Head blanking plug...

[Boink]

Your sympathies gladly accepted. My 1275 first developed a sputtering at high speed (or under serious load), then got a bit worse. After fooling with timing and carbs (endlessly), I finally pulled the head. Ta-da... the coolant blanking plug (that I've read Hap posting about) was recessed about a millimeter on my 12G1316 head! This is between #2 and #3, and is not quite the same as blowing a gasket between cylinders, but similar. The plug spans right across the fire-rings on the Payen gasket. Dang. Going to replace the head. :frown: The rest of the messy head is merely from me being careless removing it. It's funny... the car would run OK under a light to modest load.
I guess one can have that plug drilled and re-plugged, but this might be a good time to go to a very clean 12G940 that is ready to go with non-lead seats (and fully tested).

 

[RaserX]

Not familiar with his post. Is that something that happens over time with vibration? Or a misnset plug from the factory? It looks like it would cause bleed over predetonation to me. Which makes sense with what you discribe. The hard the load the worse the bleed over. Is that something I need to worry about on my all stock 1973 - 1275?

 

[Boink]

Not sure, but there are several posts out there (here and in the MG Experience forums). Evidently, the plug can work its way in, and, yes, strang detonations will happen. I don't think it's a super common condition, but something to keep on the look-out for. In serious cases, there will be a bad leak of coolant (but at least that will be an easy diagnosis). I think mine was possibly assembled without the PO knowing what was there (because it was there from the get-go, only it got progressively worse over several hundred miles).

Found 2 posts (that include tips from Hap on this):

https://www.mgexperience.net/phorum/read.php?1,2096990,page=1

"Ok with you doing retorque and seeming to get a seal I doubt this is your issue, but it's worth mentioning and something alot of folks don't know about. On the A series heads, for sure the 1275 head, there is a brass plug that barely intersects the fire rings on #2 and #3 cylinders that sometimes can with heat countersink itself into the head casting, if it does this, then you will get water directly into cylinder #2 and #3, I guess in the last 15 years I repaired atleast 3 1275 heads that the plug drifted upward, or was completely gone."

In my case, there was no water issue but a cross-fire between cylinders.

https://www.mgexperience.net/phorum/read.php?3,2112483,page=1

"This has probably nothing to do with your issue, but I thought I would share with the group. When rebuilding a A series head, one should pay close attention to brass plug in the head surface where it bolts to the block, there is brass plug there, it intersects #2 and #3 fire rings of the head gasket, and it known to counter sink, even fall into the head, if it is not there, then it will dump water directly in to #2 and/or #3 cylinders, and most folks would never recognize it's absense if not there, if it is countersunk from the deck surface of the head it could cause a head gasket failure. I'm doing a 1275 head rebuild right now and the owner ask me to not do a resurface if the head was straight, howeverr I found a issue that required resurfacing, I first check the combustion chambers to confrim they were stock, they were, but this brass plus was countersank about .007' in the head, so we did a .010" resurface to elminate the counter sunk plug issue. I've had tow cases where these plugs were totlay missing and in that case developed a nice repair for this, I thread that hole in the head deck for NPT plug, I use a brass square peg NPT plug, Install the brass NPT plug with JBweld on the treads for seal, cut most of the proud section of the plug off with a cut off wheel, then resurface the head until it cleans up where the plug was install, in this case truely better than new, because the threaded plug is never going to move."

Again, in my case, the plug settled in about a 1/16th of an inch, and the gasket dimpled in right there but with no coolant issues (and this is right across the fire-rings). Not good.

 

[58Custom]

I have posted about this as well. I had this problem but I had a good used head that I cleaned, lapped, resealed and used. The brass or bronze plug expands at a different rate than the cast iron and over time repeated heating & cooling cycles will push the plug into the head reducing gasket sealing in the area until gasket failure occurs. Gasket replacement is a temporary fix. This plug should always be replaced when the head is rebuilt and it should be flush or very, very close to flush if a used, non-rebuilt head is to be reused.

