Retitled: Gypsie's rebuild

[hambone]

Check the good vendors and see what they have available - CB Performance etc. Maybe even Raby knows.

[Gypsie]

chuckle...I'm sure Mr. Raby knows. I have looked through his site and find that he uses very special valves that are made primarily for him but not much is mentioned by who or specs.

I have been looking around and am stumbling on some spec issues. I have been corrected about what I thought was advice about enlarging the exhaust valves. The advice was actually to just make sure that I get high quality exhaust valves. I like this for the stockness of it (in case there is a roadside cylinder swap with an out of the box AMC head)

I see exhaust valves listed as 33 mm heads and some 38 mm heads.
http://www.tunacan.net/t4/reference/cylhead.htm
http://www.importperformanceparts.net/i ... srace.html
Perhaps the larger ones presume that the valves have been upsized or I am missing something.
I will keep looking around.


Ok so I was stumbling through the threads and saw this line
"A fresh engine does not have much in the way of blowby making it into the crankcase past the rings and valve guides, a used high mileage engine will."

Mhmmm, think I. Makes Sense... Then I remember that in my checking/testing processes I have pulled my crankcase breather off and even at idle I have a pretty steady puffpuffpuff of air blowing out of the hole, even at idle.

In theory a newly rebuilt engine should have no blowby. I am a bit concerned and will look closer at this when I get home. geez...

[Gypsie]

Heads removed and being inspected and TRW exhaust valves installed.

redid deck height and Head CC calc's.

DH for all four: .059"

CC for all four: 56 cc's

with stock bore and stroke (94mm X 71mm) i get 8.4:1

Agreed?

Isn't this a good CR?

Now what if I said I have dished pistons? I have not cc'd the dishes but I am opining around 12cc's I can do it if need be.

I am not certain about the crank being stock and have presumed it to be so. Any tips on gettin' a stroke measurement. Seems like I could measure to the bottom of the stroke and subtract the DH to come up with that.

I am certain about piston size.

I did find that the #3 had a head leak though I could find no glaring issues with the mating surfaces.

Could it be that the permatex at the base of the cyl's could have compressed/squitted out over heat cool cycles to allow enough room for this cyl head leak?

is there anything I can put on the top lip to help with a seal when I reassemble. I seem to recall seeing someone mentioning something that could be used....

One more thing. Any ideas about the puffing from my case vent? Should I be pulling the cyl's off the pistons to see if rings have lined up to allow this blowby phenom? the intake sboot and plenum are wet with oil and a couple cyls had some oil pooling (teaspoon or so).

The breather valve is NLA. Ideas about rebuilding? I have a spare...(they both 'leak' ie are no longer 'one way')

[Westy78]

8.4 is on the high side. Shoot more for 7.8-8.0. Piston dish cc must be included in the calculation for accurate deck height figures.

[Gypsie]

I have been seeing that the 94 dished mahles have 12 cc's in the dish. I will measure tonight to confirm. That would put me at 7.3:1 with current conditions. This would explain the low compression. (And this is where i likely screwed up my original calcs.....doh....)

So to get to close to 7.8:1 I need to thin down my base shims. ( they are .040" now.)

So if I use the .010" that will put DH at .029"

Thereby leaving me at 7.7:1

Better? too shallow? piston slap?

Or...
No ring that will leave me at .019" and a CR or 7.9:1

Seems way too close. I am thinking that the only way to increase CR with these parts is to add material in the Head. I think I will just have to live with 7.7:1 if that is what I end up with.

The calcs will be redone when I get the heads back but I think I am catching on....

[hambone]

Busy bee, you.
I think 7.7 is good, I believe mine is 7.9. At least you checked.
Hambone Springs?

[Westy78]

7.9 would be fine and not using the shims is one less point for leakage.

[Oregon72]

It seems I remember AC Ranch saying that a finished deck between .040" to .060" was right where you want it and the closer to the .040" the better, but also that you never want to go less than .040". If your heads were not flycut too deep, only then can you eliminate the head gasket. After you determine whether or not your heads were flycut too deep, you can then adjust your deck with the appropriate base shim (goes between the case and cylinder) thickness. My final compression ratio came to 7.5 and my deck was .045". Good god did I toil over this crap when I built my engine - still had fun though.

[Gypsie]

Jasan: I was hoping someone would say that the base gasket could be omitted. That would increase some CR and eliminate a leak point.

Troy: Thanks for posting.

When you measured your CC's, did you have sealing rings in place?

at 7.5:1 you are just above where I figger I am currently. Just before dissassembly I had 90psi in three of my cylinders (which seems low) and no evidence of head leaking except from the number three cylinder (which gave a 30psi reading).

I am wondering if I should plan on disassembling pistons/cyls to inspect and assure that the rings have settled in good? (they were replaced this last time around even though the pistons and cyls were new with less than 500 miles on them.)

Trying to find out why I have low compression.

I am wondering if anyone has an opinion about how much compression readings go up or down with each .1:1 (i.e how much difference would there be between 7.3-:1 and 7.4:1?)

Just to clarify, I am using new cast AMC heads with a step cut (ie a ring around the interior of the head that is the mating surface of the cylinder and the head that mimics a sealing ring). The heads have never been fly cut. I haven't measured the thickness of the step an will when it comes back from the shop.

I too have heard that .040"-.060" is desired to prevent the piston slap phenom that occurs during engine operation if the deck height is too low. The theory being that during operation the piston is thrown to it's furthest reach out from center and the gaps in the connecting rod bearing to crank and wrist pins to connecting rod are eliminated. That and the changes in gap due to heat caused expansion of parts.

Here is a question: Does the .040"-.060" measurement include the thickness of the head sealing ring?
Edited to add:

And the answer, as found in one of the many emails I received when I was going through this process last fall, is yes. I believe that I have found my main culprit for low compression. Bad math and loose marbles....

I will double check the math when I get the heads back with their new exhaust valves in them in case they change the cc's (likely not that dramatically).

I think I will be able to eliminate the cylinder base ring and that should leave me with .019" DH (that and the .040" cylinder head step will put me in the .040"-.060" DH zone), which should put me in the 7.9:1 CR zone...

-Gypsiewithclumpsofhairinhand...

[Westy78]

Jasan: I was hoping someone would say that the base gasket could be omitted. That would increase some CR and eliminate a leak point.

They aren't really a gasket but a shim to fine tune your deck. I didn't use any of them. I also didn't use any head gaskets. Just lapped the cylinders into the heads and used some Permatex copper spray gasket.

[Gypsie]

They aren't really a gasket but a shim.

Absolutely. I was shooting a little loose with language there.

The spray gasket stuff is what I was looking for for the head side mating surfaces.

Now if the heads could bet finished....

[Bleyseng]

You don't use the spray gasket stuff on the head to cylinder mating surface. Lap the cylinders to the heads so the seal is perfect without any gasket as you want it to be flat. Its kinda like the valve to seat surface as you want it to seal as it moves.
You are sure they aren't flycutting the sealing surface on the heads? That's standard practice on head rebuilding to make sure the heads are flat again. Double check all your measurements by CCing the heads and take into account if the step is still there to figure out your CR. Then adjust it with base shims...

[hambone]

I've always installed the cylinder to head mating area dry, after lapping.

[chitwnvw]

Can you use feeler gauges to judge the deck height?

[hambone]

Yep. Under a socket extension.