Head flatness tolerance

[Mark Serj]

Hi all

I am inspecting the heads removed from a motor that was running OK albeit smoky. The heads show a max .0025" gap between center bores using a calibrated straight edge, and in the form of a uniform bow from front to rear. No across the heads bow exists. I have no previous experience in this so would be grateful for some comments as to acceptability. as regards the block I can't get .002" feeler under the straight edge anywhere

Finally having read about skimmed heads potentially altering cam timing, I'm curious as to how this can be because ignoring other issues like piston to valve intercourse, couldn't the head be lowered by any theoretical amount and the rotational relationship between the crank and the cam remain unchanged?

Cheers

Mark

[Motorsport Micky]

You could lap a thou or so off on a large sheet of glass placing the head onto a large sheet of 200 grit and see how the material removes, being alloy it should remove OK, I think our famous Flying farmer has already done some of this.

Micky

[Mark Serj]

Hi Micky,

Yep I was planning to do this if it was thought feasible by experienced people like yourself Micky so thanks for that. Looks like a visit to my bric a brac shop is needed tomorrow!

Cheers

[mole42]

I have a suitable piece of glass if you want it.

Richard

[Motorsport Micky]

Old mirrors are cheap. large enough and available at house clearance centres for a couple of quid.

Micky

[Guest]

I have a guy in Germany that went and bought a large block of marble. DEAD FLAT.

He had a Jag XJR V8 with a defective block, which kept blowing gaskets.
He put a large sheet of abrasive one side, and spent about 6hrs going at it until he had removed enough metal for it to be perfectly flat.

The engine remained in the car, and he refitted the head.
The engine runs perfect now, no more worries.

[Motorsport Micky]

I guess masking up the engine block "wet" surfaces might be important doing it that way.

Micky

[Mark Serj]

Thanks for all the input chaps. I went and bought the oldest and heaviest (therefore thickest) mirror I could buy at my 2nd hand local shop. Didn't take my straight edge out with me as was in a hurry to get a sunday lunch in and forgot it. Knew I should have left the mirror there for another time cos it's got an 8 thou bow in it! Bet all the other newer ones are worse! anyway I'm going to wait and see what it looks like post heating up for valve guide removal/installation. Shame I can't take it into work and use our surface tables, they are flat!! Thanks for the offer Rich, I my take you up on it later. I've been matching the manifold to head ports this aft, and doing a bit of head flowing work. Reminded me of my bike racing days but with loads easier access!

Regards

Mark

PS has anyone any experience of a reputable tuftriding outfit & crank grinders in the Bristol area please?

[Guest]

PS has anyone any experience of a reputable tuftriding outfit & crank grinders in the Bristol area please?

crank grind.
South Cerney engineering.
Ashton Keynes,
Swindon

(tufftriding/nitriding - do it in Birmingham)

Balancing again in Swindon area on another thread.

I have a lot of respect for Swindon (pigtown) based engineering.
They used to make steam engines there!

Honda made cars there.
(They even made all the MG bodies at PSF, but that's no recommendation! Most people don't know that.)

[jpyke]

...

Finally having read about skimmed heads potentially altering cam timing, I'm curious as to how this can be because ignoring other issues like piston to valve intercourse, couldn't the head be lowered by any theoretical amount and the rotational relationship between the crank and the cam remain unchanged?

...

Mark

No... because the chain doesn't shorten (or at least, the length of the links x however many links are in the tension side between the two sprockets is unchanged). For sure, the tensioners can all be adjusted to take up the slack, but the end result is that the camshaft will be rotated the more the head is skimmed. There's a little slop on the cam sprocket retaining bolts which can be used to get better alignment but unless the heads are really well skimmed and you're trying to get the best possible performance I can't see it's worth worrying about.