This recession issue is accelerated when the engine runs hot. The more heat, the more expansion resulting in more recession.

We would be wise to help all members who seek engine rebuild info by mentioning this issue in any engine rebuild thread.

 

[Boink]

I have posted about this as well. I had this problem but I had a good used head that I cleaned, lapped, resealed and used. The brass or bronze plug expands at a different rate than the cast iron and over time repeated heating & cooling cycles will push the plug into the head reducing gasket sealing in the area until gasket failure occurs. Gasket replacement is a temporary fix. This plug should always be replaced when the head is rebuilt and it should be flush or very, very close to flush if a used, non-rebuilt head is to be reused.

This recession issue is accelerated when the engine runs hot. The more heat, the more expansion resulting in more recession.

We would be wise to help all members who seek engine rebuild info by mentioning this issue in any engine rebuild thread.

Excellent points Tom (and I'm surprised how many people don't know about it). If I keep the head, I will insure that the plug is done over and done well. Thanks.

 

[Boink]

<mutter mutter>
Stripped the head apart... found sticky exhaust valves (seriously so) in #2 and #3, which makes me wonder if that area got really hot and THAT caused that blanking plug to recess. Either way, those valves were darn sticky. Furthermore, my straightedge reveals some warpage, and given this head has been seriously skimmed before, it's toast (given I have about 5 things wrong with it).

On a positive note, the valves, springs and even guides look very good. Perhaps the head was poor to begin with and when the trouble-some valve guides were stuck in there with some distortion. I could fix it all, but the prior skimming and was just too extensive. I see I can get a nicely preparted 12G940 with APTFast, which I might consider (hardened seats, fluxed and blasted). Decisions...............

 

[Hap Waldrop]

I can repair the head if you decide to go that route

 

[Hap Waldrop]

I can repair the head if you decide to go that route

 

[DRH]

You guys got me thinking......My 1275 runs fine up to about 3400-4000 rpm say on a long trip. Then, while still running down the road, it starts missing. It sounds like it is running out of gas--like maybe only two cylinders are hitting. It sounds like it is starving out. So, I back off the throttle just a little and it starts running good again. I've always thought it was not enough fuel, maybe the float bowls were dry and I've meant to stop to check that but haven't. I did add a 5/16 fuel line but I'm not sure it helps out. Boink, what kind of 'missing' did you have? Did it 'go away' as you lowered the rpm? I'm not getting water in the oil nor loosing any water anywhere I can tell.

 

[Boink]

You guys got me thinking......My 1275 runs fine up to about 3400-4000 rpm say on a long trip. Then, while still running down the road, it starts missing. It sounds like it is running out of gas--like maybe only two cylinders are hitting. It sounds like it is starving out. So, I back off the throttle just a little and it starts running good again. I've always thought it was not enough fuel, maybe the float bowls were dry and I've meant to stop to check that but haven't. I did add a 5/16 fuel line but I'm not sure it helps out. Boink, what kind of 'missing' did you have? Did it 'go away' as you lowered the rpm? I'm not getting water in the oil nor loosing any water anywhere I can tell.

This is nearly identical to my experience (and that of a friend I was helping). The "miss" was a sputtering with loss of power that would go away if I let up. I did the float-bowls several times (first thinking flooding because I did have some flooding due to Grose-jets and then starvation which I could sort of similate by briefly putting a rag over one carb intake), but no luck with carbs. I seem to recall a smokey bit of exhaust (not blue) during the sputtering. [As an aside, the timing is excellent and I have a Pertonix distributor.] The interesting part is that I had NO coolant-related issues... just the eventual discovery of the recession of that coolant blanking plug (as in my photo above when I pulled the head). To my way of thinking, this would have created a similar deal to having the gasket "sort of" go out between #2 and #3. My theory is/was that only under serious load would it fully communicate through that blanking area of the gasket.

