Cylinder Head flatness and roughness

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HypnoToad
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Cylinder Head flatness and roughness

Post by HypnoToad »

I've currently for the cylinder head off my '84 Excel and it's recommended to measure for flatness before putting it back on with a new gasket.

I now have all the needed parts, straight edge, feeler gauges etc. but can't find a spec anywhere in the manual that states how flat the cylinder head needs to be? I've only checked the Tech data and Engine sections so far however.

EG. Is 0.005 inch gap lengthwise considered out of spec? Would a 0.002 inch gap be OK?

I'd rather not skim if it doesn't need skimming.

EDIT: I assume it's recommended to measure and skim with cam carriers removed also?

Pete Boole
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Re: Cylinder Head flatness and roughness

Post by Pete Boole »

Also try to get the head on a surface plate to check for twist as well as straightness. I don't recall seeing any data for straightness and certainly not roughness. I think if I found any hint of out-of-true I'd get it skimmed. It's hard enough getting the head gasket to seal properly at the best of times! Let us know if you come across any data.

Pete

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rbgosling
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Re: Cylinder Head flatness and roughness

Post by rbgosling »

Yes, I'd say it should be checked as dismantled as possible.

I don't know of any designated limit. When I did my engine rebuild I just had my head skimmed as a precaution, just to make sure. I had a very fine skim, not enough to affect compression ratio by any meaningful amount.
"Farmer" Richard

1990 Lotus Excel SE (Lilith)
2022 MG MG5 EV (not due to be a classic for quite a few years...)

gusexcel
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Re: Cylinder Head flatness and roughness

Post by gusexcel »

The 'bad' warp is generally across the short way. A small amount from left to right does not go away when you torque the head down, but you could have a fair bit of lengthwise warp without having a sealing problem. Now whether the cams would turn is another story

A straight edge across the short way on the head with feeler gauges.

I'm gonna guess .005 is too much and .002 i think I would bolt back on, but maybe I am crazy

They warp the short way by getting overheated and there being no bolt in the middle to hold the head down
SO if it didn't overheat I would not think you would expect to see it.

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Alan_M
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Re: Cylinder Head flatness and roughness

Post by Alan_M »

Have a look at the file attached to this post from Tim Engel. I think it will help you.

https://www.facebook.com/groups/2377984 ... 703934760/

Oldtimer_Basis
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Re: Cylinder Head flatness and roughness

Post by Oldtimer_Basis »

Outch... I guess I was very lucky, then, because I just cleaned the Surfaces and bolted everything back together again. I thought: Hey, that´s just soft Aluminium. It will surely bend it´s Way back in Place as soon as I tighten the Bolts.
When I read all this I would have brought Head and Block to the Machine-Shop for resurfacing instead. A little Bit less height won´t do much harm, I think. Maybe, 0.5 Points more Compression or so...
I don´t think that I would touch the Cam-Carriers, because then you definitely have to readjust the Valve-Play. I´m sure there still is enough Room between the Valves and the Piston @TDC, even with a little Bit Height scraped off...

But that´s just my Gut-Feeling - which you shouldn´t rely on without measuring.

Greetz,
Raf

Esprit2
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Re: Cylinder Head flatness and roughness

Post by Esprit2 »

Go here:
https://tinyurl.com/45kwkm4y

This page is from the 93-97 Service Notes simply because I had it handy. The flatness info applies to all 9XX 4-Cylinder heads.

"IF" you have the head milled to flatten it, make sure the machinist knows the bottom & top of the head must be parallel. The crankshaft, camshafts and tensioner bearing roller must all be parallel. If their centerlines converge, the timing belt will tend to run off center on the pulleys toward the convergence point. The top and bottom of the head must be parallel.

Also, if the head is warped, then it's not just the bottom... the entire head is warped like a banana. Just milling the bottom of a SOHC ro DOHC head won't address the top. A flat cam carrier won't seal well against the top of a warped head. And the cams will end up pointing somewhere other than parallel to the crank.

When flattening a SOHC or DOHC head, it's best to flatten it as best you can before machining it (brute force). Then use only the lightest cut to true the surfaces the last little bit. Just buzzing the bottom is for OHV pushrod & rocker heads.

Regards,
Tim Engel

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