Bent Valves, That is the Questions


As some may recall, about a year ago, I was in the process of installing a professionally rebuilt head in the 71 XJ6. Towards the end of the process, I may have made a mistake that resulted in bending the valves on the rebuilt head.

The head had been tightened down in place and I was in the process of reattaching the chains on the two sprockets that are in front of the camshaft. I was working on the right side sprocket first. I had installed one of the two bolts that hold the parts of the sprocket together. I was, however, unable to install the second bolt as the parts would not line up. I was hopeful that if I rotated the head, the holes would like up with that hole. When I told the list of what I had done, I was advised that most likely had bent the valves. Short of removing to head to inspect the valves, is there any way to determine what if any damage I may have done?

Frankly, I was so demoralized at what I had apparently done, that I could not think about it until now. It’s time now to at least determine where I am in this process.

I guess the first question is, short of removing the head, is there a way to determine if any of the pistons are bent?


One way to see if you have any bent valves is to perform a valve clearance check while turning the engine by hand observing if the tappets rise and fall easily following the camshaft lobes and that the valve clearances are to spec. If a valve is bent, even a tiny bit, it will interfere with the valve stems and get stuck. It is my opinion that if the valves open and close easily following the lobes on the camshaft and all of the valve clearances are within specification it is unlikely that you bent a valve.
If your engine passes the valve clearance check when turned by hand, then further confirmation can be achieved by doing a compression test on all cylinders using the starter to crank the engine to verify that you have good compression on all six cylinders.



What @Paul_M_Novak said!

My suggestions regarding valve clearances check and compression check assume that the camshaft timing has been properly set with the correct procedure and tools.
If the camshaft timing is set wrong then you can bend valves if the valves and pistons make contact. This us possible in an XK engine because it is an interference engine.



Ya, what @Paul_M_Novak said. He’s spot on.

Ways to bend a valve.
Short version
The valve clearance between the piston and the valve seat is off. OR the cam timing was off enough for the valves to collide.

Long Version

  1. Cam timing off at the time the engine was turned over.
  2. improper clearance between the head and the top of the piston. This can be accomplished too much being taken off when the head was resurfaced. or a cam with too much lift - performance grind. Though with the XK and high lift cams; you’re more likely to clip the edge of the tappet bucket sleeve first.
  3. Piston coming up too high in the bore. too much taken off the top of the block in order to make it flat.
  4. Weak or broken valve spring.
  5. Seriously mal adjusted valve clearance (between the cam and the tappet). meaning a way too big shim keeping the valve from seating properly. or the shim isn’t seated properly under the tappet.
  6. Valve is sitting too high in combustion chamber - Valve not seating properly in the valve seat.

How to check
valve contacting the piston
Pop out a plug, check the top of the piston with a borescope. They can be had at any DIY store or amazon for under $100.00 store. Amazon has borescopes for Andoid (based) phones for under $30.00 US. Just check the size of the business end to ensure it will fit down a spark plug hole.

cam timing
If you’ve got the Good Book of Green (aka Service manual) umm… The Series I is brown…
The service manual has cam adjustment & timing spelled out very well. The cam tool isn’t an absolute necessity but is HIGHY recommended. The timing on both cams can be checked after the fact. Also check for slack in either of the cam chains - There shouldn’t be any. If there was slack in the chain anywhere - upon a couple of hand revolutions; it can affect the cam timing. I’d check it again and look to see if the cam chain is loose on either sides of the cam sprocket.

I did a post on adjusting / checking the cam timing on a post I did several years ago “Another XJ that won’t start” I think that’s the name of the thread.

Anyway keep us posted on your progress.

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Where in the process are you, Lou…?

As Paul says; you must not(!) rotate the engine for further testing unless chains are fitted to both cam sprockets and the valve timing is properly set…never, never never.

Obviously; a compression test will then reveal bent valves - but cranking an engine with possibly bent valves is unsafe.

Provided the valve clearances were set properly, as they should with a professional rebuild; turning the engine by hand while checking valve clearances is as good a test as it gets - as Paul says…

Of course, head must be removed to replace bent valves…

xj6 85 Sov Europe (UK/NZ)

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as above, plus it is actually quite hard to bend valves with Jaguar dia, stems whilst turning the cam or engine by hand, especially when the valves are cold, more modern engines have very thin stems and do bend quite easily,

For the sake of any future readers of this I have to disagree. I found it very easy to bend a valve on my 3.8 when resetting the clearances. It simply required one episode of rotating one camshaft after forgetting to loosen up the other one. It did allow me the pleasure of learning how to remove the head and replace a bent valve though.

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You have me a bit confused. or merely me confusing myself!!!

You mention rotating the head?? And bending the piston.

But, I do get your concern.
I concur with the advice you have received here.