I also think I had sticky exhaust valve guides in the same area... because if they got hot, they may have temporary gripped the valve-stems there. I suppose I'll never know what the cause was (because those guides will be fixed one way or another). Regardless, IF that head blanking plug gets worse and worse, two things could/will happen: 1) gasket failure between #2 and #3 that won't go away by letting up much, and 2) the plug could even fully recess and THEN you'll get coolant-related issues.

My take.

P.S. - forgot to add that I would certainly eliminate fuel starvation by checking the flow of fuel at the line to the carbs (and through the filter). I'd also be sure you are fully filtering fuel and that nothing has gotten stuck in the supply to the gets. You could check the fuel level in the floats just to be sure that the carbs aren't briefly getting ahead of the floats. I did all this but to no avail.

 

[DRH]

Mark, looks to be the same as me. Please let me know after you get the head back on and if it solves the problem. I thought I could drive mine until It begins to sputter then shut off the ignition and coast to a stop and then check the float bowls. If they are empty, then I might have found the problem--fuel starvation. Don

 

[Boink]

Mark, looks to be the same as me. Please let me know after you get the head back on and if it solves the problem. I thought I could drive mine until It begins to sputter then shut off the ignition and coast to a stop and then check the float bowls. If they are empty, then I might have found the problem--fuel starvation. Don

Will do, Don. I hope to get the car all back together soon though that could be 3 weeks (as I'm just waiting on making a decision about the head - this week). I did as you, would let the car sputter and quickly kill it and look to see if there was: a) float bowl flooding (though you will likely smell that as gas will come out the overflow), or b) if there was puddling in the carb at the jet. No luck in my case. I finally decided that starving was harder to explain unless there was crude in the bottom of the bowl or in the jet-tube.

What I found striking was that with a rag briefly over the throat of a carb (with air cleaner off), it similated the problem, thus the thought the carbs were starving. However, I have renewed HS2s and they are perfect... and, as I previously reported, I've verified that fuel to the carbs is clean and with a good flow-rate from the pump (so it's not the tank - and, yes, I tried the gas-cap off test, no luck).

Obviously, when I finally pulled the head, all seemed clear... and that oddly felt good. :laugh: So, the only thing I will be able to confirm for you is that it WAS head-related (because it might have been a combination of sticky valves and the blanking plug); but either way with that, one pulls the head and finds out. It's really not a difficult thing to do (though I'd not done something like that for 15 years and was rather chuffed to have just jumped in - and as a geezer in training it only took 2 hours, which is about an hour too long). I'm also going to really verify TDC while the head is off. :laugh:

 

[nomad]

If the car was cutting out at speed I would think that if you hit the clutch and cut ignition at the same time then check to see if your carb float bowls are full that would give you a way to eliminate fuel starvation as the cause.

Just a suggestion.

Kurt.

 

[Bayless]

Mark's suggestion of crud in the jet or passage is still a possible cause. It really wouldn't take much varnish or other restriction to limit the fuel flow under high demand, even though the bowl is full.

 

[DRH]

Well, for the time being .....I'm going to let the sputter problem be a hillbilly governor. It's pretty hot here now and cooler weather will force action I'm sure. Anyway, I never liked to rev an engine too high and 4000 is about enough for me. Thanks Boink....I'm still curious about your solution...when it comes.

 

[Boink]

Spoke with Hap today. What a great guy... very knowledgeable (as I think we all know) and full of ideas. And all this with the fact that he won't be doing my head. Trouble is that he's a bit backed up and I need the head by the time of the Portland ABFM (in just 3 weeks). My local shop can do a 2-3 day turn around for a complete valve/head rebuild (though for more money). Hap generously gave me lots of advice!

So, I should know if my problem is solved (though I'll never fully know if it was the plug or center exhaust valves sticking). They're going to thoroughly clean/blast (and prime) it, re-plug all the brass plugs (4?), skim the head, new seats, new bronze guides and re-use my valves/springs/keepers. I'll only have to paint it (with some paint I'll have to acquire from Moss).