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Triumph TR6 - Shaving the Cylinder Head

Now that my '74 block, head, crank and rods have been cleaned and magnafluxed (all perfect) I am ready to start the rebuild. I previously posted some of this so will try to be brief. I've taken Steve's (and others)advice and will be shaving the head to gain some compression to help out the new GP2 Goodparts cam. Can anybody advise what I can or should shave to optimize performance but also to avoid having to shorten pushrods? I am looking at 20 to 30 thousandths but has anybody done this and what is the best route to go?

Thanks,
Bob
1976 - TR6
Bob Evans

You may not have to shorten the pushrods, you may even need longer ones. But first, did you establish the swept and unswept volume of the cylinder along with the volume of the combustion chamber before deciding how much to take off the head? If not, you've still got some work to do. Otherwise you wind up with whatever you get for a compression ratio instead of something planned. If you are going to start modifying things, you really need to know what you are building. Go back to the archives on determining all of the volumes and calculating the compression ratio.

Bear in mind that the odds of needing a shorter push rod are rare these days as most of the TR camshafts available are actually reground, so the lobes are are not as high from the camshaft centerline as they would be a new camshaft.

To determine if you need to either shim the rocker pedestals or shorten push rods (two means to the same end) you will need to have the head finished, assembled with valves and installed, then valves adjusted. If possible, adjust the valves such that the rockers are 90 degrees to the push rods at half of the lift.

Mark the top of the valves with a Sharpie, then install the pushrods and rockers and adjust as indicated above. Turn the engine over by hand a couple of times and see where the rockers are hitting the valves. If the ink is only removed from the center area of the top of the valve stem, then you are OK and need to do nothing. If the ink is removed from the inboard side of the top of the valve stem, then you need to either shim the rocker pedestals or shorten the pushrods. If you only remove ink from the outside, then you need longer push rods. Clean off the ink when you are done.

There is not really a nice straight forward way to do this except by a certain amount of trial and error. Having said that, I think there is a better than even chance that you will have to do nothing as far as the push rods are concerned. The rationale is that sure, you shaved the head, but then the lobe height on the new cam is reduced by some amount and there is a reasonably good chance that the push rod length is washed out between the two things.

One last thing, if you do wind up shimming the rocker pedestals, remember to put the oil hole in the shims otherwise you kill the oil flow to the rockers.
SteveP

Thanks Steve. I'll be taking all your advice to the machine shop and asking them to help work out the details.

Cheers,
Bob
Bob Evans

I had the head shaved on my 1970 TR6. The swept volume on all these cars is the same, so one calculation will do for everyone. I had 50 cc combustion chambers and calculated that 0.055" off the head would make 9.5:1 compression. I went a little conservative and took 0.050 off for about 9.47:1. One of the steps involves measuring the deck height, Hilariously, every cylinder was different. I told the rebuilder this and they reassured me they would do something about it. The engine will never run smooth if every cylinder has a different compression ratio! I found the formulae for calculating head shave on the net. It took me a couple of tries to get it right, being an iterative process. When I was comfortable with the results I ran the calculations backwards to check my numbers (from new chamber volume to compression ratio and back again).

The cam was reground by Shadbolt in Vancouver. I asked him to make it breathe a little better but retain that good low RPM torque I love so much. I have to say I am thrilled with the combination. With the cam and shaved head and the valve stem seals and the pertronix ignition and the platinum spark plugs and the dynamic balance and the new valve springs it pulls strong and has a sharper note than before. Everyone that has driven it has been impressed with the power.

I have the timing set at 10D BTDC and run premium fuel with the vacuum retard disconnected to avoid detonation .

I did not have to do anything for the pushrods.

I strongly suggest getting a calculator and running the numbers for yourself. Mistakes here can be costly.

Don
sixtoy

Isn't there a difference for cylinders that are not stock anymore?
don
DON KELLY

I shaved my 76 head shaved .1" Made a helova difference! That may be a much for an earlier engine but the '76 low compression engine it was great. Needed priemium then.
skikir

I must disagree with the statement that the swept volume is the same. It is not uncommon to find variances from one engine to another or even within the same engine. There are common replacement parts used in rebuilds to throw in another wrinkle. How many people out there in TR6 land have oversize pistons? The swept volume of a standard bore vs an overbore will be different. All other things being equal, a bigger bore means a higher compression ratio due to the increase in swept volume. The difference may be small, but if you are operating close to the edge.... One size does not fit all. You have to know the stroke, you have to know the bore, then you can come up with a swept volume.

