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MG MGB Technical - New engine Rear Welch Plug Failure

Had my 67 MGB GT out to put a few miles on the new engine. Pulling a small hill passing a car when temperature gauge starts climbing fast. Had been at 190 the entire trip. Pulled over and turned her off. Found that the rear Welch Plug failed. (The one part I did not put in the car, the machine shop installed the plugs).

Not looking forward to pulling the engine again, so I am going to give Chris's "Betson Bolt" a try.

Next question is, I read that a blown head gasket could cause an increase pressure to the cooling system. I am using a Payon head gasket, APR head bolts and re torqued the head after the 20 minute break in run and three more start-ups to temperature. Any tips for checking the head gasket?

PS, No water in the oil.
Bruce TD4139 Cunha

Bruce, you probably caught it in time before the head gasket had a chance to blow. Install the Betson Bolt, along with a new freeze out plug, and take the car out for a test drive. I think that you'll find that it's okay. RAY
rjm RAY

I have seen on many occasions welch plugs fitted incorrectly
A lot of people just knock the centre in with a punch which doesn't really do much
The whole idea is to 'almost' flatten the whole plug, which causes the od of the plug to expand to be a tight fit in the hole
Just punching the centre does not a lot of expanding
I'd be checking the rest of'em

willy
William Revit

Most likely cause we have found is detonation knocking the plugs out especially the rear one, maybe the giveaway is going uphill overtaking implying some welly. Try less advance?



Peter
Peter Burgess Tuning

Over 25 years ago I had my engine rebuilt by a well known local performance engineering company, like you the only thing I didn't reassemble myself were the core plugs. The one by the alternator blew out on the first journey. When he came to replace it he said that there was probably no damage because the paint hadn't burnt off!!
I decided to pull the engine to replace the rear one after that.
c cummins

Thanks all. Been away from home, but should have the fix in place this week, so I will let you know.

Pete. Interesting thought. I am running an VP12 cam but have not noticed any pinging under load. Will recheck the timing.

I did get lucky in that the plug blew out to the outside, so it was easy to grab and remove. I think William hit it correctly. There is a slight dimple in the center but I don't believe it was enough to really push the edges out as they should.

I also see that the shop used some type of silicon sealant around the plug. I have always used JB weld (epoxy paste) for these plugs.

Bruce TD4139 Cunha

Hi Bruce

One often cannot hear higher rpm pinging. We strap the plugs into the block a la MG Factory MGA competition mods.

Peter
Peter Burgess Tuning

That welsh plug looks like someone tried to install it with a large socket, which works on most of the welsh plugs found on U.S. cars, but not on the ones MG used. It wasn't punched in the middle as it should have been and IMHO is what caused it to fail. I installed the ones in my '67 B, back in '93, after a complete rebuild. They're still firmly in place even after twelve years of running with a supercharger. I occasionally encounter pinging while going up a steep hill, but can dial back my timing while under boost, from inside the car with a boost timing retard system fitted to the ignition system. I fitted my welsh plugs with a bit of Permatex Aviation sealer around the perimeter of the plugs. Never had a failed plug or a leak. RAY
rjm RAY

Hi Ray

We have seen the problem enough times on many different B engines to play it safe and strap the plugs, we have been doing this for 12 years or more now. The problem for me is I have to guarantee my work and also do not want the embarrassment of a plug exiting the block. You may be sensitive and able to look after your engine, not all folk are like that and can and do push things with poor engine settings. All I do is pass on the potential problems and solving and preventing we come across. I think we have built around 120 engines since we started strapping them and not lost a plug. We have seen and helped with perhaps 30 occasions when folk have lost plugs. I have seen around 5 engines pop a plug on the rollers and not found a problem with the way they were fitted. It is more common with high CR large bore engines. I think Goldseal may have bored the core plug holes out on all the recon engines to take cup type coreplugs, this also prevents plug failure.

Peter
Peter Burgess Tuning

"Goldseal may have bored the core plug holes out on all the recon engines to take cup type coreplugs"

Interesting, I wondered why mine had cup-type when so many talk of the flat type.
Paul Hunt

Peter B. - pardon my ignorance, but what do you mean by "we strap the plugs into the block..."? - is there a photo somewhere you can refer me to? T.Y. john.
J P Hall

A picture

Peter

Peter Burgess Tuning

Peter, that system is also used by several B race engine builders here in the States. Set it and forget it. RAY
rjm RAY

Right - got it! Thank you. John.
J P Hall

One other fix I came across is on that was discussed on the MG experience web. Seems the MGB uses the same size welch plug as the Dodge Hemi. This unit was designed for the Dodge, but fits perfectly on the MGB.

A bit pricey, but another alternative.

http://www.hothemiheads.com/dodge/main_caps/freeze_plugs_stainless.html




Bruce TD4139 Cunha

Here is a picture of one installed on an MGB

Bruce TD4139 Cunha

Peter. Could you give some details on how you put these in? Drill size, depth, tap used?
Bruce TD4139 Cunha

Hi Bruce 4mm thread M4 x 0.7 (IIRC), 3.3 mm drill. We drill right through, tap carefully without tapping compound as cast iron self lubing. When we fit the cap head screws we seal with a blob of silicon sealant. Use washers between the ally bar and the core plug to put a little pressure on it.

Peter
Peter Burgess Tuning

Got the Betson Bolt lathed out and installed today (Thanks Chris for the information). I did this one out of steel stock on my lathe and went with a 1/2 course bolt.

Put JB weld on the plug and have it tightened in place.

Thanks all for the assistance on this issue.

I think that doing Peter's fix for at least the rear plug, should be a standard part of a rebuild. If not, at least the Betson bolt should be a strongly recommended part to be installed.

Bruce TD4139 Cunha

Bruce, let's hope that this is the last of the adventures that you have to cope with on your freshly rebuilt engine. RAY
rjm RAY

Thanks Ray. You know how it goes with a new engine. Now I have to get to that comfortable place where I have full confidence that all is correct.

Going to the Dixon British show on the 17th, so that should give me a bit more confidence.
Bruce TD4139 Cunha

While I agree that its worth strapping welsh plugs for competition, a welsh plug will not come our if installed correctly which none of the ones shown so far have. A plug when installed properly will be almost flat with no dent in the middle. A plug of this size if flattened on the bench will grow approx ,050" and thats a lot of grip, far more than the bucket type plug. Not trying to be picky but it must be done right. Denis
Denis4

"Not trying to be picky ..."

Being picky, it's 'Welch' plug - http://en.wikipedia.org/wiki/Core_plug#Welch_plug
Paul Hunt

Thanks Paul, cant spell but I can fit "welch" plugs that dont fall out. Denis
Denis4

"Core" blimey!!!
Allan Reeling

Denis4
100% agree
William Revit

This thread was discussed between 28/04/2015 and 18/05/2015

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