Help with Head Gasket selection

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gzig5

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The car went in to a professional shop to have some accident and rust damage repaired correctly. So to do that I had to blow it apart. Seems like a great opportunity to take apart the motor and finally find out what I really have. It is a forged crank, 6/70 casting 340 and it turns out that it still has the stock pistons and hasn't been bored over yet. WooHoo!! It has been rebuilt because it has Dorman freeze plugs and the J heads have modern seals, but looks like they took it apart and put it back together. Just barely can feel the beginning of a lip at the top of the cylinder and still a little cross hatch left.
I'm going to put it back together with some ProMaxx 171 heads, PRW 1.6 rockers, B3 correction kit, and a Comp XE275HL hydraulic flat tappet cam that I already have. I had Jones spec a nifty SFT cam for me, but it was done with a manual trans in mind and he said it won't work with an automatic an my converter. The manual conversion isn't in the budget for a while so I'm going with what I have. Car is 73 Cuda, 727/2500 stall, 3.55 gear, 3600lbs and is to be road course/street oriented, not drag race. Also getting a new Kevco pan and a new balancer.

I need to select a head gasket to work with these closed chamber, 63cc heads. My pistons are .025-.027 out of the hole, stock Chrysler pistons. Block may have been decked. Using the Diamond piston compression calculator I'm getting 10.1 static using a .055 gasket and assuming 6cc for the piston eyebrows, which was a guess. Is that 6cc close? That gasket gives me .030 quench and if I go to .065 gasket I end up with 9.85:1. I want to have as much compression as I can keep with 93 octane. Looking for recommendations. I think .030 is about as tight as I want to go on the quench? I'm just starting this today so I still need to go back and verify the chamber volume and the height above deck with an indicator instead of the feeler gauge I used this morning. I'm sure I'll have a few more questions after this.

edit...The head surface seems very smooth, no tool marks, so I think I can use a Cometic? Will verify once it is cleaned up better.
 
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What you are calling quench is also your piston to head clearance. You need to measure your pistons again to see precisely how much out of the hole every piston is. You used your shortest measurement to get .030 piston to head clearance, but your higher .027 only gives you .028 piston to head clearance. i would not risk running it that close on a nice standard bore 340.
if the stock pistons are staying in place and highest one is .027 out of the hole, and I was doing this, I would mill the heads to get 58 cc chambers, then run a .065 thick Cometic gasket with 4.06 specified bore. That brings you back to a 10.43 to 1 compression without running the piston to head clearance really tight. Need to check piston to valve clearance before milling the heads though, too. But that is my novice guess.
 
Cometic can get you any thickness you will need. I also like the Felpro 1008 which is 0.039 compressed but it sounds like you may need a little thicker that this one.
 
If you already have those ProMaxx heads, please post a picture of the chamber side of the head, with a wide enough picture that we can see the coolant passages around the chamber as well.
 
This is the pic off their webpage. Mine are buried in the basement at the moment. Need to get them out this afternoon.
SB-Mopar-Chamber.jpg

Like I said, I was using a feeler gage and mill vise parallel. I've been cleaning the block all afternoon. I'll get the surface gage and test indicator on it later. I assume I need to rock the piston to get the worst case?

Something funky is going on with the crank and connecting rods. Pics later, but 6 of 8 have a copper colored coating on them, but the cyl numbers coincide with the hole. Number 1 and 7 are typical steel colored but 1 has no number and 7 is stamped 8. Obviously they aren't all original. There are some interesting marks on the crank counterweights that make me think that crank was in a wreck at some time. Wouldn't surprise me. Car is a mess with hidden issues, motor didn't run, trans blew up 50 miles after I got it running. Trans and motor don't match car and trans had external weights on the converter from a later car....and on and on.

How much over bore size should the gasket be? They have all kinds of diameters, some in stock, some not. Can I run 4.040 on same size bore or do I need a bit of a gap on the gasket? The top of the pistons are beveled so I doubt they would hit if they sit centered.
 
No, you need some gap around the pistons and bores. The gasket and the bores are not guaranteed to be perfectly registered on the exact centers. Next step up in Cometic-land is 4.080", IIRC.

As for the rods, the copper colored ones may be reworks at the factory, or may be reman'd rods. If the latter especially, the rod lengths can be off, so that means to check EVERY piston height.

When checking with the dial indicator, and wanting to be precise, I will check all pistons to deck on top, bottom, front and back and lay it out on a drawing. Tedious, but you will know exactly how your deck sits. I will rock the pistons back and forth when checking the tops and bottoms and take the average.

