The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: zitman on 07.11. 2009 19:02
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hI ALL,
Well the A7 head gasket has gone again (third time). This leads me to ask 2 questions:
1 - Can anyone recommend someone to skim the head, preferably someone in the west midlands?
2 - Composite or copper head gasket? Its composite at the moment.
Cheers
Z
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Its composite at the moment.
Thats one thing thats not helping! You cant beat good old solid copper, and properly anealed as well.
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lj is right copper is best
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The plot thickens!
Got the head off this afternoon and it was obvious that the gasket had been blowing. It was also noticeable that most of the rocker box bolts were less than tight!
Inspection of the head revealed a burnt exhaust valve:
(https://www.a7a10.net/forum/proxy.php?request=http%3A%2F%2Fwww.mavericksolutions.co.uk%2FHostedImages%2FHeadWithBurntValve.jpg&hash=9ff70e861c4cda11438ca555869931b8b0cb81c7)
I have ordered 2 new valves this afternoon and will sort a new copper head gasket tomorrow.
Any thoughts as to what may be the cause of the valve damage? Is it likely to be linked to the head gasket problem? I had noticed the bike was a bit down on power of late (which is no surprise given the state of the valve). The colour of the plug seems OK to me which would imply the mixture was OK. Could it be timing?
All opinions appreciated.
Ta
Z
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colour looks good so I agree with you about the mix and if the timing was out you surley would have noticed it kicking back on start up or overheating with normal running and again one or the other would show up on colour of the combustion chamber, me I would say poor quality valves or to many miles on unleaded with out any additives
Richard
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Hi Z,
Hmmmmm, its a long time since I saw the mice attack a valve like that *eek* *eek*
It may have happened because of lack of valve clearance due to valve/seat recession
caused by the already mentioned unleaded fuel!
Also when the head gasket is leaking on that side it would cause overheating *sad2*
Looking at the inlet valve side in the photo you can see the head of the valve is recessed in the seat!
It looks to me from the photo that you should completely recondition the head, you will also have to
ensure the head is flat before replacing it on the bike, there appears to be damage between the combustion chamber and one of the holding down boltholes *sad2*
HTH
John O R
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Your photo shows some anaomalies.
For instance, it seems the head gasket is displaced; there is little gasketed surface between the the combustion chamber and the pushrod tunnel; likewise, between the chamber and the #6 headbolt (Service sheet No. 204), or bolt #4, Service sheet No.205. Are you using the A7 head gasket or one for an A10?
IMO, you can't do better than the laminated head gasket, unless you're going racing - in which case everything else better be in super condition. I've used both in both situations. Roland Pike stated that the head gasket needs to be doweled to keep it in place; I did this by placing a 1/8 inch roll pin at the center of the cylinder bores, and drilling the gasket accordingly. Correct head bolt torque is essential; I use 40 lb ft with the laminated gasket, and a smear of Permatex Ultra around the oil drain holes. A flat head head surface is likewise essential.
As for the burnt valve - looks burnt in two places. What about the other valves? Per John O R, recession of the seats without a corresponding adjustment to rocker clearance is indicated.
David
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in my defence I was assuming all settings, i.e. tappetts were correct but upon looking again I can see the valves are recessed a bit, BUT mine have been pocketed for some time and I am aware of that (which is why I took the alloy head from the plunger A10 so I could use it on the S/R but it turned out to be an A7ss head)and the last time I checked the head all the valves were intact, so surley the valves being pocketed will not be the main cause?
Richard
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Hi there, It seems you have a bit of work on, but kaput valves themselves would not make the gasket blow.Are the head bolts pulling the head down firmly or,(as in many instances on old engines) are the bottoms of the threaded holes in the barrel full of an accumulation of 45+ years of old gasket/gasket goo/dirt and general muck that has become compressed into the bottom of the holes by successive tightening of the bolts over the years, causing the bolts to bottom out before they pull the head down sufficiently?I always dig `em out with a scriber point using Plus Gas or similar to loosen the crud up before blasting it all out with compressed air-its amazing what gets lodged down there.Get a good torch, and clean `em until the bottom of the holes (drill point shape) can be seen clearly, run a tap down if needs be, and put all bolts fully in by hand(should be all nice and free, measure under the heads with all relevant washers included and measure the head thickness at all corresponding points to make sure the bolts (are they the right ones-length/length of thread etc) will pull it down with room to spare in the holes.Its worth doing as it will eliminate any chance of chasing around in circles later, best of luck, Bob.
