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Head Gasket Impression in Head #758888 12/11/18 9:42 am
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perfect.tommy Offline OP
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Every time I have removed my head, there is an impression left from the head gasket just in front of the centre bolt. It is usually about .0015" deep and that is the only place on the head where the gasket has left an imprint. Also suspect, is the 5/16 centre hole is not completely round anymore, wallowed ever so slightly laterally.

My head gaskets are annealed "dead soft" gaskets from MAP. Correct torque setting of 15ft/pounds is used on the centre bolt. Bike is a 1970TR6.

What would cause this issue?



1970 Triumph Tiger
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Re: Head Gasket Impression in Head [Re: perfect.tommy] #758937 12/11/18 6:18 pm
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perfect.tommy Offline OP
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My immediate thoughts are that the head bolt does not stretch enough to accommodate the thermal expansion of the aluminum head in this area: one of the hottest areas of the motor.

However, it may be an error in my workshop practices. I know John and RF must have seen this in their years, and hoping they can correct my oversights. The head is not warped, just a slight impression from the head gasket, and only in front of that centre bolt.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #758947 12/11/18 7:26 pm
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How often do you remove the head and for what reason?


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Head Gasket Impression in Head [Re: perfect.tommy] #758997 12/12/18 3:31 am
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tridentt150v Offline
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Can you vouch for the deck of the barrels? Have you measured them with a straight edge...just to rule it out?

Re: Head Gasket Impression in Head [Re: tridentt150v] #759000 12/12/18 3:45 am
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perfect.tommy Offline OP
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Barrel deck has been surfaced. They originally had a low spot brand new "out of the box".


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759003 12/12/18 4:34 am
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tridentt150v Offline
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OK, its not sitting hard on the PRT and when you tighten it you are pulling the head metal down/distorting it are you?

Re: Head Gasket Impression in Head [Re: tridentt150v] #759004 12/12/18 4:56 am
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perfect.tommy Offline OP
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As I said earlier the head is not warped, there is an indent from the head gasket. But since you are curious, crush height on the pushrod tubes is set between .025"-.030".


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759007 12/12/18 8:18 am
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tridentt150v Offline
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yeah and I should have thought about it, its an impression, not a raised bit.

Re: Head Gasket Impression in Head [Re: perfect.tommy] #759059 12/12/18 5:01 pm
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I know you said they're MAP gaskets. Sort of rules out a bad pattern on the gasket. Have you triple-double-checked the head surface for signs of warping?

Is there a performance issue from this .0015" indent (exhaust gases/oil blow-by, compression loss/leak)? If not, perhaps this is a common problem on 9-bolt heads and no one else has noticed it? Or at the very least, a "problem" you can live with?

Perhaps converting to 10-bolt head would solve the problem. But what, exactly, is the problem? Just grasping at straws here.....

Cheers,

Steve



'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
Re: Head Gasket Impression in Head [Re: JubeePrince] #759075 12/12/18 6:31 pm
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perfect.tommy Offline OP
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I meticulously check the head with a Starrett straight edge whenever the head is off. The impression is significant enough to have me resurface the head. .002" variance is definitely enough to cause a leak, so i don't take chances.

In previous years i made the mistake of taking the head to the machine shop to have this done. First time they walked it over to an industrial belt sander. I didn't like that. The second time i requested they do it on the Bridgeport with .005" off max. It came back with .020" off and an invoice for $100. I now do all re-surfacing myself at my own shop with some sandpaper on a surface plate. This allows more control and as little metal removal as possible. Just the high spots.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759096 12/12/18 8:56 pm
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btour Offline
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It is not a good idea to do that. It changes too many things. If the head is warped, it can be straightened.