[Guest]

No... because the chain doesn't shorten (or at least, the length of the links x however many links are in the tension side between the two sprockets is unchanged)...the tensioners can all be adjusted to take up the slack, but the end result is that the camshaft will be rotated the more the head is skimmed. There's a little slop on the cam sprocket retaining bolts which can be used to get better alignment but unless the heads are really well skimmed and you're trying to get the best possible performance I can't see it's worth worrying about.

This is all wrong.
The more you skim off the heads the closer they get to the centre line of the crankshaft.
That means the more RETARDED the cam timing will be.
(it's the same effect as stretching the timing chains).

Effectively speaking that means the exhaust valve clearance at just before and at TDC will be closer and closer to the piston, because the inlet valve won't have opened so far....
however being as the INLET valve is so large, and inclined off the vertical, it means the clearance for the inlet valve to piston in practice becomes more and more critical.

As you will notice on any late (domed piston) engine if it's been revved a bit hard there's digs in the crowns of all 8 pistons.... If you close down the clearances further (by skimming head and block) it's certainly gonna have late timing as well as pretty critical clearance issues.

[jpyke]

This is all wrong.
The more you skim off the heads the closer they get to the centre line of the crankshaft.
That means the more RETARDED the cam timing will be.
(it's the same effect as stretching the timing chains).

Effectively speaking that means the exhaust valve clearance at just before and at TDC will be closer and closer to the piston, because the inlet valve won't have opened so far....
however being as the INLET valve is so large, and inclined off the vertical, it means the clearance for the inlet valve to piston in practice becomes more and more critical.

As you will notice on any late (domed piston) engine if it's been revved a bit hard there's digs in the crowns of all 8 pistons.... If you close down the clearances further (by skimming head and block) it's certainly gonna have late timing as well as pretty critical clearance issues.

I think you'll find I was saying "no" the OP's query that the cam timing was *not* changed by the fact the skimming moved the camshaft closer to the crank. I.e. I agree with you. I made no mention as to whether it was retarded or advanced, so "this is all wrong" is rubbish.

As you know (assuming you've seen my resto thread) I've had to go to special efforts to pocket the tops of my pistons to avoid collisions and made vernier sprockets to correct my timing, but then I have heads which are skimmed right down to the valve seats so this is unusual. What I was getting at is that if the heads are only skimmed within the somewhat conservative recommendations in the ROM, then I didn't think there would be a problem. And the small loss in power probably wouldn't be a worry to most. (Obviously I'm making assumptions on these last two points).

[Guest]

What I was getting at is that if the heads are only skimmed within the somewhat conservative recommendations in the ROM, then I didn't think there would be a problem.

And the small loss in power probably wouldn't be a worry to most. (Obviously I'm making assumptions on these last two points).

If you skim the heads you will raise the CR.
Normally speaking this would give an increase in torque.

I think I posted a simulation graph of changing the cam timing up to 5 degrees each way.
In general if your retard the timing you lose torque at the bottom end and gain a little at the top.
Advancing it tends to have the reverse effect, but these are generalisations.

You can also change the cam timing by changing the flow of the ports.
This is an exact correlation of timing off the seat and lift, but most people don't see it that way.

Actually it's totally off topic when talking about flatness, but of course Neil has commented already about the tendency of the Stag block/head to drop one end anyhow.
If you have a head shaped like a dutch edam and a block shaped like a wensleydale, how do you find a slab of ham the right shape to make a decent sandwich?

[Mark Serj]

I'm hungry now......

[marshman]

Yes the timing will change as the head comes down if you don't change the position of the cam sprocket, but surely when you reassemble the engine/heads etc. you line all the timing marks up and then refit the chain. If you can't get the chain to fit without moving the cams from their timing marks you try the cam sprockets in all their different positions and if necessary redrill/ fettle the holes? The only potential problem I see with heavily skimmed heads is possible contact of valves and pistons. If the situation is that bad simply fit a "saver shim". I think it is wrong to say it changes the timing.

Would it not be more correct to say that it may make it impossible to get the timing correct without modifying the holes in the cam sprockets?

Roger