Cylinderhead has been ground too often?

[Karl100]

Every time you grind cylinder heads, they become slightly flatter. How do I recognise whether a head has been ground too often?

Thank you!

[Goldstar]

IMG_9852.jpg Look where the valves are relative to the edge of the combustion chamber. These heads had a skim of 8 thou (0.2mm) to get them straight and won’t take another skin even with thick gaskets there was a faint touch on the piston crown so I fitted 50 thou (1.25mm) saver shims. Car running nicely - well it was until the owner called me to say he had a problem starting it (unrelated gearbox inhibitor I think).
IMG_9851.jpg

[jbuckl]

4.427” here
IMG_4638.png

[jbuckl]

http://ldpart.co.uk/shop/shop.php?sid=sid4177fc35209449b739ee3aab39715fd5&c=ex&page=145

[Goldstar]

I’ve seen that before Julian but I remember seeing somethingTony Hart wrote in the tech bulletins back in the day that it wasn’t accurate because there was no machining datum for the measurement in this diagram and therefore that all the heads could be different. Can’t remember the exact technical detail but that was the gist of what he wrote.

[marshman]

I’ve seen that before Julian but I remember seeing somethingTony Hart wrote in the tech bulletins back in the day that it wasn’t accurate because there was no machining datum for the measurement in this diagram and therefore that all the heads could be different. Can’t remember the exact technical detail but that was the gist of what he wrote.

The measurement given is from the bottom of the cam journal to the head face. I think the cam journal is a good proxy for a machining datum. If it was more than a few thou out then the shims required under the cam buckets would be out of the "normal" range.

That diagram has been published and accepted many times both on this forum and over on the Dolomite forum as it is applicable to the 1850 and the Sprint. It also ties up well with heads I have measured.

[jbuckl]

I’ve seen that before Julian but I remember seeing somethingTony Hart wrote in the tech bulletins back in the day that it wasn’t accurate because there was no machining datum for the measurement in this diagram and therefore that all the heads could be different. Can’t remember the exact technical detail but that was the gist of what he wrote.

Ok … the datum is the cam bearing bore….

I can dig out the standard nominal from some data I have.

Empirically, the inlet valves must be clear of a skim on worst case… will need thick head gaskets though.

FWIW, by that stage I’d be doing a dry build and checking valve to piston clearances using bluetac or similar….. on a 720 degree rotation.

sava shims are a get out of jail way forward though.

[Goldstar]

Ok … the datum is the cam bearing bore….

I can dig out the standard nominal from some data I have.

Empirically, the inlet valves must be clear of a skim on worst case… will need thick head gaskets though.

FWIW, by that stage I’d be doing a dry build and checking valve to piston clearances using bluetac or similar….. on a 720 degree rotation.

sava shims are a get out of jail way forward though.

It was the dry fit cranking that showed up a little kiss on the ones I did recently.

Can the cam bore be a proxy measurement? Each head will be different.

marshman i accept it has been accepted here but that doesn’t make it correct or easy to measure accurately. personally I prefer to rely on the method flying farmer describes which Julian paraphrases above but whatever works for each individual is also ok.

[marshman]

Of course. No one should just put an engine together without checking everything as you go. Agree it is not easy to measure but it is a reference measurement. Other factors come into play as well if the top of the block has been skimmed as well. Yes Bluetack or plasticene (remember that!) is a useful tool/indicator and should always be used where clearances are tight - just because valves are not making contact on a cold "dry fit" check doesn't mean they won't in a hot engine revving at 5000+rpm

As stated I would say that yes the cam bore can be used as a proxy for measurement as it is the valve hitting the piston we a re concerned about and the valve length is fixed - yes I am aware that shims are used so that is a variable.

The original poster asked how you can check if a head can still be skimmed, the document gives you a good guide. As you say even if it has been over skimmed you can always use saver shims. As I said I have checked 3 pairs of Stag heads and 2 dolly sprint heads and the measurements tied up with that document.

