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Coolant tap on TR3 block

Moss Motors
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DaveC David C
CA, USA   USA
I have now purchased two block taps from Moss Motors, and one from Victoria British and they either don't fit or they leak.

Does anyone know exactly what thread size is needed, as I'm just going to put a blank in the block.

Thanks in advance for help!

dave

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Jacad Avatar
Jacad Gold Member Barry Shefner
Montreal, QC, Canada   CAN
1959 Triumph TR3A "Loose Wheels"
1976 Triumph TR6 "The Tweetster"
David,

The TR register guys have a thread on just that subject https://www.tr-register.co.uk/forums/index.php?/topic/39723-drain-taps/ BTW before giving up, you may want to try disassembling and lapping the brass tap which may remove enough interference material for the tap to seal correctly. FYI someone in that thread said that the Rad thread is BSP (British Standard Pipe Thread) and that the block thread is UNC (Unified National Coarse) so that would be the type of thread to use for the blank.



Barry
59 TR3A 0TS57675LO - "Loose Wheels"
76 TR6 CF54266U - "The Tweetster"
Website: Triumph TR2-TR3-TR4 www.trtriumph.com/ (sorry for not keeping it current for the past couple of years)

darrellwalker Darrell Walker
Vancouver, WA, USA   USA
1966 Triumph TR4A "Christy"
1981 Triumph TR8 "Kate"
I don't recall the thread on the block, but I do know it was a standard size (as Barry motions above). I used a bolt with a copper washer to replace the tap. Make sure it isn't too long, or it will hit the liner.



Darrell Walker
66 TR4A IRS-SC CTC67956L
81 TR8 SATPZ458XBA406206
Vancouver, WA, USA

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DaveC David C
CA, USA   USA
Thanks, all. I think I'll seal the tap from the inside as a temporary solution and then get a blank or bolt to replace.

DonP Avatar
DonP Don P
Frankfort, IL, USA   USA
I used some Teflon plumber's tape when I put mine in during the rebuild. No problems so far.

I use it on all of my coolant related connections that involve bolt together fittings.

6TTR3A Silver Member Frank Conklin
San Diego, CA, USA   USA
I tried lapping mine. Worked for awhile.
I gave up & filled the hole with solder. Now it looks like a tap
but is actually a plug.
Frank

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Something I've not seen mentioned here : The block tap is supposed to have a sealing washer between it and the cylinder block. Which leads me to believe that it uses straight threads rather than tapered pipe threads (just the opposite of what Barry said above).

IMO it's important to see whether it is leaking at the connection to the engine block, or from the outlet of the valve. If the connection to the block is leaking, then a new washer (or two, or three) should be all it takes to seal it up.

If the outlet of the valve is leaking, then disassemble the valve and use a bit of fine abrasive (eg rubbing compound) to lap the tapered cylinder into the tapered bore of the valve. Also check that the spring on the bottom supplies a reasonable amount of force to hold the tapers together. I've had to replace that spring a couple of times, but my original valve still works.



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Jacad Avatar
Jacad Gold Member Barry Shefner
Montreal, QC, Canada   CAN
1959 Triumph TR3A "Loose Wheels"
1976 Triumph TR6 "The Tweetster"
Randall,

This is extremely perplexing,

Revington TR describes the brass tap for the engine and rad as follows:
Brass Drain tap with a parallel 5/8"UNF thread, used in the engine block of TR2-5 to drain the coolant. This tap can be used on later TR's instead of the standard blanking plug, or indeed in any application where the thread is correct.
There is another similar tap used in the bottom tank of the radiator part number 061864, this one having a similarly sized BSP tapered thread.


Then in the part header for part 061478 they refer to it as a 1/2" UNF https://www.revingtontr.com/product/061478/name/drain-tap-cyl-block-1-2-unf and the fibre washer as WASHER FIBRE 1/2 ID x 3/4 x 0.062" ... a little confusing to be sure. Is it 5/8" UNF or 1/2" UNF?????? not having a tap handy, I'm unsure.