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: Head Gasket Impression in Head [Re: btour] #759097 12/12/18 9:02 pm
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perfect.tommy Offline OP
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Hey gang,

Please read my post before you chime in... the head is NOT warped.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759108 12/12/18 11:01 pm
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Possibly the barbarian with the belt sander did the initial damage in that area, and a 20 thou skim was the minimum to retrieve a gasket surface around the cylinders?
Just use it, setting pushrod tube crush accordingly is all I'd suggest.
Alternatively, you could keep skimming the head until it is just a gasket itself!

Re: Head Gasket Impression in Head [Re: perfect.tommy] #759114 12/12/18 11:27 pm
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perfect.tommy Offline OP
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I don't think you are understanding the problem. And the industrial belt sander incident happened with another head anyway.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759117 12/12/18 11:55 pm
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Triless Offline
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Well, trying to understand this, my thoughts are that the purpose of a gasket is to accomodate very minor imperfections. This is why copper gaskets are annealed to soften them. Wether a copper gasket is "pre annealed " or not, I always anneal them myself, anyway !
If it is only an impression being evident, with no sign of leakage or blowby, I think you may be worrying un necessarily !

Re: Head Gasket Impression in Head [Re: Triless] #759130 12/13/18 1:44 am
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Originally Posted by Triless
If it is only an impression being evident, with no sign of leakage or blowby, I think you may be worrying un necessarily !


+1

Steve


'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
Re: Head Gasket Impression in Head [Re: NickL] #759143 12/13/18 8:10 am
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perfect.tommy Offline OP
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Some of us are interested in a higher level of workmanship than yourself. I don't think you should be so quick to impose your lower standards on others. But thanks for your opinion all the same.

The head has leaked between cylinders during some previous assemblies. So this is a valid point of enquiry. I don't think that developing a dent in the jointing surface is part of a normal engine's service.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759144 12/13/18 8:18 am
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Triless Offline
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Because Nick has a laid back attitude to life, that does not reflect on his high standards of mechanical work and his immense knowledge of such. He is a bloke I really pay attention to what he says. Don't confuse his humour with real ability .

Re: Head Gasket Impression in Head [Re: perfect.tommy] #759161 12/13/18 12:52 pm
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Exactly as Nick says, old turds.Many here would drop a turd if they saw how I work on these things using impact wrenches and "that's good enough" LOL...If the head surface is flat and smooth, the cylinder deck the same...and combustion blows past the head gasket, you have a problem..Otherwise there is no problem..If you took off the head multiple times? because of a combustion or oil leaks and found the indent despite have flat surfaces, you have to look closely around the center bolt area...Metal is moving when the bolt is tightened..And the center bolt torque is not extreme so there may be a :thin" spot on the head that distorts when tightened down..


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Head Gasket Impression in Head [Re: NickL] #759162 12/13/18 12:53 pm
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Yes, dab hand with a knockchrometer ! Silly bugger !

Re: Head Gasket Impression in Head [Re: perfect.tommy] #759171 12/13/18 2:22 pm
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I am having a hard time following just what was done to this head and this barrel. This head was done in a bridgeport with 20 thou off? And then you have taken sandpaper to it? These barrels were resurfaced? How do you know where you are at now?

Has a stud been put in wrong way round and pulled some metal up round it?

Do a search here on John Healy and head straightening. It is much better than resurfacing. I can understand why you feel attacked. I have found that when I do something that I think is best, and it turns out to be wrong, I feel terrible and defensive.


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: Head Gasket Impression in Head [Re: btour] #759190 12/13/18 7:19 pm
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perfect.tommy Offline OP
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I appreciate the recommendation to John's posts. I have read most of John's posts on this forum, and have printed every piece of literature he has posted on Vintage Bike and have them in a binder in my shop next to my workshop manual. I was hoping he would chime in here on the subject, or RF Whatley.

I am familiar with straightening warped heads, as this is common practice on heads with overhead cams (I have read John's posts on the subject as well). It would be relevant here if the head had been bowed to due to excessive pushrod crush, or uneven bolt torque. But what we are talking about here is an indent the size of a nickel in an area where these heads are susceptible to leaking. Especially when you move up to a 750cc barrel. There is not a lot of gasket material around that 5/16 bolt!

