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MG TD TF 1500 - NPT to BPT Radiator Drain

How can I change the threaded drain hole in my TF radiator from 1/4" NPT to 1/4" BPT? Looks like the original BPT drain tap was replaced with a NPT drain. Don't know if the radiator threads were re-tapped or if brute strength did the job.

Thanks,

Lonnie
TF7211
LM Cook

Fill it with silver solder, drill, and retap.

OR, since there is only 1 TPI difference in thread pitch, you might get away with the brute force method as long as you use some thread sealant.

But since the original 19 TPI threads may still be there, you might find that the BSP fitting will fit with the above mentioned sealant.
Lew Palmer

Thanks Lew. I kinda thought the same. I’ll run a borescope in the opening look at the threads.

Lonnie
TF7211
LM Cook

Lonnie,
You can always fit an adapter rather than use of brute force.
https://www.ebay.co.uk/itm/Male-NPT-to-Female-BSP-in-BRASS-American-European-Male-Fem-Extension-Adapters/111430856008?hash=item19f1cbed48:m:mQN-439SWl_xwA0P8gEA90g:rk:3:pf:0

Regards
Declan
Declan Burns

My TD had a regular US type petcock with matching nut soldered to the tank. I supplied a correct threaded nut to the radiator shop and had them solder it on to the tank for the original type drain. George
George Butz III

I received this message today regarding the drain tap threads in my radiator from my engine builder and TF race car driver George Pardee …

"Caution!
BPT (aka BSPT - British standard pipe tapered) is a tapered thread, as is NPT (although they are not the same thread pitch) and seals by jamming the tapered threads.
BSP (aka BSPS - British standard pipe straight) is a straight thread which seals on a washer (fiber, copper, or stat-o-seal) compressed by a shoulder on the fitting.
I am certain that the radiator drain tap (like the block tap) is BSP/BSPS. Confirm this with a micrometer or dial caliper - diameter of the threaded portion should be constant except for the first 1 and 1/2 threads; at the end of the threads there should be a 90 degree shoulder).
If the new drain tap will engage the threads in the radiator, it is worth trying to re-tap the radiator with a BSP/BSPS tap."

I measured the drain taps. He's right, the drain that a previous owner installed has tapered threads. The peak of the threads are about 0.523" near the tip and tapers to 0.537" near the base.

The new reproduction drain has straight threads that are uniform from tip to base at about .0514". (I believe that George meant BSPP instead of BSPS. BSPP - British Standard Pipe Parallel)

Good explanation:
https://www.ralstoninst.com/news/story/the-difference-between-npt-bspp-and-bspt-seals

Lonnie
TF7211

Left: Reproduction TF radiator drain. Parallel threads. Fiber washer
Right: Aftermarket drain that I removed. Tapered threads.

(TD uses a straight radiator drain. TF uses a curved drain.)



LM Cook

I believe (hope) that I repaired the threads for the drain tap in my TF radiator. I tapped the brass opening with a BSPP tap. The opening wasn't butchered badly by the NPT drain, so my new reproduction BSPP drain fits tightly. I'll dab some thread sealer onto it to be safe.

I bought two G (BSPP) 1/4"-19 HSS 55 degree taps on ebay, hoping that at least one would be acceptable. Both included free shipping from China. Photo below.

Left: $1.88
Right: $6.99 -- this is the one that I used. Decent quality. Longer threads. Included case. Worked well.

Thanks for your help and advice.

Lonnie
TF7211


LM Cook

This thread was discussed between 11/12/2018 and 13/01/2019

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