Fly Cut Head Question

[lespaul]

I usually hang out on the main forum, but thought I would come over here with a specific rebuild question.

78 SC 3.0L. I am in the middle of a top end rebuild due to exhaust side head stud breakage on an engine with 35K miles on it. The heads have minimal erosion damage from PO driving with broken head studs. My machine shop says he will make a minimal fly cut on the heads to clean this up. We are guessing this will be in the 0.1 to 0.15 mm range. I asked whether I should shim up with thicker gaskets and he said there was no need.

I am putting on the standard 0.25 mm gaskets. So, if I would shim up, are there 0.35 or 0.40 mm gaskets available? Or do I stack a 0.1 mm or 0.15 mm on top of the 0.25?

Plus's with no thicker gaskets = some slight compression gains.

Negative seems to be some reduced clearance from piston to head and/or valves but at that small fly cut, is that a real risk?

I suppose that there may be some negative impact on the alignment of the cams with the chain housing opening but is that a real concern?

I would appreciate any observations/advice from those more experienced here.

Thanks

[911pcars]

It sounds you're within the normal parameters of rebuilding specs. You should have adequate valve-to-piston clearance, minimal uptick on compression ratio, etc.

Do you know the previous history of the engine?

The total cylinder, cylinder head, cam housing "stack" height could have an affect on how the camshaft centers in each chain box opening. Ideally, the chain box mounting surfaces should be milled an equivalent amount as the total removed from the stack above.

For example, if the total stack height is reduced .25mm due to resurfacing of the parts above and/or as a result of truing the crankcase spigots, then .25mm should be removed from each chain box mounting surface. The alternative is to install thicker cylinder base gaskets.

Sherwood

[lespaul]

Thanks Sherwood. I believe this is the first rebuild -- thus, no previous fly cutting or resurfacing.

The chain box concern is legitimate and worries me. I will ask machine shop about that once I know exactly how much he fly cuts off.

[cgarr]

I think porsche limits head cutting to .25mm so that tensioners and chain boxes remain stock there is enough room if that's all taken off. Now for some of us adjusting boxes etc is not a big deal and I have taken heads down .40mm on some race motors but other accommodations need to be made.

[911pcars]

Just to piggyback on Craig's comments. My previous engine had sufficient material removed to warrant milling the chain boxes. In addition, the reduced stack resulted in the timing chains becoming too long for the normal range of the chain tensioners to accommodate. Per B. Anderson's one sentence suggestion in his book, I sourced slightly oversize idler arm sprockets German Precision, Ted Robinson).

Sherwood

[Walt Fricke]

Contact EBS to see what cylinder base copper gasket thicknesses they carry. I think they have a 50, which avoids stacking (which some think is bad practice) after the kind of standard fly cutting. Better to keep close to the stock dimensions to avoid messing with the crank to cam spacing.

But the cam housing can accommodate a certain amount of difference. The large rubber O ring can squish on one side and still do its job on the other.

If concerned, start with the stock base gaskets and do a trial fit. Slip in cylinders 1 and 3, no pistons yet, and bolt on those heads. Put the cam carrier on and use a few of its fasteners. Then put on the chain box with standard gasket, and snug it down. Install a cam (any cam) and see if you can install that round aluminum cam end plate - the one with the 3 6mm bolts - with the O ring on it. If you can work it in and snug it down you should be OK.

[lespaul]

Great advice and suggestions all! Much appreciated. Thanks. I will report back on how this shakes out.

Brad

[safe]

I cut the heads 0.15 mm on my 3.6, didn't need to touch the chainhousings. The O-ring fit just fine. Porsche specifies that you can cut the heads 0.2 mm, at least on a 3.6.

[911pcars]

Originally Posted by Walt Fricke

Contact EBS to see what cylinder base copper gasket thicknesses they carry. I think they have a 50, which avoids stacking (which some think is bad practice) after the kind of standard fly cutting. Better to keep close to the stock dimensions to avoid messing with the crank to cam spacing.

But the cam housing can accommodate a certain amount of difference. The large rubber O ring can squish on one side and still do its job on the other.

If concerned, start with the stock base gaskets and do a trial fit. Slip in cylinders 1 and 3, no pistons yet, and bolt on those heads. Put the cam carrier on and use a few of its fasteners. Then put on the chain box with standard gasket, and snug it down. Install a cam (any cam) and see if you can install that round aluminum cam end plate - the one with the 3 6mm bolts - with the O ring on it. If you can work it in and snug it down you should be OK.