I add one. it was going to be two, but rethought and deleted one!!

Get the simple gear and do a leak down test on each of the six. Even a slight bend and the cylinder will leak down… Might add a bit of oil to seal the rings and create a false read…

Just make sure the cams are timed properly before turning the engine, CW, not CCW!!!

My first car, a 23 t refused to start after a long period. Billy and I pulled the head and turned the engine and found one valve stuck open, Not an interference engine by far. lots of room in there for all. A whack with a hammer and some crank over by hand and it resumed open and close.

Put the head back on. tied it on to Billy’s T. Gave it a tow… yahoo, as soon as I engaged the clutch, it fired.
I drove it back to our drive way. First time i ever drove a T!!!


I have to second that, with unfortunate personal experience.

You can EASILY bung a valve, turning by hand, a hemi-headed engine.

Don’t ask.

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Certainly relevant, Carl - but requires some nous for a decisive diagnosis.

Even the slightest bend of the valve stem will prevent the valve from sealing properly. And as the angled valve head will hit the piston at its rim - no great force is required…

xj6 85 Sov Europe (UK/NZ)

We’re all quick to jump on the band wagon about the whole bent valve thing So I’m revisiting this thread from the very first post.

I want to make sure I’m clear with where you are at.

  1. You rotated the cams independent of each other?
  2. or rotated bottom end (lower rotating assembly) independently of the cams.
  3. When you installed the cams, did you have the cam positioning tool in place so both cams were as they should be?
  4. Do you have the timing chain installed? If so
  5. how far off was the bolt / cam alignment?

Here’s more thoughts on the subject.

  1. If the cams are installed, and the positioning tool was used and the cams did not move and the bottom rotating assembly did. While the bottom was rotating did you meet up with some resistance? like a stop? if so that would indicate valves colliding with the piston. This would be the only way you could bend a valve IF I say IF; both cams maintained their alignment / orientation using the cam positioning (lock) tool; and cams were locked down.
  2. If you didn’t run into some resistance while turning the engine over by hand, AND the cams were position correctly using the positioning tool. I wouldn’t think you bent a valve. BUT I do think the timing will be off and depending on how much the bottom end was rotated will determine if the engine will run properly and it could result in a bent or burned valve providing you could get the engine to run with the cam and crank being out of time.

Mark goes on another tangent…
Like I mentioned in my previous post. if you don’t have a borescope - get one. they’re cheap and extremely handy. If you had an android based phone; there’s a borescope that plugs into the phone and it’s really cheap - under $30 bucks on cheap.
I have 2 of them. the one I picked up at harbor freight for $49.00 works the best for car stuff - the camera fits down the spark plug whole - it has a mirror attachment that permits you to look up the other direction. very very handy tool.

At this point, for whet ever it’s worth. there is a snap ring on the cam gears that keeps the cam gear teeth in line with the cam. by removing the snap ring it provides the ability to position the bolts of the cam independently of the cam gear teeth.


for what ever it’s worth.

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While a borescope certainly is worth while, Mike - I’m not sure that it will detect subtly bent valves with some certainty?

However, clarifying exactly what he did is vital…

xj6 85 Sov Europe (UK/NZ)

I agree! With all the worries, I‘d simply pull the cams, then the head, look over everything and then start from clean.

I also had the issue of the holes not lining up and it was a little trouble but nothing that takes more than a day to get right with a helper or crane for the head! :slightly_smiling_face:


Yes, with the cams off, one could detect a valve not in place!!

All should be fully closed. Visual or by GENTLE tap of the hammer.

Any valve not seated is bent! Can be detected.

If all are seated, Yahoo, got lucky!!!

Reminds me of my XK’s demise. Compression test, zero in all holes!!!


Take it one step further. With the cams off rent or borrow the equipment to do a leak down test. It is similar to a compression test except you are not cranking the engine to build compression, You are using compressed air to pressurize the combustion chamber, then seeing how long it will hold pressure. If it won’t hold pressure at all, then you most likely have a bent valve.
Also, do you have any idea of the condition of the lower timing chain? They wear much more than the top chains that are mostly supported by the large gear sprockets. The lower chain, hydraulic tensioner and guides will all wear and stretch. When the lower chain stretches too much, the tensioner can actually pop out and fall off. With to much slop in the chain it could be possible to bend valves by turning the engine backwards.

When a valve strikes a piston it will usually leave a witness mark or a dent or mark in the head of the piston. That is what I would be looking for.
If you wanted to go to the trouble of removing the manifolds, you could use a bright LED light source by the valve and look for a light leak if a valve was bent.

This was the depression in the pistons on my 3.4 when the cams stopped rotating.

That is indeed a witness mark left by a valve hitting the piston!