Cylinder head flatness - is milling really necessary

[rcb280z]

A head that's "bent" down doesn't loose any material that would be lost if milled. 

 

A head that's "milled" looses material requiring more head work. Shimming cam towers etc. But you know that already so I think I misunderstand the "essence" of your question. Sorry.

Edited October 18, 2015 by rcb280z

[EuroDat]

I have heard over here a lot of machine shops won't touch a warped overhead cam head until its been in the oven on a straight plate. Its just too risky. Its needs to be straightened, depending on how its warped, and the lightly skimmed.

You have about 0.15mm which doesn't sound that bad for a six cylinder head. It's a bow over a long distance compared to a four cylinder or a V6.

Machining it flat will cause cam flex, depending on how much warp it had, which could eventually brake the cam. The only thing you can do to fix that is a cam bore alignment.

Common for an old car is just put it back on "as is" and a little more torque on the inner bolts like chickenman said.

I have seen a technician at work hobbying on his old Opel inline six head.

He used a piece of 200mm channel steel and drilled 6 holes using an old head gasket as template. Mounted the head on it using spacers under the outer four bolts. That way he could get his straight edge under the head to check it. Then tightened the center two until he went just passed flat actually creating a reverse bow inthe other direction. Left it under a bar heater for a day, loosening the two center bolts and checking it every couple of hours with the straight edge and then let it slowly cool down.

He managed to get it within spec so the shop would skim it for him and it was pretty bad when he started. You could try something like that on the block.

[Mark Maras]

 It would be interesting to know if the cam spins freely with the head off & on the engine. It might lend some clarity to all of our guess work. That said, I'd still put it on as is. It seems like the worst that could happen is a blown head gasket. I would want the engine to have a very good cooling system to give the head gasket half a chance.

[EuroDat]

Checking if the cam spins freely may not be enough. The can journals have a maximum clearance of 0.1mm which would give you a fear amount of tolerance.

The problem is that the cam doesn't run in this condition, but runs with the valve springs forcing the cam upwards. This will press the cam up towards the top of all the journals and causing the cam to bend ever so slightly per revolution. Its not much and won't have any immediate effect, but fatigue will eventually take its toll and break the cam somewhere in the middle. Then the back end will punch a couple of valves through the pistons......

 

I made a quick exaggerated sketch to explain it better. 1. is free spinning and 2. is under normal conditions.

 

I agree with Mark. Its Bob's spare engine, so I would use something like A4 printer paper folded double under both ends of the head and tighten it down on the centre bolts. That would give you a very slight reverse bow and leave it like that until you want to use it.

[Patcon]

But if the head is fully assembled the springs are in place so doesn't that mean if it spins freely untorqued it will only get better as it gets torqued and flattens out?

[Mark Maras]

I can't help but think torquing the head down flat would return the cam clearances back to normal. 

[djwarner]

The force required to distort a head casting 0.007" would be surprisingly small. If you consider the pitch of the thread on the head bolts as an inclined plane, the torque is multiplied by 10-20 times when applying clamping force.

Assuming the head was flat when it was clamped in place prior to the overheating and no maintenance was done to the head after it was removed, you have a reasonable chance of restoring the cam tower alignment.

[EuroDat]

I was generalizing on why not to shave a ohc head. A warped head is not good, but shaving it to get a flat surface is only going to solve one problem and create another.

If Bob would use the head as it is now it shouldn't be a problem for the cam. The headgasket between 3 and 4 might be a possible issue, but you could compensate by overtorqueing the center group of head bolts.

Bob's head in not that bad. I have heard 0.15 is ok to use on a six cylinder. The opel head I mentioned was a lot worse, somewhere around 0.3mm might have been more, but it 4 years ago.

The point I was trying to make is: Shaving an ohc cylinder head should not be done to correct warpage. It needs to be straightened first. Some machine shop don't realise the difference between an ohv en ohc heads.

[madkaw]

Last time I mentioned here ,the fact I was going to straighten a head ,it was considered crazy talk. The fact is there is few shops that will do this.

Usually just told to find another head.

Bolt it down with some shims and throw it in the oven with the chicken dinner and see how it comes out. Cook it low and slow though!

[Mark Maras]

 OK. Now that we're finally coming around to your solution, did you ever try it? I remember the talk about it but I think everyone was concentrated on milling & the unintended consequences at the time.

 Rolling the idea around in my head, oven size came to mind. I know my oven isn't large enough to slow bake a six cyl. head & if it was I would have to send my wife out of town. But any powder coating business has an oven that could. Temp 400 - 450. Sound OK?

[madkaw]

No, I never did try this. It seems all that I have read says 400-500 for 4 or 5 hours. I was also considering a powder coating shop for the job. Need to make a hefty fixture to bolt down the head to.

My head is probably. 030 out, so it would take good size shims.

I came across 2 more good heads so there was no need to rush into this fix. I'm going to use my warped mn47 for the flow bench so it doesn't matter much

[Mark Maras]

Just watched a guy on UTube straighten a warped six cyl. head. .007 brass shims under the ends, baked it 5-6 hours at 500` and let it cool in the oven. He baked it twice until the cam towers were aligned then sent it out to be surface milled.