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Hi all,
To my great disappointment I find the inlet valve on Left Hand No. 2 cylinder is riding. All other valves are ok.
It seems very strange to find only one valve riding.
Can anyone advise if this is common and what the cause might be?
I would have imagined that exhaust valves were more prone to recession and if a major overheating event had occurred more than one valve would be involved.
My desire to pull the head off is low at the moment. I am also in trepidation of breaking the chain tensioner and would greatly like to avoid touching it.
Is it practical to just re-shim the valve in question?
I have read as much as possible including the incredibly helpful Kirby book but am still a little confused.
what does the special tool that keeps the cam in place look like? Can it be made or improvised ?
Is it possible to slide the cam sprocket forward onto it's 'hook' without letting the chain tensioner go?
Is it possible to maintain tension on the chain so the sprocket can be slid back onto the cam without letting the tensioner go?
Is it possible to calculate the size of the new shim without fully reassembling?
i presume the circlip on the sprocket keeping the cam wheel on the splines does not need to be disassembled during the operation?
Any advise or experience would be greatly appreciated.
Alan
Hi Greg,
Riding is an expression commonly used to describe a valve without any clearance so it is held a little open even when it is on the base of the cam.
that is to say there is zero tappet clearance or worse, the valve is being held off it's seat causing compression loss and further damage
hope this helps
Al
To replace valve shims the head does not need to come off, but the camshaft need to be removed. Which means the chain has to be de-tensioned and the cam sprocket removed.
If you have one valve that is riding it could be either a valve head or worn seat. Before I pulled the head I would remove the cam and check leakdown on that pot. If you're lucky it'll be just a worn shim, earlier shims were not as hard a s the later 6.0L ones. I have seen shims with significant wear on the valve stem side.
Thanks Alan. Are you 100% sure the valve stem is actually always in contact with the cam? because a valve being slightly open is more likely to be crud on the stem or valve/seat interface itself, rather than a tappet shim or valve stem-end fault. For it to be caused by a valve stem-end or a tappet shim, a somewhat unusual breakup of metal would have had to occur. (but see Warrjon's poinrts above)
If it is crud on the valve/seat interface, I recommend a full blast, high rev in low gear, 30 minute drive!
If it IS a material failure in valve tappet shim, then I think it is possible to fix it without the tensioner being broken, Grant Francis is the gut to tell us how to do it.
If the valve stem is shot, then of course it is a head off job...
Hi Greg and thanks for your input.
Alas, it is not crud under the seat as was my initial hope.
Not only is it impossible to insert a feeler gauge of any thickness between the cam base and the bucket but also the bucket is not free to rotate. The other 23 buckets can be easily rotated when on the base. Whether it is riding by a micron or a mile I cannot tell.
I like your idea of pressurising the cylinder with the cam out to check whether it is seating.
As an aside, I was wondering at what temperature valve clearances should be checked. Obviously as the head is aluminium it would have a different coefficient of expansion than the steel valve. I presume that in other words the valve clearance should increase slightly as the engine heats up. As it is bitterly cold here in the Snowy Mountains I would expect slightly tighter clearances.
Inlet valve head "mushrooming" is NOT uncommon, BUT, mostly limited to the good old days, AKA 1960's, and I saw plenty of it during the apprentice days.
Why only ONE, NO idea, stuff sometimes happens.
Biggest issue, is the head usually gets to a point it falls off.
I put all new valves in mine, and some of the Inlet were showing very slight signs of this :mushrooming".
Short cut with that tensioner, shape a broom handle, BASH it down the inner face of the chain AT the tensioner, so as to wedge it where it is, then take that head off and fix the valves, remove said broom handle prior to starting the engine, BUT, what about the others, mmmm, gets scary now, sorry mate, but fact it fact, and age is a mongrel with any engine internal component.
Greg has a good point there, when I pulled my 6.0L down every (and I mean every) valve leaked some so bad the fluid poured out the port. It was a combination of crud on the head/seat and valve stems being oval.
If a valve is not closing 100% as designed, by crud etc, that would give an INCREASE in the clearance at the camshaft, NOT a decrease,
Valve stretch is the only "thing" that will reduce the gap at the camshaft.
So the question I cannot answer is: "how can the valve get longer ?
Can the stem stretch in use?
Can the valve seat wear that much?
I have no idea and would like to know!
Never did any valve work on this engine. But, a "lot" on other much less complex critters.
A lot of red line work does that. don't ask!!
The issue lies in the valve head and/or seat. Deterioration in one or both and the valve seats further into the head. Thusly clearance at the cam goes away,
Don't keep driving, it will get worse til something gives way. Not good.
Hi all,
And thank you everybody for your helpful input.
Huge revelations may yet be at hand. The cam is out and strange and wonderful thing may be discovered.
On first observation I am slightly hopeful, though, as they say in the army, 'hope for the best but expect the worst'.
I will revisit the 'The Heart of Gold' later today and continue the investigation.
I am having trouble posting pictures to this site, any helpful hints?
Al
So the question I cannot answer is: "how can the valve get longer ?
Can the stem stretch in use?
Can the valve seat wear that much?
I have no idea and would like to know!
Greg,
Valves do stretch, and that is a fact.
The valve head very gradually gets pounded out, and instead of being basically "flat" across the face, it gets a hollow appearance.
Valve stem stretch is rare, but not uncommon, and usually in modified engines that rev at insane speeds, and/or stronger valve springs are the usual culprits.
Valve head seperation is also common, and that is generally from age.
The PreHE is not known for this, anymore than the HE is.
A win at last!
Hi all,
I discovered the reason for the riding valve. It turns out the shim under the bucket was misplaced.
All advise received on this topic was greatly appreciated, particularly Grant Francis's tip on using a modified piece of wood to lock the cam wheel in position.
im sufficiently chuffed and surprised by the discovery that I will write a seperate blurb to describe it.
Al
06-25-2018, 10:48 PM
Riding inlet valve 1978 Double Six
