TR2/3/3A thread pitch for TR3 water tap in block

[TexasKnucklehead]

It appears my block tap has been replaced with a plug. Aside from the hole being plugged, I don't think the threaded plug matches the threads in the block. The book calls for a water drain tap PN 61478, but I can't find that in the standard triumph hadware (decoder) book. I have cleaned enough rust away from the threaded hole, that water could flow, but I'd like to chase the threads. The plug was barely threaded in and the threads on the plug appear damaged.

The plug appears to be 1/2" 18tpi. What should it be?

 

[eschneider]

my notes from the last time I went through this - 1/4" BSP (British standard pipe, non-tapered thread) OD=0.518"

It's close to 1/4" npt pipe (18tpi, 0.54" OD taper thread) which looks like what you pulled out.

 

[MGTF1250Dave]

Aloha,

I can't help with the thread, but the sediment plugging the hole can most likely be knocked free with an awl or small punch. It seems to be common for the tap to become blocked over time. I suspect that is because coolant flow at the rear of the block has reduced velocity and solids (rust and minerals from the water) precipitate out of the coolant and collect there. I've always been able to break up the stuff so that if will flow out after the tap is removed. I use hand tools to root around in the block through the tap hole. I've never had to use a hammer doing this. The solid thing you are most likely to encounter is #4 cylinder.

 

[TexasKnucklehead]

Dave,

For sure, the sediment in the hole was easily removed. I have no way of knowing if it leaked before, but seeing only 1 or 2 threads holding the plug in place makes me wonder if the sediment was the only thing keeping it from leaking. Also, when you look at that hole with the liners removed, from the inside, you can see it's very near the bottom of the sleeve -near the Fo8 gasket, and it's not exactly round. I was unable to get a good picture showing the slope, but the hole is pointed downwards. I suppose I'll be needing a tap to start the threads, and another (bottoming tap) to clean the remaining ones, because the hole doesn't go clean through the block. If I drilled it out, it would end up inside the Fo8 area.

 

[TexasKnucklehead]

For the record, the correct answer is 1/2" 20 tpi. I was able to bottom out my hand tap where I could see it from the inside of the block. It is key to note that the center line of the tapped tap hole, roughly lines up with the lower line of the recessed area for the Fo8. I guess the designers wanted to be sure all the water could be drained from the block when the tap was opened. Once all the crud is removed, you can see inside the threads to the bottom, and it looks like half a hole at the back. The second picture was taken straignt on, with a light inside the block.

I used nothing but a dental pick and tap to remove the crud, but some of it was really hard.