Then you have this description from Anglo classic car parts https://www.angclassiccarparts.co.uk/home/triumph-5981/tr2-tr3-tr3a/heating-cooling/water-drain-tap-tr2-tr3-tr3a brass tap with THREAD SIZE IS 0.5 INCHES (Diameter) X 20 NPT (National Pipe Thread) AFAIK such an animal does not exist, 1/2" NPT would have 14 TPI (Threads per Inch)

If I assume (never assume nuttin) that Revington is referring to 5/8" UNF the 3/8" BSP size which would be the closest to 5/8" UNF and they compare as follows:

Size Major Diameter (mm): Minor Diameter (mm): TPI
5/8” x 18 UNF 15.839 14.351 18
3/8" BSPT tapered 16.662 14.846 19

If the size is indeed 1/2" UNF then
Size Major Diameter (mm): Minor Diameter (mm): TPI
1/2” x 20 UNF 12.667 11.328 20
1/4" BSPT tapered 13.157 11.445 19

Obviously if the threads are not the same between the tap and the block, one could expect coolant leakage around the threads so perhaps an incorrect thread on the brass taps currently being supplied is the cause of the leaking complaints that folks are reporting???/



Barry
59 TR3A 0TS57675LO - "Loose Wheels"
76 TR6 CF54266U - "The Tweetster"
Website: Triumph TR2-TR3-TR4 www.trtriumph.com/ (sorry for not keeping it current for the past couple of years)



Edited 3 time(s). Last edit at 2019-01-31 08:11 AM by Jacad.

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Yeah, I've seen a lot of that kind of thing. Seems to especially affect cooling system parts. Just try figuring out the thread on a standard mechanical temp gauge probe!

I dug through the cooling parts bin, but didn't turn up an original tap. However, I was able to try some precision rods (a transfer punch set actually) in the hole in a block over in the corner. A .435" would go in easily, .451" not at all.
So clearly, 5/8" is out of the question.
NPT is sized by the nominal inside diameter of heavy pipe, so 1/2" NPT takes a tap drill of around .700". Obviously that is wrong, too.

I found a chart that shows 11.6mm tap dril for 1/4 BSP, which is .457" so that's close.
But another chart shows the minor diameter for 1/2 UNF (20 tpi) to be .446" So that's my favorite for the right size.

Next step of course would be to measure the thread pitch. But I didn't try that.



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Jacad Avatar
Jacad Gold Member Barry Shefner
Montreal, QC, Canada   CAN
1959 Triumph TR3A "Loose Wheels"
1976 Triumph TR6 "The Tweetster"
I suspect that you are right that the 1/2" UNF 20 TPI is likely the correct size of the hole in the block. Appears that some folks got it wrong with the 5/8" Dia. and 1/2" NPT designations which seem to have been repeated/compounded

These folks seem to agree http://www.leacyclassics.com/61478.html
and Quick update, a 1/2" fine thread (20) bolt fit perfectly. Got a stainless steel one, and a copper washer. http://www.britishcarforum.com/bcf/showthread.php?38994-Engine-block-drain-valve-replace-with-plug

Sometimes these thing can get pretty frustrating trying to figure out what is correct or incorrect. It would be interesting to see what thread size was on the replacement taps that folks have been complaining about to compare.



Barry
59 TR3A 0TS57675LO - "Loose Wheels"
76 TR6 CF54266U - "The Tweetster"
Website: Triumph TR2-TR3-TR4 www.trtriumph.com/ (sorry for not keeping it current for the past couple of years)



Edited 1 time(s). Last edit at 2019-01-31 11:14 AM by Jacad.

andy303 Avatar
andy303 Gold Member Andrew Blackley
Chardon, OH, USA   USA
Interesting discussion. In the MGB Moss catalog on line there is a pdf explaining why the drain taps leak and what can be done. Basically they all leak as sold and either you do the hand finishing or use a plug. There is, however, no such disclaimer on the TR external engine page.


