Cracked heads and pre-cups

[rjschoolcraft]

I've found that both heads are cracked in every combustion chamber between the valves. I've reviewed the repair procedure found here (http://www.thedieselpage.com/members/gmheads.htm) and am investigating. I also found that the pre-cups are all cracked, fairly significantly. More evidence of extreme temperatures... Here are some pictures:

Pre-cups from #6 (right) and #8

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000758.JPG

Close-up of #6 outside

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000761.JPG

Close-up of #6 inside

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000765.JPG

Close-up of #8 inside

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000772.JPG

As you can see, #8 is about to lose a chunk. I plan to replace all 8 pre-cups.

Here are two shots of the left bank head. These cracks are typical of all combustion chambers.

Chamber #1

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000776.JPG

Chamber #3

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000777.JPG

The manual says not to resurface these heads because of extremely close piston to valve clearances. However, if I'm running 18:1 dished pistons, that shouldn't be a problem should it?

[gmctd]

You'll need to check JK's piston configuration, see if the un-dished circumference area interferes.

Iirc, the Pen pistons are reduced only 0.055" in crown heighth.

Fel-pro and others make a 0.010" thicker head gasket, meant for resurfaced heads.

I've been wondering if the late heads were really reworked in that area to improve coolant flow, and if so, should be possible to install the larger exhaust valve from the '82 heads.

[grape]

808.14 cc's per cylinder stock + 39.5 chamber space (gasket, dish, and head)= 21.46 to 1

By my math if Pen took .055" off of the whole crown of the piston they'd be down to 16.8 to 1 with a stock thickness head gasket.

[gmctd]

Does that 39.5cc include the pre-cup volume?
And is "dish" the 'cup' in the piston crown?

Actual compression is listed variously as 21.1 or 21.2, and the later engines were variously listed as 20.1:1, is point of my question.

That looks to be roughly 1 point per 0.010" reduction in 4"dia crown heighth, right?

With the 0.010" offset pistons, and the +0.010" head gaskets, 19:1 should be within economic reach, if the gasket fire ring would hold up under severe useage.

The hydraulic lifters should compensate, since the valve-springs are 0.010" further away, and the 'regulating' relief is by valve-spring pressure against oil pressure.
Shouldn't lose any valve lift.

[dieseldummy]

RJ, If you have your valves out yet have you noticed hairling cracks extending from the seats outward toward the head gasket? Every set of 6.5 heads I have seen in person tore down have these cracks in them... Except one set that is currently in my pickup that I found as a result of luck...

[eracers999]

According to techs at victor and fel pro the .010 taller gasket should NOT be used if your plans are to run in the hi boost end.

Kent

[Kennedy]

RJ,

What is the compressed thickness of you old head gasket at the fire ring?

Piston to valve clearance is easy to correct for surfacing. The valves and seats need to be ground anyhow. The installed height of the valve will take care of this.

PS. GEP told me that they have actually been pushing the piston up CLOSER to the head as this helped with emissions...

[rjschoolcraft]

I measured them in several places on both gaskets with a dial caliper. The thinnest reading was .044" and the thickest was .050". A larger range than I would have expected.

[Kennedy]

I forget, yours are later model production. I was thinking you'd say .039"-ish...

[gmctd]

So does that mean the late engines are at 20:1?

[rjschoolcraft]

I don't remember if I've posted this before or not, but my heads are finished.

Here's a picture of the left head:

http://www.schoolcraftpowertrain.com/Pictures_&_Data/Failed_6.5/IM000780.JPG

Notice the radial cracks coming out of both valve openings.

[tom.mcinerney]

1.) Great fotos, Ron. The color in addition to the resolution is outstanding. Is this in daylight? My colors are usually washed out.

2.) Regarding the issue of head milling VS piston excursion/clearance: On a used set of heads that i nearly attempted to rebuild, it became apparent that the valves had pounded the (malleable iron?) seats into submission, so that the valves were seating too low (high?) in the head to meet specs. [Or, 'like J.K. said'!]. Resurfacing the head would help this issue.

3.) My heads and the used ones i got were trash. Peninsular sells new OEM great price.
***
Good luck with your solution.

[rjschoolcraft]

Tom, thanks.

If you notice in the upper left corner of the picuture, I was illuminating the surface of the head with a trouble light. The camera then used its autoflash.

[tom.mcinerney]

Ron--Assuming your engine is still offline, i was going to ask if you'd thought over the stock coolant flow thru the heads.
[Pls forgive me...i missed the blast/past discussion More Power enabled last year due to my misplaced password--this may be way obsolete.]
I spent a while looking at the coolant passages in the block and heads {tuff to do when assembled}, and thru head gaskets. It seemed like the flow in the middle of the head upper range could 'stall'. I should say i have no engineering familiarity with 'flow', and no familiarity with IC engine cooling, either. I like the fact that the heads are interchangeable, but by modifying the head gasket, coolant flow might improve. In a discussion here last year someone mentioned that it might be good to block off the large passage from block-to-head at the forward (thermo/outlet) end of the heads. This would force the coolant to flow forward in the upper head passages, rather than occassionally dithering about.
I myself had a similar image when staring at the head/block/head-gasket. Somewhat larger holes thru the gaskets at the rear cylinders might also help. And rather than block off the forward big aperture between block and head, just partially restricting it to maybe 1/2 or 1/3 OEM area might be adequate.
Since last years discussion it's occurred to me that optimal efficiency cooling may be engineered with sensors and programmed valves in the future, allowing PID control rather than the reactive mechanical thermo.
I think the precups are sacrificial, will just fail after a while. Since GM has successfully pioneered the use of cast steel and now cast/malleable iron crankshafts they should be respected for the trick of using induction-hardened valve seats. It doesn't take a lot of coolant flow to keep temps within reason, but things get out of spec fast when the flow doesn't.

Just thought now would be a good time to poll your opinion on this issue, forgive all the dribble!

[rjschoolcraft]

I spend some time and think through what you wrote while looking at my disassembled parts. I have been planning on opening the rear passages up (currently blocked off with plates) and using 1" heater hose to encourage more flow to the rear of the heads.

[gmctd]

The block does not have the large matching coolant passage at the front, just above the coolant entrance from the waterpump, just below the outlet to the coolant crossover t-stat manifold.

The heads must be cast with that large passage at both ends, so a single casting can be used for right and left banks.

Same with the outlet passages to the t-stat manifold - identical, both ends.

[tom.mcinerney]

Ron--
[i just failed to find last summer/fall discussion in a 'search']

I had plumbed my heater to the rear head port, rather than the thermo mtg on the forward port (thermo xover).
More Power was good enough to comment that :
1) the idea of drawing coolant from the rear of heads arises about yearly, and 2) it is a bad idea, insofar as it magnifies the cooling problem, rather than relieving it. . . I returned to the oem plumbing config.
Some people are runnung a 1/4 NPT , or less, nipple from the rear ports.
JD/GMCTD appears to report in the above post that the large holes in the block/gasket/head, up forward, do not align. If that is the case {I've learned to trust his judgements/observations}, then the flow should be stable and attention should remain focused on thermo/fan/pump/exchangers.
I think flow is designed to pump cooled radiator- output to block/cylinder area; it then gradually percolates up into head, while main flow is from block-to-head at large aligned aperture(s) at rear , then this preheated coolant flows around valve chambers forward to the outlet where enters thermo, back to radiator. My (faulty) observations led me to speculate that the coolant exiting the pump into the block could take a 'short cut' up thru head into the thermo crossover [rather than flowing aft around cylinders, then up into head, then forward to exit].