To Shave or Not To Shave? (the Cylinder Head)

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timberstone
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To Shave or Not To Shave? (the Cylinder Head)

Post by timberstone » Sun Mar 01, 2020 7:36 pm

Lately put a stock cylinder head (meaning that it had NOT been "shaved") on rebuilt TC XPAG engine that had 0.020" piston overbore and a mild Crane cam. Was disappointed with the performance.
Though have not put the car on a "Rolling Road" or Dynamometer, in order to determine the power output, the road testing and trials with it just did not seem to match what I remembered from the previous engine that I think had a stock head and was 0.060" overbored and had a stock cam. Carbs, distributor and coil remained the same.

Do I want to keep the cylinder head with "Stock" ("Unshaved" -- though it has been polished, ported and cc'd)? Do our cars really lose too much performance -- given modern fuels -- by not getting the head shaved at least some?
If so, what is a good amount to shave? (I am not yet considering going back to the 0.060" overbore or more performance camshaft.
Octagonally yours, Boxley (Robert and MGTC0820)

Duncan M
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by Duncan M » Sun Mar 01, 2020 9:30 pm

Rebuilt engine and stock low compression head? Add a supercharger and call it a day.

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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by frenchblatter » Sun Mar 01, 2020 10:57 pm

Won't do any harm skimming (we don't shave on this side of thE pond) to raise the CR however research how much you can take off before the valves hit the pistons.
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Steve Simmons
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by Steve Simmons » Sun Mar 01, 2020 10:57 pm

Assuming the cam is timed correctly, ignition timing is good (should be around 7-degees BTDC), valves aren't too loose and everything else in order, then raising the compression would be the most economical way to make additional power. However since uncut heads aren't exactly growing on trees anymore, I would suggest taking another route. Ordinarily I'd say find another head that was already skimmed but you've done work to this head so I'm guessing you want to keep it. Another way to raise compression would be with high top pistons. I'm running an uncut head with +30 high top pistons and the compression is about 9.1:1. I have a skimmed and highly re-worked head going on that will take it up a bit more, to around 9.3:1. A supercharger would definitely give a noticeable boost in power but it isn't the cheapest option. It is, however, the easiest. If you don't have any objection to swapping heads then I think that's the way I would go. In my view it's the best balance between economy, ease of labor, and potential gains.

I don't think it's possible for the valves to hit the pistons in an XPAG but they can certainly hit the block! That's the only reason why my new head isn't installed yet. I need to notch the block first and I'm not prepared for that particular bit of surgery just yet.
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timberstone
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by timberstone » Mon Mar 02, 2020 5:47 pm

Have the luxury of the still "unshaved" head that is now on the block.

The other head has been skimmed an amount which raised the compression close to 8:1. It also has been polished, ported and cc'd. Did not use it in the rebuild because I wanted to see what the engine would put out in close to "original" form -- even though it has the 0.20" overbore and a Crane cam.
Since I found out, and found it not up to expectations, will swap the heads and keep the uncut head "original" head (though it is polished, ported and cc'd) for another project.

Please advise whether I may "shave" off more from the head and still use 87 Octane fuel (ethanol or non-ethanol)?
Not sure whether I want to put in larger valves.........

Never will a supercharger be.

Octagonally yours, Boxley (Robert and MGTC0820)

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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by Tom Lange, MGT Repair » Tue Mar 03, 2020 5:16 am

Like Steve, I suspect you have some impediment in your present engine, and that just putting on a hotter head will not bring it up to your taste. A stock 1250 is a peppy little engine when timing is correct and the distributor has been rebuilt. My guess is that you have something amiss in the valve timing.

I would not hesitate to bump compression up to 8:1 or 8.3:1 in order to get a bit more power. But I would use better fuel if you go above that, and listen closely all the time for detonation - your greatest enemy.

Tom Lange
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by frenchblatter » Tue Mar 03, 2020 10:17 am

I use 10 degrees BTDC ignition timing on mine (instead of the book TDC)
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by stephen stierman » Wed Mar 04, 2020 2:19 pm

With an unskimmed head, what sort of numbers should you get when you do a compression test?

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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by frenchblatter » Wed Mar 04, 2020 3:22 pm

I doubt you'd notice much difference betwen the standard CR and a higher one. If you do the test using the method of throttle wide open and three turns of the engine (to ensure one of the strokes is a compression stroke)
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by stephen stierman » Wed Mar 04, 2020 6:32 pm

Norman, I think you would notice a difference. My head is unskimmed as I remember and I am getting around 125 lbs., a bit less in number four so am thinking of pulling the head and looking at the valves and skimming to around 9.1 to make a little better use of the fuel available and getting some better performance.

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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by Steve Simmons » Wed Mar 04, 2020 7:37 pm

It's been a while but I seem to remember numbers around 125 for stock and I think mine at 9.1:1 tests about 145 or so. My head is not skimmed, but I have high top pistons. I think with the high tops and a head skimmed 30 thou it was around 165. These numbers are purely from long term memory so I could be completely wrong.
1949 TC8975 / XPAG 9609
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by frenchblatter » Wed Mar 04, 2020 11:16 pm

I stand corrected. Sorry chaps, ben a long time since I did a compression test (even though I have a tester!)
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timberstone
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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by timberstone » Thu Mar 05, 2020 3:21 pm

With the unshaved head the compression readings on all four cylinders is 145 pounds. The head is stock thickness but has been ported, polished, cc'd and had the guides replaced with silicone bronze and the valves and inlet/ outlet seats ground to match. The cylinder bore is 0.020" overbored due to a milling anomaly.
The stock compression ratio is 7.25 to 1 on this cylinder head. But since the bores in these cylinders are over size 0.020", the compression ratio increased to 7.35 to 1 (according to chart in W.K.F. Wood, p. II-1, 1968). Not sure how much the increased bore capacity factors in.
According to the factory "Special Tuning" manual (dated June 1949, page 4) the Stage 1 Tune specifies removal of 3/32" from the stock cylinder head to raise the compression ratio to 8.6 to 1. ("The standard head is 76.75 mm thick; the finished thickness should be 75.37 mm. after machining.")
An online post states: "Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel." So even with the slightly increased bore capacity, there ought to be a cushion for running 87 octane fuel. "Higher octane provides no additional performance for cars that are designed to run on regular gas." (Car Talk)

Octagonally yours, BOXLEY (Robert and MGTC0820)

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Re: To Shave or Not To Shave? (the Cylinder Head)

Post by Warmuthb » Sat Mar 28, 2020 2:45 pm

I would not shave or cut a stock head. Uncut heads are rare. Check other things and/or install other speed parts. Once cut, it can never go back!
Brian

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