies, one metric, one SAE .... both sets even the metric has a 1/8" & 1/4" NPT pipe thread that would clean those threads up. Just saying running a tap through the threads would be my first choice. If it starts ok and goes 1/2 way before it stops .... I suspect the threads are dirty and caused the slightly smashed thread. As Don says, this is pipe threads and should get tighter as it goes .... I'm just guessing you will have some sort of witness mark on the plug to see how far it was installed before. You will know if it is going back to where it was. Quote Link to comment Share on other sites More sharing options...
Bingster Posted October 17, 2023 Author Report Share Posted October 17, 2023 It's been on the engine since I bought the car 15 years ago. It came off pretty easy after a good tug or two. I didn't know that about the hole getting narrower. I tried WD40 and motor oil on the plug but no dice. The big thing I don't want is for the plug to get stuck in the hole, and open up a can of worms of dropping metal filings on #6. Since brass is softer than the block I guess the threads on the block would take precedent over the brass threads on the plug. But it's a bit scary. Re-threading is probably the way to go. Quote Link to comment Share on other sites More sharing options...
Sam Buchanan Posted October 18, 2023 Report Share Posted October 18, 2023 Probably shouldn't try chasing the threads unless you REALLY know what you are doing. Mess up those thread and you have a real problem. Quote Link to comment Share on other sites More sharing options...
Bingster Posted October 18, 2023 Author Report Share Posted October 18, 2023 I don't know what I'm doing. At this stage everything is a learning curve. So, your advice? Quote Link to comment Share on other sites More sharing options...
Bingster Posted October 18, 2023 Author Report Share Posted October 18, 2023 I saw on one of Keith's (Garage) videos that his #6 TDC pipe plug only goes down halfway. Threads are showing. Maybe mine is okay after all. Quote Link to comment Share on other sites More sharing options...
Sam Buchanan Posted October 18, 2023 Report Share Posted October 18, 2023 I would get a new steel plug, lubricate it with teflon sealant and thread the steel plug into the head. 1 Quote Link to comment Share on other sites More sharing options...
TodFitch Posted October 18, 2023 Report Share Posted October 18, 2023 6 minutes ago, Sam Buchanan said: I would get a new steel plug, lubricate it with teflon sealant and thread the steel plug into the head. I would hesitate to use teflon tape on a pipe thread going into the combustion chamber. It seems unlikely that teflon will hold up to the heat and combustion products. Quote Link to comment Share on other sites More sharing options...
Solution Doug&Deb Posted October 18, 2023 Solution Report Share Posted October 18, 2023 It’s okay for some threads to be showing. The plug is tapered. Snug it down and you’ll be fine. Quote Link to comment Share on other sites More sharing options...
kencombs Posted October 18, 2023 Report Share Posted October 18, 2023 1 hour ago, Doug&Deb said: It’s okay for some threads to be showing. The plug is tapered. Snug it down and you’ll be fine. Excellent answer! OP, take a look at the threads on the plug, there should be a clear indication as to which ones were outside the head. Is that where it stops now? IF so, you're done! Quote Link to comment Share on other sites More sharing options...
Sam Buchanan Posted October 18, 2023 Report Share Posted October 18, 2023 10 hours ago, TodFitch said: I would hesitate to use teflon tape on a pipe thread going into the combustion chamber. It seems unlikely that teflon will hold up to the heat and combustion products. I wasn't referring to tape, just some type of lubricant such as a teflon thread sealant. The point is to get the plug to thread into the crusty threads far enough to seal via thread interference. Quote Link to comment Share on other sites More sharing options...
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