Shaving the head, how much is too much?

[mojorising84]

Quick question for the hive mind, how low can you go on the head? The tech wiki lists no more than 60 thousands from stock but looking at the head I have now, which is already shaved 60 thou and is 22 thou warped, I still see quite a bit of meat left. The head at one point had a catastrophic head gasket failure and was unfortunately left with moisture in the combustion chambers. I was planning on carving out the head a bit to remove the oxidation and reduce the compression ratio to compensate for the probably additional 25 to 30 thousands needed to get things square. Am I in danger of hitting an oil or water passage?

[redroadster]

What engine ? If 2000 the warping affects the cam of course

[mojorising84]

Yeah that's good info to have. R16.

[keith0alan]

In my experience very few shops will bother to cut the head square. Measure the thickness of each corner and set it up on the mill so that all four corners end up the same thickness. Obviously, the back of the head will also be warped. This will lead to cracking the valve cover. Remove valves, etc. and mill the back side flat also. Sit the head on the engine with no head gasket using a couple studs in place of head bolts to locate. Turn the engine over. If the pistons bump the head then remove head material where they hit. A little grease on the piston crown will help mark the spots. This is a very labor intensive process. This carving of the combustion chambers will leave the squish area perfectly matching the piston crown allowing you to run lots of spark advance. Once the head matches the pistons then measure the combustion chamber volumes and match them by removing material from the valve pocket area. I have carved a lot and never had a problem with hitting water jackets or ports. Also the port side will also be warped. Mill it flat also. When assembling the engine be sure to measure the piston to valve clearance. Shouldn't be a problem with stock cam. Check the rocker to valve tip wipe pattern. I haven't had a problem but you might need to shim up the rocker shaft towers.

[JT68]

Right good info there, especially about the top side! So many valve covers are busted because folks don’t realize the head top is bowed and they keep tightening the cover to stop a leak until “CRACK”!

The rule of thumb is .030” inch off the bottom (head gasket face) max if you are running stock domed pistons and standard head gasket. (After that the pistons will hit the head unless using our mini-dome pistons)

You can cut about .05-.06” of the HG surface with no combustion chamber if using our .100” gaskets.

You can cut about .100 off if using flat top pistons or domes+ machining the combustion chambers, but with domes the CR can get out of hand.

If you cut more than about .100 you will get into the arena of needing shorter custom made push rods. We also have shims for raising the rocker pedestals if needed, but only so far with that.

AND we can mill the combustion chambers to clearance them and even up the CR, but as mentioned that gets pretty labor-intensive.

At some point, it makes a lot more sense to just buy a better head.

[redroadster]

You can double the head gasket with aviation sealer between

[JT68]

That’s a very temporary fix because the sandwiched fire rings won’t seal compression long term-especially if the compression ratio is increased. It’s not a GOOD solution, but may work for a while. Ultimately compression will blow through to a water passage and fail.

Also all roadster heads are prone to leak at the rear oil galley passage, doubling the gaskets doubles the odds of a leak there too.

As mentioned, a better head is always the best answer.

[mojorising84]

Thanks all! I appreciate the input. I think I will work with what I have for the moment for a few reasons. One, I got the head for free so to me it's worth some experimenting. Two, I've got a 3d scanner coming soon so I hope to be able to scan things and see how they look before and after. Hopefully over the Christmas break I can do some CAD work and run some CFD. If I'm able to get some results I'll definitely share them here.

[JT68]

I doubt you will run into any passages if you keep the carving to the minimum required- just enough to clear the pistons with a bit of a safety margin. (Don't hog it out deep in any one spot.)

The thicker gasket is not much more costly than 2 high quality head gaskets and saves tons of work:

https://www.datsunrestorationproducts.com/shop-31

[redroadster]

That’s a very temporary fix because the sandwiched fire rings won’t seal compression long term-especially if the compression ratio is increased. It’s not a GOOD solution, but may work for a while. Ultimately compression will blow through to a water passage and fail.

Also all roadster heads are prone to leak at the rear oil galley passage, doubling the gaskets doubles the odds of a leak there too.

As mentioned, a better head is always the best answer.

That was 15 yrs ago ,still no problem &I've got high domes , used to drive 500-700 miles a yr
I talk to the machine shop to heat straightened it , but they had a 19 yr old just mill it
At the U haul RPS I worked they occasionally did it on rebuilding engines with a steel shim plate between them ,I had dealers that found out about it wanting that shim so they could do it on many of their trucks ,454 &460
You can put a S steel shim on the trued deck too, no sealer ( I was skeptical they at the plant had done it before know what works ) with 1 gasket
Having them made cnc plasma machine ,needs a 4x4ft steel so $$$

[JT68]

Regardless of the N=1, Most double gaskets do fail and it is an definitely an amateur technique. Professional engine builders never recommend it and I certainly would never do it on any customer engine.

Sure it works sometimes.

Even with the minimal use of 500-700/year, I'd bet its leaking compression or water somewhere without your knowledge.

It is certainly OK as a temporary fix IF it doesn't leak somewhere. 50/50 - but also a PIA to re-do if it does.

It also should be disclosed to anyone buying the car or engine since no one I know would want to buy a roadster engine (or any engine) running two head gaskets lol. Who wants to inherit that problem?

With engine builds and roadsters in general "Cheap is the enemy of good". Be a pro, do it right. Those are words to live by.

[Gregs672000]

JT, it looks like you have developed some additional head gasket options that we're not previously available. Thanks! Before I machined the pistons a bit more I would have strongly considered a copper or thicker composite gasket as an option, but I think my compression is more reasonable now.

[keith0alan]

What doing a thicker head gasket doesn't do is correct the out of shape squish area. On Brian's race car the head was carefully matched to the piston domes. We kept adding more spark advance and getting more power. The thing just wouldn't ping. That of course what the squish band area is for.

[JT68]

Correct, if you are going to do it right, it is best to machine the CC shape so that all four are fairly similar and fairly close in CR-that takes many hours. Regardless, if the head had been cut a lot the CR will get too high for street gas (especially 87 octane) if you use a standard gasket. The only solution for that is the thicker gasket, another head, or reducing/eliminating the domes. (most street guys don't want to run 97/100 octane fuel).

[JT68]

JT, it looks like you have developed some additional head gasket options that we're not previously available. Thanks! Before I machined the pistons a bit more I would have strongly considered a copper or thicker composite gasket as an option, but I think my compression is more reasonable now.

Yep, thank you. We wanted some robust solutions and these work pretty well. Several guys have used them to prevent having to start over on another expensive head. One saved a race engine that kept blowing Nissan gaskets-those guys were very pleased I've used each type on test engines as well.

Cutting off the piston domes is certainly a good method for correcting CR and that is a good plan, but on the U20 the thicker gaskets give the added benefit of taking up chain slack.

Also, we just came out with some .088" gaskets for heads than have been cut a bit, but don't need .100"! I'll put them on the site soon.