Using thickish cylinder base gaskets to avoid a possible off-center fit of the cam in the chain box may upset the deck height which might require further mechanical adjustments to re-establish the desired deck height spec. Remember to calculate gasket crush of the head gasket and cylinder base gaskets. The amount of fussiness depends on whether it's "good enough" or if "best" fit is the goal. YMMV.

It's a good time to confirm all surfaces are on the same plane (crankcase spigots, installed cylinder and cylinder head height) so all cyl. head sealing surfaces are flush with the cam housing (potential oil leak path).

.... or if borderline material removal from the above parts was calculated, just continue to final-assemble the piston/cyl. assemblies, cyl. heads and cam housing, then trial fit each cam and chain box to determine exact adjustments to perform on the respective mounting surfaces.

Sherwood

[Walt Fricke]

Sherwood - what engines other than the 2.0s had head gaskets which might crush? Seems to me that the CE rings are just not a factor here, as they are designed to crush until the surfaces mate. Or the 3.6s different?

Thicker base gaskets, if they match the flycutting done on heads, should leave you with the same squish height/piston to head clearance. The fact that the deck height may be greater is made up by the head moving down.

Because the head is hemispherical, flycutting is going to move the flat part of the head mating surface in a bit. If that is significant, the piston edge can be relieved, or the edge of the head dome can be relieved to keep things where they need to be. But it seems that with the tiny cuts involved you don't have to worry about this geometric effect.

[911pcars]

Originally Posted by Walt Fricke

Sherwood - what engines other than the 2.0s had head gaskets which might crush? Seems to me that the CE rings are just not a factor here, as they are designed to crush until the surfaces mate. Or the 3.6s different?

Other Porsche engines unknown. The effect gaskets have on the overall height of the "stack" may or may not be relevant.

Thicker base gaskets, if they match the flycutting done on heads, should leave you with the same squish height/piston to head clearance. The fact that the deck height may be greater is made up by the head moving down.

Hmmm. If the deck height is "x" with a standard cylinder base gasket, then a gasket is installed to restore the flycut amount ("x" + .25mm), then the deck height changes. If the builder knows that flycutting the heads is the only material removed from a virgin engine, then one could assume with some certainty that the factory stack is restored by an equivalent cyl. base gasket. Not sure how many engines fall under the virgin category when the engine is possibly 30+ years old.

Because the head is hemispherical, flycutting is going to move the flat part of the head mating surface in a bit. If that is significant, the piston edge can be relieved, or the edge of the head dome can be relieved to keep things where they need to be. But it seems that with the tiny cuts involved you don't have to worry about this geometric effect.

Perhaps. but w/o checking, some builders might assume it's "good enough". Years ago, I purchased what seemed to be a pro-rebuilt race engine with unknown history. Upon dis-assembly and inspection, I noticed the cams were no longer centered within their respective cam housing openings.

After changing the crank, rods, machining the crankcase and heads, I could have trial-assembled the entire long block just to determine the needed cyl. base gasket thickness, but at the time, it either seemed like a lot of wasted labor or something I may have discovered only after going through all the final build motions up to installing the chain boxes. Probably the latter as it was my first build.

My options were to adjust the chain boxes at that point or start over by removing the entire top end and cylinder assemblies and install a 0.040" thick base gasket (or whatever) and in my case, further reduce the compression ratio of an already Porsche-optimistic set of RS cylinders. The deck heights were already set for standard cyl. base gaskets.

From this discussion, a builder might pursue one of three paths to ensure the cams are centered in the cam housing:
1. Calculate known amounts of removed material to re-establish the factory stack height, then compare with the relevant chain box dimensions. BTW, not sure there's a spec for the factory stack height. It's probably published somewhere, but I haven't stumbled upon it.
2. Trial-assemble the long block to calculate the needed cyl. base gasket thickness.
3. Assemble the long block (with gaskets, sealers, etc.), then adjust the cam boxes accordingly.

I think I may have exhausted the hell out of the topic. My apologies, but welcome further discussion and scrutiny. I can learn something new everyday.

Sherwood

[snbush67]

If your engine is in an untouched state then 0.1 - 0.15mm is a frogs hair, nothing to write home about, this change will likely reduce your head volume by less than .5 cc. Potentially it may raise your CR a bit but you will still be under 9.5:1 on a stock US 3.0L motor. There should be more than enough clearance, likely you will see 2mm of squish height, you can go much closer without concerns.

Yes you can stack cylinder base gaskets, I think they come in .25, .50 or 1.0 mm thicknesses.

Measure everything twice.