Head Gasket blow.

[twolitre]

Just over 1000 miles ago I had the barrels liners on my Triumph 5T replaced and bored back to standard.
Although I had no issues with running in I was getting a little bit unhappy with how it was settling down and with the power compared with my 5TA. Even allowing for it being a heavier machine.
When the head was removed it revealed evidence of "blow by" on the head gasket, though not actually blown. And before anyone asks, it was new and annealed before fitting in case it was old stock. And the head sequentially tightened with a torque wrench to Triumph specs.
Can anyone enlighten me as to how proud the lining tops should be above the gasket face on the cylinders? There seems to be very little "nip" on the gasket before the liners bottom (or should that be top? ) in the head recess.
Careful measurement leaves a gap of 0.013in when the head is placed on the barrels and the solid gasket at 0.039in is only 0.026in thicker than the gap available.
Is that sufficient for a satisfactory seal?
Checking faces as far as possible with an engineer's straight edge reveals no distortion problems.
Incidentally, I was surprised to find the thickness of the old gasket to be exactly the same as the new. Also, I must admit that I had not tried re-tightening the head.
Jim.

[wetdog]

is this the all iron engine ? which year , my 6T all iron is flat at the top of the barrel , am i thinking of the right engine a twin ?

[Rex]

This sound like the ally barrels to me. Are you asking how proud the liner/spigot stands off the block?
 Personally I'd re-anneal the gasket, retorque the head and use it. If the power is down but the gasket hasn't actually blown then you need to look elsewhere for any issues.

[wetdog]

why did they leave a spigot on this engine , ariel do this and you lap the head on but I can see no way of doing this on a twin , is the gasget there just for the oil ways tro the head and barrel , only just starting to find my interest in triumph twins again and I had forgot how well built they are .

[twolitre]

The standard 5T has iron barrels. The (shrunk in) liner stands proud like a spigot and the gasket is designed to locate on the "spigot", keeping all bolt holes aligned perfectly.
Rex, I am trying to find out how far the liner should stand up above the cast barrels top surface where the head gasket sits.
Fairly obviously I did not have the equipment to fit new liners etc. and left it to an engineering firm (now disappeared!). I trusted them to fit the liners properly and never took any measurements myself.
I feel I need to hone one bore slightly,before re-building,  because one piston is really an unhealthily tight fit at the top of its bore, where it needs a push (even with no rings) to help it all the way through.
Of course, with ring gapping being checked lower in the bore and pistons fitted at the cylinder barrel bottom, that problem escaped notice. Amazingly there is no sign of siezure pick-up.
I wonder why that engineering firm has "disappeared"?

And Wetdog, the spigot (if that is the right term) or collar might be a better term seems to serve the same purpose as dowels might to locate the gasket. In fact they could be considered as huge, hollow dowels.
There are no internal oilways or other passageways on a Triumph pre-unit twin cylinders area. Oil to the rockers being delivered via external pipes tapped off the engine to oil tank return oil line.
Jim.

[wetdog]

how does the oil return from the head ? is yours fitted with the external returns to the lower section of the push rod tube ?

[twolitre]

how does the oil return from the head ? is yours fitted with the external returns to the lower section of the push rod tube ?

Yes.

[wetdog]

what year is this motor , why do they have a spigot/collor , on bsa the head gasget is the foil type to get the correct nip (2 to 4 tho i think ) but with a solid copper gasget ? if you reface the head the seal maybe on the spi/col only and the gasget not doing much , as for gasget location the head bolts do this on the 6T

[wetdog]

in april 1956 the spigot hieght was reduced from 3/16 to 1/8  ref service sheet 153 Triumph,

[twolitre]

Thank you Wetdog. You have answered my question.
The lip on my pre-1956 (1954actually) is 0.165 in. as posted earlier. i.e. 40 thou more than the service sheet quotes. Though not the 0.3125 in. original height quoted.
It seems in line with my suspicion. I will reduce it to 0.250 in tomorrow and continue the re-build.
I did feel that the lip was too high to allow all the tightening torque to be transmitted to the gasket.
Jim.

[wetdog]

this must be amost imposible to get right , to high and hot spot , to low on spi/col (so head gasget useless) also if the liner is not nipped it may move , i will look out for this , i wonder if triumph changed this as performance incressed , to show any sighns of blowing the spigot is not seated in the head . i have always known "blow by" as a ring problem but this maybe local to the midlands

look at the numbers you are stating closer before altering

[twolitre]

I used the term "blow by" because the gasket was not blown, but evidence a gas escaping past it was shown by a darker part of the solid gasket surface. That led to a suspicion that the "nip" on the gasket was insufficient. Piston blow by is NOT a problem after 1000 miles.
Since the gasket is outside the protruding SHRINK FIT liner all that really matters is that the liner protrusion is not sufficient to prevent gasket compression. Which is the reason for trying to determine the recommended dimension.
I don't think that a small gap will have much effect except to perhaps lower the compression ratio by an infinitesimal amount. I think it is important that there IS a small gap (measuring "thous") to ensure gasket sealing. I don't see that a "hot spot" is at all likely.
Jim.

[wetdog]

you have never worked on BSAs im guessing , dark areas on a solid copper gasget indicates a blown head gasget (caused by gass escaping , at least in the midlands , i have never seen a solid copper gasget blown apart , have you ) the small gap you refere to should be avoided it will cause a hot spot , piston blow by can occur very early in a rebuilt or new motor (if over reved normally) and is usually accompanyed with pick up ...........  its only my experience you dont have to take it . look at your figures before altering anything , or better still stick to soldering nipples , the all iron engine is very forgiving and almost idiot prof

[twolitre]

Oh my God!
I have tried to explain you the solid copper gasket HAS a dark mark on it indicating gas leakage!
Hence suspecting lack of pressure on the gasket caused by too much liner sticking out above the gaslet face on the barrels. I was only trying to find out whether the company which re-lined the barrels and bored them had wrongly positioned the liner.
I never mentioned nor believed that piston or ring leakage is a problem on this engine.
THe mass of the liner sticking up is too large for a hot spot, especially as it is adjacent and touching the solid copper gasket. Copper having an extremely high heat conductivity. 
Thank you for the information you have supplied. I will take it from there. I would rather accept the Triumph service sheet recommendation than your speculation.
Jim.

[john.k]

You re right in saying the liner should have the correct protrusion,but never the less,by replacing the head without the gasket,and measuring the gap( if any ) between will give an indication of the problem.Correct sized and parallel cylinders are certainly necessary to avoid trouble.If you don't have an internal mike,you can check a cylinder bore by using a piston ring,measuring the gap with feeler gauges.The gap should be constant throughout,and the same in both bores,assuming the pistons are the same size.Ive had a lot of success sealing leaking head gaskets with a thin smear of silastic.I know Ill be accused of heresy for saying this, but I have been a engine reconditioner for forty years.Regards John........   PS solid copper gaskets must be very soft to seal at all,and should have some sealant applied.