There are also other issues with head skimming - 1) it can make getting the cam timing spot on more difficult and result in having to redrill the com sprockets to get the adjustment necessary and 2) it results in the stud hole exit getting closer to the bolt holes which can cause them to bind against the sides of the stud holes causing distortion when its all tightened down as they no longer line up exactly with the block - seen this on a couple of heads- solution is to open the holes out slightly - which in my view is a good idea any way to lesson the chance of them seizing in the holes.

[jbuckl]

Of course. No one should just put an engine together without checking everything as you go. Agree it is not easy to measure but it is a reference measurement. Other factors come into play as well if the top of the block has been skimmed as well. Yes Bluetack or plasticene (remember that!) is a useful tool/indicator and should always be used where clearances are tight - just because valves are not making contact on a cold "dry fit" check doesn't mean they won't in a hot engine revving at 5000+rpm

As stated I would say that yes the cam bore can be used as a proxy for measurement as it is the valve hitting the piston we a re concerned about and the valve length is fixed - yes I am aware that shims are used so that is a variable.

The original poster asked how you can check if a head can still be skimmed, the document gives you a good guide. As you say even if it has been over skimmed you can always use saver shims. As I said I have checked 3 pairs of Stag heads and 2 dolly sprint heads and the measurements tied up with that document.

There are also other issues with head skimming - 1) it can make getting the cam timing spot on more difficult and result in having to redrill the com sprockets to get the adjustment necessary and 2) it results in the stud hole exit getting closer to the bolt holes which can cause them to bind against the sides of the stud holes causing distortion when its all tightened down as they no longer line up exactly with the block - seen this on a couple of heads- solution is to open the holes out slightly - which in my view is a good idea any way to lesson the chance of them seizing in the holes.

100%

sorry haven’t dug out the numbers yet,
but will try to and also the cam bore to fire face tolerance as well… guessing that will be tighter than expected…. It’s an important one.

[richardthestag]

Just to lob in my 6 penneth, I have tried the cam - engine face method, measured 5 times and got 4 different results! My Vernier calliper is reasonable quality. Keep measuring and take the average is my suggestion. Gave up with the ball bearing and just measured to the bottom of the cam bearing and the flat of the head face, moving stuff until I got the minimum measurement and then recorded it.

In my most humble of opinions saver shims are the absolute last resort and usually are the death knell for any further skimage of the head.

Thick head gaskets are great at reducing compression ratio back to where it should be at but beware that you need to retorque them after the first full heat cycle. They compress far more than the standard items.

Now, slight curve ball. was recently tasked to shim the cam on a tr7 head that had been skimmed. Initial inspection filled me with the horrors

20241119_135405.jpg

Observe how the edge of the valve seat has been skimmed into the face. when the valve was installed it sat below the face the the head and would have protruded into the combustion chamber.

Spoke with the respected machine shop who did this work, and does do this for Robsport iirc and they said perfectly acceptable but only because the tr7 has recess in the piston crown to accommodate the valve. suggestion was to shim, use a thick tr7 head gasket and use it.

This head was not great to be fair and I think had seen it's last skim, and yes I did recut the exhaust seats before I rebuilt it

[marshman]

Not sure what you are doing different to what I do when you measure things Richard, but my measurement results, be these heads, cylinder bores (slightly tricky with verniers!), crank journals or the tappet shims discussed in an earlier thread, are repeatable and comparable between Vernier calliper and Micrometer - I do have a set of large micrometers that I can check/measure up to 6" (150mm). I know they are repeatable because I always measure / check at least twice. You need to have the ball or short length of round rod in the bottom of the cam bore as it is a curved surface and trying to measure with the jaws of a Vernier without, even using the "knife edge" section, is a waste of time. The results will vary a lot if you are just a fraction off axis.

Yes saver shims are a last resort, but I thought that was the point of them.

That TR7 head really is pushing it though, be interesting to see if it lasts in use.

[jbuckl]

Iirc tr7 inlet valves are 2mm bigger than stag inlet valves.

so if the picture was of a stag head with similar skim, the proximity of the skim would be less concerning in appearance.

btw a stag head can be modified to take tr7 valves…. But pistons need a cutout adding.

i once saw a fixture to help do the cutting… for a pistol drill!

sorry I don’t know the depth of cut…. But it’s probably a bit variable in reality.