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Old 07-06-2017, 07:13 AM   #1
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Lehman 120 head machining

Manual says not to machine head. Amercican Diesel I believe said up to 0.009.

Machine shop says way more (up to 0.040) typically done with no issues. Shop is very experienced in modifying gas engines for tacing but has good experience with marine diesels too.

Any experiences or strong opinions on head machining then the whole replacement process?

Other than super clean surfaces and dont use any abrasives to clean?

Anyone have this done or research it with different info?

Thanks....
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Old 07-06-2017, 01:41 PM   #2
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Bump....any tips at all?
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Old 07-06-2017, 02:07 PM   #3
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I have no experience in that regard with this engine.
But......
I would ask ADC why THEY put the limits at .009".
Is it a clearance issue?
Does it change the compression ratio to where it over stresses the internals?
Etc.
They may have the "local knowledge" that the performance guys don't have about this particular engine.
How far out of flat is the head? Or are you anticipating?
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Old 07-06-2017, 02:16 PM   #4
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Originally Posted by psneeld View Post
Manual says not to machine head. Amercican Diesel I believe said up to 0.009.

Machine shop says way more (up to 0.040) typically done with no issues. Shop is very experienced in modifying gas engines for tacing but has good experience with marine diesels too.

Any experiences or strong opinions on head machining then the whole replacement process?

Other than super clean surfaces and dont use any abrasives to clean?

Anyone have this done or research it with different info?

Thanks....
Here's my take on it: The factory says not to do it. Ok, why? Is it a clearance issue? A compression issue? A material thickness issue? Or maybe something none of us has thought of yet?

That said, unless one is trying to change compression or some other characteristic, its best to take off as little as possible. If .005 cant clean the surface and make it flat within spec, maybe its best to opt for a different head.

Ken
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Old 07-06-2017, 02:16 PM   #5
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Anticipating, and wondering where ADC got the 0.009 when the manual they refer me to says no.

I have a feeling like many things Lehman, there has been evolution without written updates all located in one spot other than Brian now...though there are others familiar with Lehmans that have similar or even diffetent answers.

If the machine shop takes off more than what I am comfortable with, we will sit down and see just what the numbers wring out. They may be insignificant or to limit comoression, etc, I will put stops on the engine controls....or break down and buy another head.
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Old 07-06-2017, 02:48 PM   #6
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Anticipating, and wondering where ADC got the 0.009 when the manual they refer me to says no.

I have a feeling like many things Lehman, there has been evolution without written updates all located in one spot other than Brian now...though there are others familiar with Lehmans that have similar or even diffetent answers.

If the machine shop takes off more than what I am comfortable with, we will sit down and see just what the numbers wring out. They may be insignificant or to limit comoression, etc, I will put stops on the engine controls....or break down and buy another head.
I think you hit it on the head with the evolution theory.
We don't know but perhaps someone asked the Ford engine engineers what limits there were on machining a warped head (this happens often in eninge rebuilding) and being ultra conservative they responded with .009". They could NOT respond with a number that might not work, they had to give a number they were positive would be ok. Perhaps they even tested it. And maybe at the time that was enough to work and the question never came up again.

Some day we won't have the option of buying a new head, then what?
Yes we will machine the head .010" or whatever is necessary and using reasonable skills give it a go.
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Old 07-06-2017, 04:05 PM   #7
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Manual doesn't say NOT do it, it simply says NOT RECOMMENDED.
Claims it might cause fowling between valves and pistons. I suspect Brian has sold enough head gaskets and heard enough anecdotes to feel comfortable with .009.

I'm guessing that engineers over designed the engine and if .009 will give you a flat head then go for it. The valves might protrude a little more into the combustion chamber so that might account for the potential fowling. .040 might also work, but clearly the less skim the better.

Have you measured the flatness of the head to see how much it's off?
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Old 07-06-2017, 04:10 PM   #8
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Correct on the not recommended part, went back and reread it.
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Old 07-06-2017, 08:35 PM   #9
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Are the head gaskets on these engines composite or just steal. It would not be unrealistic to stack two steel head gaskets on top of each other
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Old 07-06-2017, 08:50 PM   #10
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Greetings,
Mr. 30. Stack 2 head gaskets? I know this is acceptable with some gasoline engines but I really don't know if it would work with the much higher compression ratios of a diesel. Maybe one could find/make/have made a single thicker gasket???? Beyond my technical knowledge for sure...
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Old 07-06-2017, 09:51 PM   #11
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Are the head gaskets on these engines composite or just steal. It would not be unrealistic to stack two steel head gaskets on top of each other
Just what I was about to suggest, as a cure if the head planing threw up problems.
It`s not like the Ford Lehman has domed pistons likely to collide with 4 valves per cylinder driven by quad overhead camshafts.
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Old 07-07-2017, 05:11 AM   #12
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Many advise not using regular rags to wipe things down because of dust and lint even for gas engine gaskets.

It would be hard to believe stacking 2 would be OK, especially with the metal ring around each cylinder.
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Old 07-07-2017, 07:39 AM   #13
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I would not use 2 gaskets.
I don't have the service manual available. What is the spec for flatness?
You may be within that right now and all this discussion is moot.
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Old 07-07-2017, 10:31 AM   #14
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Just talked to Bomac and they said 0.015.

More numbers....

Jay, couldnt find a spec because the manual I have says machining not recommended, and any bowing or warping, the head must be replaced.
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Old 07-07-2017, 11:30 AM   #15
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Just talked to Bomac and they said 0.015.

More numbers.....
Greg has probably rebuilt more Lehman's than anyone.
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Old 07-07-2017, 12:02 PM   #16
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Just blows my mind how there is no definitive answers to some simple questions. Especially amongst the experts.

Sure after 50 years of changing specs, there might be some confusion.

Some enterprising person might do well with a Lehmans For Dummies Book....
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Old 07-07-2017, 12:03 PM   #17
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Just talked to Bomac and they said 0.015.

More numbers....

Jay, couldnt find a spec because the manual I have says machining not recommended, and any bowing or warping, the head must be replaced.
The manual I have says the same thing. The concern stated in the manual is that it is possible for the piston to contact the valves, but obviously there must be *some* room and folks like Bomac must have a lot of experience in this. Do you know if your head is warped and if so how much?

Ken
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Old 07-07-2017, 12:06 PM   #18
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Waiting for the machine shop report.
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Old 07-07-2017, 12:20 PM   #19
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Should be no issue shaving the head. It won't change compression as the thing is flat anyway.

The thing that changes is valve face position. If you shave the head the valves will now be closer to the pistons. On non-firing TDC, both valves are cocked open slightly and they are very close to piston.

In the manual there should be a spec for valve face depth WRT head deck. If after the shave that number is out of spec, the valve seats need to be ground a little to get it in spec.
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Old 07-07-2017, 12:22 PM   #20
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Milling the head will have negligible effect on compression. But if you take off more than 0.009" or maybe 0.015 if you believe Bomac, then I do believe you have a risk of valves interfering with the piston. The consequences will not be good.

You could put a little putty on the top of the piston on one cylinder and lightly oil the valve head to keep it from sticking, assemble and tighten the head and then rotate the crank, remove the head and check the clearance. The putty should have depressions made by the valve heads and you can cut a groove with a razor blade to see how much putty is left.

David
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