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I always dig `em out with a scriber point using Plus Gas or similar to loosen the crud up before blasting it all out with compressed air-its amazing what gets lodged down there.Get a good torch, and clean `em until the bottom of the holes (drill point shape) can be seen clearly, run a tap down if needs be, and put all bolts fully in by hand(should be all nice and free, measure under the heads with all relevant washers included and measure the head thickness at all corresponding points to make sure the bolts (are they the right ones-length/length of thread etc) will pull it down with room to spare in the holes.Its worth doing as it will eliminate any chance of chasing around in circles later
Phew! What a lot of work... But very good advice and a very worthwhile job I'm sure.
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One of the good reasons for using a solid copper gasket is because it helps to prevent burning exhaust valves.If you use a composite gasket it thermally insulates the cylinder head form the cylinder block so the head will run hotter.A copper gasket allows better heat transfer and lets the cylinder barrel do a bit more of the cooling.
I used to have an all iron Royal enfield single that used to burn out ex. valves untill I stopped fitting the (composite) head gasket .I did many thousands of miles after that without looking at it again
This probably only applies on alloy engines.But I still only ever use solid copper.
Mike
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The recessed valves looks to me like over ambitious use of a valve seat cutter / grinding by a previous owner.
I bet that the seating faces are getting on for 1/4 wide rather than the nice face which they should be. It looks like the valves have been ground to a "knife edge" thats why the ex valve has burnt.
New valves and hardened seats will be required.
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Hi All,
All relevant points, what I was saying was that the leaking gasket (more than once) caused over heating which contributed to the burnt valve!
Valve/seat recession, excessive cutting/facing/grinding is more gradual,
Excessively wide seats lead to burning valves due to the relative drop in spring pressure as the area of the valve seat increases,
After many uses the head bolts also stretch and lose their elastisity,
As well as new head bolts I spotface the seating area of the bolt heads and make special 0.100in. thick washers which are 0.750in. OD, this spreads the load over a wider area,
The original bolts and washers dig into the alloy head surface and also squeez the alloy in around the shank of the bolts, so affecting the torque setting!!!
On composite gaskets, (on Triumphs anyway) they normally require several torqueing down sessions, after first startup, 50 and 500 miles. Real PITA
Regards
John O R
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Plenty of possible places to drop in with a head gasket question, but I saw Chaterlea John as the last poster here and that set my mind.
I don't want to bore anyone, so I'll put this in terms of a recent popular topic.
If Boris Johnson was a composite head gasket, would you put the seams of his suit against the barrels or against the head? Is there a right way to resolve this political controversy? Boris has already received a nice application of Coppertone and, though he won't be in the sun, he will be taking some heat.
Richard L.
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P.S. Yes, it's a real question about composite head gaskets.
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As my original composite head gasket question is about to slip from recent posts after 3x "Another Scam," 2x "Spineless Boris, " and 6x "Corona Virus" comments, I figure I should try to ask it again to see if anyone cares to opine. When using a composite head gasket, is there a prefered direction for the formed seams - towards the barrels or towards the head?
Richard L.
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If you have an alloy head, put the seams towards the barrel. Don't want to mark the softer alloy. With an iron head, it doesn't matter.
Now about scams, spineless Boris and coronavirus...
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Thanks, BS. I was starting to wonder if I'd been group ghosted. My A7 is an iron head. I think I'll put the seam upward based on the unsubstantiatable (first time I've ever used that word) assumption that the head is not as flat as the barrels and the seam will help with that.
Richard L.
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Hi Richard,
Is the gasket a new repro or an old stock item ?
I remember in times past when car engines had copper/asbestos gaskets they were usually marked "top"
If I am correct it was lip up
I know from the BSA C10/11/12 forum that the modern composite gaskets are crap and blow after a few miles in spite of re tightening after the initial heat cycle
Either way I prefer solid copper gaskets.. Lani at Copper gaskets unlimited (Arizona) ,, coppergasket63 on eBay
makes bespoke gaskets at very reasonable rates
John
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John,
I should have asked sooner, like a year-and-a-half when I ordered my first composite gasket. I put it aside and barely looked at it. A few weeks ago I sprayed it with copper, still without paying enough attention. I went to put it on the barrels and, no-voila, it's for an A50. (Reeeally asleep at the wheel because they don't look alike). Searched and searched for who sold it to me so I could complain about it if not return it. No joy. Can't figure out where I bought it. Maybe a swap meet. Found a solid one advertised as A7. Nope, showed up as A10. Maybe I'll keep it for the A10. Found the current composite version from a reputable seller. It didn't look very new, having respectable shelf tarnish. I suspect this new build will come apart before too long because I'm rolling the dice on piston clearance (being a bit too big). Obviously the composite won't be reused. Figured I'd have to settle for an A10 gasket, but will definitely follow your advice on one of the suggested sources for bespoke.
Richard L.
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If you use an A10 gasket, the small gap around the top of the bores will soon fill with carbon. Had to do that 40,000 miles ago on my A7...