Attachments:
434-240_460-020_470-240_INFO_web.pdf    106 KB

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Don't you love how Moss tries to make defective parts seem "normal" ?



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Jacad Avatar
Jacad Gold Member Barry Shefner
Montreal, QC, Canada   CAN
1959 Triumph TR3A "Loose Wheels"
1976 Triumph TR6 "The Tweetster"
It does appear quite odd that they would produce a repro item that defeats it's purpose.... Knowing full well that their reproduced part will leak, they intentionally install an internal threaded bolt to stop any fluid from leaking out just so that visually the part appears to be functional and correct???? REALLY!

I have a brass valve installed on my engine block that has been there since at least 1995 and likely is original and during the past 23 years I haven't yet noticed that it leaks. I guess that either someone actually stood there and laboriously lapped the parts OR it was well manufactured in the first place!



Barry
59 TR3A 0TS57675LO - "Loose Wheels"
76 TR6 CF54266U - "The Tweetster"
Website: Triumph TR2-TR3-TR4 www.trtriumph.com/ (sorry for not keeping it current for the past couple of years)



Edited 1 time(s). Last edit at 2019-02-01 10:02 AM by Jacad.

Fred Winterburn Avatar
Ripley, ON, Canada   CAN
On my 56 TR3 engine, I'm very sure it is pipe thread, not UNC. See the picture. I used teflon tape for a better seal. I kept the original valve and lapped it with some compound to make it seal properly and it hasn't leaked since. That was about 10 years ago. I also diverted some flow directly from the pump discharge to the rear petcock and this lowered the head temperature at the rear by 10 degrees F. If anyone is interested in trying that mod, let me know and I'll make a post showing what I did. Fred

In reply to # 1593411 by Jacad I suspect that you are right that the 1/2" UNF 20 TPI is likely the correct size of the hole in the block. Appears that some folks got it wrong with the 5/8" Dia. and 1/2" NPT designations which seem to have been repeated/compounded

These folks seem to agree http://www.leacyclassics.com/61478.html
and Quick update, a 1/2" fine thread (20) bolt fit perfectly. Got a stainless steel one, and a copper washer. http://www.britishcarforum.com/bcf/showthread.php?38994-Engine-block-drain-valve-replace-with-plug

Sometimes these thing can get pretty frustrating trying to figure out what is correct or incorrect. It would be interesting to see what thread size was on the replacement taps that folks have been complaining about to compare.


Attachments:
Connection from pump housing to rear petcock.jpg    26.4 KB
Connection from pump housing to rear petcock.jpg

CJD john durant
Southlake, TX, USA   USA
I went through 4 valves trying to solve the leak problem. The problem is they machined the surfaces that should be ground. You can emulate the grinding by lapping...but then you are on a very fine edge, between lapping the machine marks out enough to seal, and lapping so far the cone valve will no longer fit in the body.

If they were meant to be lapped, then the cones would have been machined larger than they are. The way the repro’s are provided the cone sits exactly where it needs to sit for a good joint. So, as you lap the cone gradually drops into the body until it no longer fits and cone will not tighten as it should. So don’t lap and they leak. Do lap and you may have to go so far as to render the assembly unserviceable anyway.

Any way you look at it...these current repro tap valves are poorly machined and are non-functional.

My end solution, at least until they start making them right, is to lap them so that only about 50% of the machine marks remain unlapped. I then grease up the cone and body and re-assemble. It then works...for a while anyway, until the grease is all gone. Then you add more grease and it’ll seal back up. The grease only seems to go away if you use the valve. As long as you leave it closed it’ll last indefinitely.

The grease does serve a secondary purpose, in that the main failure mode for the “original” valves was not leaking, but rather the cone seizing to the housing. So greasing the valve, if it isn’t a permanent solution, will at least prevent seizure?!?



John
Southlake, TX

'55 TR2



Edited 5 time(s). Last edit at 2019-02-01 11:48 AM by CJD.

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