Author Topic: Blown top gasket  (Read 2240 times)

Offline madsens

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Blown top gasket
« on: 20.09. 2012 15:22 »
Hello Gents

On my way back from BSA IR2012 my A-10 1962 suddenly started to sound like an old tractor, and lost power. I stopped and went home by train  *sad2*

Got the old girl home by SOS international (long story, maybe later) and have finally got the time to pull the top of.
I did the same excercise in January, and put on a new annealed copper gasket, but alas, take a look at my pictures... and some comments please...

Is this caused by not retightening, bad annealing or any other ideas? I don't want to do this every winter if possible to do it in a better way - so all advise is very welcome ;)

regards
Joergen




BSA A10GF 1962
Denmark

Online a10gf

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Re: Blown top gasket
« Reply #1 on: 20.09. 2012 16:32 »
Sorry for your trouble! Just to cut down on possible causes, apart from the annealing and possible need for retightening, were the bolts originally tightened in the (generally accepted) correct sequence, gradually increasing the torque to the (generally accepted) strength? & are the cylinder & top surfaces absolutely flat?

btw, thanks for using the click-thumbnails!

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Online RichardL

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Re: Blown top gasket
« Reply #2 on: 20.09. 2012 17:40 »
In addition to the "best practices" issues Erling mentions, it appears to me that the gasket you have there was not cut properly. Look at the meat between bolt hole and cylinder on the opposite side. The one toward the front has a very narrow bridge versus the one toward the rear. Where your blowout occured likely had this same lack of symmetry. Look at the carbon print around the cylinder on the top of the barrel. You can see that the gasket was never concentric with the cylinder. We would probably all like to know where you got that gasket so we don't buy similar. If it was from a reputable source, I suggest you send them these photos (and, maybe, my comments) and see what they have to say.

Richard L.
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Offline Goldy

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Re: Blown top gasket
« Reply #3 on: 20.09. 2012 21:03 »
I would start by checking that both  the barrel and head faces are perfectly flat. if you don't have a surface plate take them to an Enginering machine shop. It will never seal of there is a problem with either face.
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Offline madsens

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Re: Blown top gasket
« Reply #4 on: 20.09. 2012 21:13 »
Erling and Goldy

Flatness of top and cylinder will be my first point of check - I did tighten up according to manual, but never got around to checking and retightening ( you know, the sad excuse not wanting to lift the rockerhouse again...)

Manosound
Gasket was bought from Draganfly together with all new bolts for head and rockerbox.

I am afraid it all boils down to laziness, I should have  retightened, and checked for flatness. I was just wondering about the "big" blowout on the gasket, do they always end up looking like that?

regards
Joergen
BSA A10GF 1962
Denmark

Online RichardL

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Re: Blown top gasket
« Reply #5 on: 20.09. 2012 21:15 »
Goldy,

Maybe I'm missing something (well, I'm definately missing something, just don't know what) but it would seem to me that un-flat surfaces would lead to leakage and that that would prevent enough pressure from building up to blow out the gasket. On the other hand, like you and Erling are implying, how can it slip away at all if it is uniformly squeezed? I might slowly be coming around to your thinking, but I still don't like the bad fit.

Richard L.
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Online chaterlea25

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Re: Blown top gasket
« Reply #6 on: 20.09. 2012 21:22 »
Hi Joergen,
I agree with all the other comments + the following
The head studs can stretch and loose there elasticity after lots of uses !!! On a lot of modern engines head bolts are one time only tightening
Make sure the threads in the barrels are clean and that the bolts thread in smoothly
A lot of tightening torque can get lost if the head bolt faces (on the head) are deformed
I spotface the head bolt landing areas flat, and fit thick washers (turned from bar) under the bolt heads
Some antisieze on the threads and under the bolt heads helps

When You annealed the gasket did you scrub the scale off ?
Use a scotch pad or similar to remove the scale and bring the gasket back to copper colour

