TR4/4A TR4 coolant leak through aluminum plug in top side of head

[chassutton]

Slight coolant leak from the plug after engine rebuild. Replaced with new plug from Moss and used "Permatex high temp sealer" but there's still a leak!!
The old plug top was flush with the head (as per factory installation as I understand it). The new one goes further into the hole by about 1/8 inch.
I've seen a few posts discussing such things as using alternate thread size plugs (12 TPI instead of 14), re-threading the head to NPI so as to be able to use a standard pipe plug (brass or iron) etc. However, nothing definitive. The techs at Moss suggest trying different thread seal materials (Hylomar?) but it seems shady to me to rely on thread sealer to prevent leaks from a pressurized system.

Ironically the engine rebuild projected started several years ago due to coolant mixing with engine oil (Cappuccino anyone?!). We assumed it was the head gasket or FO8 gaskets.

Any input, advice etc. for fixing the plug leak greatly appreciated.

 

[TR3driver]

"Definitive" is hard to come by around here; lots of opinions. I haven't done it myself (yet) but I've had several heads where the hole was tapped to NPT and a pipe plug installed. No leaks.

 

[TRMark]

I tapped to NPT and used plug from hardware store. No problems. There should be some pictures posted around here. I used a 1/2-14NPT tap and teflon tape.

 

[TR4nut]

I’ve had 3 heads rebuilt so far - all retapped for a pipe plug and no leaks. On one head the plug was a little too tall and interfered with the valve cover so I replaced with a flush style plug.

 

[chassutton]

Many thanks for the input all. Sounds like re-tapping to standard pipe plug is the way to go.

 

[jimstr4]

My head was re tapped and a brass NPT plug fitted. So far so good.

 

[chassutton]

The re-tap and NPT plug seem to have done the trick!
Again, thank to everyone who responded.