Just to set things straight, I feel neither terrible nor defensive. I am totally open to the possibility that I have done something wrong, which is why I have posted on the forum to begin with. Unfortunately, no one here has been able to provide an explanation. Instead the recommendations have been to ignore the problem, because these engines can't be rebuilt with some amount of perfection as the end goal. And I simply do not subscribe to that mentality. It is precisely that mentality (and workshop practices) that have given these bikes such a bad reputation to begin with.

As John Healy has pointed out time and time again, these bikes work quite well when they are maintained properly. Most of the problems he has encountered have been due to poor repairs by other mechanics. And John, I hope I am not speaking out of turn here by loosely quoting you.


1970 Triumph Tiger
Re: Head Gasket Impression in Head [Re: perfect.tommy] #759203 12/13/18 9:25 pm
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I suppose what would be most helpful to us would be a picture showing what we are trying to visualise from a few words.

Also a bit of a goose-chase from your irrelevant info about other heads, not making it clear that it wasn't this head being described.

I would still doubt that a 1.5 thou recessed small area as I imagine from your description would be a problem for an annealed gasket to accommodate.

However, if you are determined to eliminate it, I can only see 2 ways of doing it. Assuming the rest of the surface is flat:
Build with weld and re-surface, or a fine skim biased toward the front.

As to how it happened in the first place:

Can you remember the first time this impression was present? Had you had the head off on a previous occasion when the impression wasn't present? ie. do you know that it has appeared during your ownership?

It could be a defective casting that just barely made the grade originally when surfaced.

There could be a region of porosity in that area of the casting rendering it more impressionable.(you mentioned the bolt boss was deformed).

Unlikely, clutching straws, a hard,unsoftened patch of the gasket at some time ago? which acted as an incompressible mold.

A small long term seep of combustion gas eroding the alloy.

Overheating combined with porosity might do it?

Just free ideas

Re: Head Gasket Impression in Head [Re: koan58] #759207 12/13/18 9:53 pm
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I have seen pictures of this head on another forum ( which for some reason unknown to me I can no longer post on ), and it seems that the offending portion is still just touching, no signs of blowby or anything from when it was all tensioned down. Personally, I think Tommy is looking for total perfection, and there is nothing wrong with that ! But I believe he is worrying un necessarily. The only thing I can think of, having seen the pics, is that the top surface of the cylinder block may be slightly out of parallel to the bottom gasket face . In other words, a very slight taper towards the front so miniscule that it could be difficult to discern with straight edges. In other words, bugger all , but only the blueing picks it up !

Re: Head Gasket Impression in Head [Re: perfect.tommy] #759208 12/13/18 9:55 pm
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Here's a story the might shed some light on the subject. Once in the early 70's a friend's 750 Atlas developed cracks right above the cylinder flange. We took it to a very capable specialist. He put a strobe light on the motor while it was running and I was flat stunned by what I saw. I had been operating on the assumption that these motors were a rigid assembly, but boy, was I wrong. We could see the entire head and cylinder grow and shrink, once slowed down by the strobe under the influences of those barely contained explosions going on inside the motor. It basically looked like Jello wobbling around....

Adding to that, I once had a customer who complained that his head bolts wouldn't stay tight. I assumed correctly that he was tightening them too tight and had stretched the bolts beyond their elasticity.
New bolts and much less torque solved his problem.

What helps to understand this is that there is a difference in the expansion rates and amounts of steel, vs. aluminum. Aluminum heats up quicker and expands more than steel or iron. Once everything has achieved proper running temperature, the motor settles down. Before that, things are moving around quite a bit. There is much more movement between parts in this type of motor than you might expect.

And fret marks like you describe here would be a result of that movement.

It's basic physics. Scratching your head and musing on the design can help with understanding of what is actually happening inside that motor.

Cheers,
Bill



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