HTH
John
1961 Super Rocket
1963 RGS (ongoing)

Online bsa-bill

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Re: Blown top gasket
« Reply #7 on: 20.09. 2012 21:53 »
Apart from all the above viable reasons is it a trick of light or does the timing side piston look like it's run hotter than it's neighbour?
All the best - Bill
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Online Triton Thrasher

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Re: Blown top gasket
« Reply #8 on: 20.09. 2012 22:08 »
You have to retorque cylinder heads, several times, until a small anticlockwise twist and application of the torque wrench ( or well-judged arm) gives no advance in fastener position.

Offline benet

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Re: Blown top gasket
« Reply #9 on: 21.09. 2012 08:13 »
Hi Joergen, I have just had a similiar experience as you. I had the head planed and put in a solid copper gasket, heated the head with a heat gun after torquing and then retightened. All was good for about 20klm and then the gasket leaked. I removed the head again and put a straight edge on the barrels and measured a warp of .007" with my feeler gauge. The question i then asked myself was how does this occur and why. I understand that castings sometimes have stresses that relieve themselves over time, also that the original factory machining on the face's was quite crude by todays standards. I also think that the laminated gaskets ,which i had on previously, are a little more compressable and the solid copper one's a little less forgiving of an undulating surface. I would be checking the flatness of both the head and the barrels. Ben     

Offline madsens

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Re: Blown top gasket
« Reply #10 on: 21.09. 2012 08:44 »
BSA-Bill
I was also wondering about the different surfaces on pistons - but I recon it might be due to the timing side blowing compresion out until my sudden stop when the blowout occured - does that make any sence? I mean, when I hit the stop button the primary side was still under compression, but the timing side was not due to blowout... and that's why the timing side looks "dryer".

I will be doing the retorque stepwise as recommended, and leave to settle a bit in between.

Benet
I actually did app. 1500 km - and some of it quite fast for the old girl - would a laminated gasket maybe be more sensible or is that a new can of worms opened ?

Anyway - thanks for all suggestions and replies, I think I'm just about ready to start putting her together again  *smiley4*

regards
Joergen
 
BSA A10GF 1962
Denmark

Offline Flatboy 1950

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Re: Blown top gasket
« Reply #11 on: 21.09. 2012 09:52 »
Many years ago , when the world was still flat , there was an article in " Motorcycle Mechanics " about a racing sidecar outfit powered by an A65 engine.Blowing head gaskets was the problem & he solved it when he noticed that his cylinder top had been ground to a mirror finish. He rubbed it down on a surface plate with much coarser grit paper on it ..... the coarser finish gave the gasket something to "grip " on .... problem solved !!

Cheers , Flatboy.

Offline duTch

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Re: Blown top gasket
« Reply #12 on: 21.09. 2012 10:03 »

 So if we just flatten the planet back out again ALL the problems will be solved?? I'll go get the shovel *smile*
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Offline JulianM

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Re: Blown top gasket
« Reply #13 on: 21.09. 2012 11:45 »
Many years ago , when the world was still flat , there was an article in " Motorcycle Mechanics " about a racing sidecar outfit powered by an A65 engine.Blowing head gaskets was the problem & he solved it when he noticed that his cylinder top had been ground to a mirror finish. He rubbed it down on a surface plate with much coarser grit paper on it ..... the coarser finish gave the gasket something to "grip " on .... problem solved !!

Cheers , Flatboy.

To an extent I agree here!
After all you wouldn't use a pair of pliers with ground flat surfaces and expect them to work better than teeth would we?
Same for clamping gaskets then as long as we are within reason and don't introduce gas leakage paths.

Julian

PS.
Gasket pictured is typical of "loss of clamping force" failure.
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Re: Blown top gasket
« Reply #14 on: 21.09. 2012 22:11 »
H heated the head with a heat gun after torquing and then retightened.

Bad practice. You heat the engine by riding it.