As stated, mistakes here can be costly, best to know what you have.
SteveP

As I was saying above,restated very smartly by Steven, they all are different. When I had my head done I had to very carefully and as carefully as I could measure the volumn of each indivisual cylinder for Ted @TSI before he would even start the shave job.
don
DON KELLY

Bob, I would agree with Steve, Don and Sixtoy, that the volume of the combustion chambers must be measured before you can calculate how much to mill off the heads.

For one thing, unless you have owned the car since new you do not know if or how much a previous owner may already have skimmed the head.

Having said that, 20-30 thou is a very modest amount, so given the low compression ratio of the later TR6s, you may well be able to do quite a bit more than that.

I would suggest that when you get the head off and cleaned you look at the raised edge across the wall of the combustion chamber by the spark plug hole. It is worthwhile to blend this in more smoothly to prevent it from getting very hot and causing preignition; there may also be a small benefit to gas flow and mixture quality by making this edge less prominent. Naturally this makes the combustion chamber very slightly larger, and they may no longer be exactly equal. Here is where the detail work comes in!
For most cars it is enough to equalise the volume of the combustion chambers, the machine shop calls this "ccing" the head.If you are really fussy you might now want the machine shop to check the deck height of the pistons and match up the combustion chamber volumes to equalise (or "cc" them) so that the pistons which are slightly higher at tdc have slightly larger combustion chambers, and vice versa. This ensures each cylinder has identical compression ratio.

This level of fussiness is probably necessary only for racers, the average guy will not bother.
If you take a lot of material off the head you can buy shorter pushrods, or have them shortened, or as Steve mentioned you can have 50 thou or 100 thou shim stock used under the rocker pedestals.

Sorry to be so long winded!
Simon.
Simon Rasmussen

Goodparts (http://www.goodparts.com/)has a very complete article on CR with a chart showing head thickness for a given CR, cc volume, and the effect of oversize bores.
Berry Price
BTP Price

For me, figuring out how much metal to mill from my head was no too difficult. I don't dispute the elegant approach I've read here (e.g., CC the combustion chambers), but for most of us this is unnecessary. I simply played with Goodparts' CR calculator and arrived at a mill figure that seemed to work for my target 9.5 CR. As long as you can get an accurate measurement of the existing head thickness to determine your "unaltered" starting point (for a '72, that would be 4.550"), this approach is foolproof. Even if you are unsure if your cylinder bores are oversized, just plug in the max oversize (0.040) into the calculator and use that possibly conservative mill result.

I wound up milling 0.120" off a spare unaltered head to a final thickness of 4.430" to get about 9.5:1 CR. Upon removing my existing head, I found I had 0.030 oversize bores, so my corrected CR is closer to 9.62:1. Bottom line: a real seat-of-the-pants improvement in performance (also put in a mild Isky Z-19 cam). Have to burn premium fuel though.
Rick Orthen

Thanks Guys. That is a LOT of good information on the milling operation so I am going to take all this and consult the local shop for their advice. After they tanked the block I learned that they have actually done a few TR6 rebuilds and they do a lot of performance work for the local muscle-car guys so they should be able to help. Really appreciate the help and will post pics when the rebuild begins in earnest.

Cheers,
Bob
1976 - TR6
Bob Evans

When I first started to consider milling my head, as stated previously, I started measuring things and found every cylinder different. I also made calculations based on standard bore vs 0.060 over. What I discovered was that the differences in outcome were very small, like 9.5:1 vs 9.62:1. As I also stated, I went a little conservative on the mill and also wound up with 0.060 over pistons. Whether it came out as 9.48:1 or 9.5:1 or 9.62:1, alls I know is: it goes like stink and I like it. Again, unless you are going to blueprint the whole engine and go racing, you may as well ignore the differences in individual cylinders, Triumph certainly did!

Don
sixtoy

This thread was discussed between 19/03/2006 and 27/03/2006

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