.030" is getting a bit brave.. the local circle track guys occasionally stray as low as .025" and I just put together a 4 banger at .028" . But .035-.040" is safe. Cometic makes them in .005" increments in the range you are going to need. .005" is only going to be <0.1 points in CR.

Since you have new heads (straight and flat and smooth) I would think they will be fine. IIRC, Cometic has recently relaxed the RA requirements vs what they used to call for...

And yes, 6 cc is a decent number for the eyebrows plus a bit for the crevice down to the top ring. BTW, I am getting 10.1 with a .060" thick head gasket and the pistons .025" out of the block. DCR is right at 8... purty good for a 340!
 
Good info on the rods. Ran out of steam today but will get on it tomorrow.

Looks like the crank might have been worked on. I’m tempted to pull it all the way down but there are no symptoms of bad behavior. Runs good. Doesn’t smoke. So probably better to leave sleeping dogs lie. Full rebuild isn’t in the cards until I recover from paying for the body work.

9C80EA90-BAD3-41D1-98F7-BBBBB781AEB9.jpeg


CA95BBE9-CE80-4DD2-AED4-AFADC4A9578F.jpeg
 
mderoy340 said:
I use these on my 10.3:1 340 .025 above deck, RHS Indy X closed chamber heads. My block surface was to rough for cometics and these composites are less expensive and work.
SCE Pro Seal Composite Head Gaskets 569096
Click to expand...

I will give them a good look once I nail down the numbers. Seems like Cometic is behind the curve on delivery too. Shouldn't matter, the car will be in the shop for a good part of the summer.
 
I would not worry about the copper rods, they were dipped by the factory/OEM supplier to bring them within the weight tolerance.
 
Wyrmrider said:
what were the specs on the manual cam
let's compare so ohers do not use the wrong one
Click to expand...

Jones SFT recommendation for my application with manual trans. 3600lbs, 3.55 grears, street and road course/auto-X emphasis.
Cam# Chr340, M74365-76364-108
244/252 @.050"
.365"/.364" Lobe Lift
108 LSA

The Comp XE275HL HFT that I'm going with for now using an automatic w/2500 stall is 231/237 @.050", .350"/.350" lobe lift, 110 LSA.
 
I do ot see your lobes in Jones online lobe list either
our circle track cam
M76394-79378
304/316 lash+.006
254/260 @.050
.394/378 lobe lift

The lobe number- take the first two digits and x4 for the .050 duration
last 3 digits is the lift
do not think it would work well with an auto either :)
 
Busy week. Still cleaning. The heads that are going on were used for a short time before I got them and they have a 4.200" groove of sorts apparently from the head gasket? I can I peen or stone that down? Really not looking to have them surfaced.

IMG_3769.JPG
 
@pittsburghracer
Looks like heads have been cut for O Rings.
I believe PittsburghRacer ran into a similar situation as you have.
IIRC he installed SS wire and filed them down, even with the surface.
 
MiradaMegaCab said:
@pittsburghracer
Looks like heads have been cut for O Rings.
I believe PittsburghRacer ran into a similar situation as you have.
IIRC he installed SS wire and filed them down, even with the surface.
Click to expand...
They aren’t cut. It’s an impression. Like the gasket had a ring like those SCE gaskets do maybe.
 
MiradaMegaCab said:
brinelling?

It’s the only other thing I can think of.
Click to expand...
I think that is what Mike at B3 suggested it could be when I was talking to him this morning about the geometry correction for the rockers. Something about the kind of gasket used. I'm going to get it smooth and if I am happy with it, run it.

I just finished surveying the piston heights above deck. I've done each one three times using three different methods and I verified that the decks are very flat. They run .028, .027, .032, .035,.037, .032, .027, .026. So if I want to run between .035-.040 quench I am going to need a gasket between .072-.077". Yippee. I'll start looking but probably going to be a custom. Might squeak by with a .070 if I set the limiter at 6000rpm?
 
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Or .075". < 1 cc difference from .070"

My recent 4 banger at .028" quench is OK .... so far. C'mon in... the water's fine LOL.
 
The .010" spread really stinks. Takes me from the min to the max recommended distance it seems. I'm not putting a lot of money into the bottom end on this motor. I might pull the pistons out to see if it is a rod or two that I could easily change or have rebuilt. My concern is the strange machining on the counter weights suggest rework and I'm concerned that they somehow screwed up the throw length on a regrind. I guess it doesn't cost anything to take it apart further right? I could just continue the Bubba engineering work and throw the two tall ones in the lathe and knock a few thousandths off the top of the pistons to get them back to .030 above deck.....
 
Yessir. While you can chase your tail on this stuff, .010" difference is almost 0.2 points in compression so it is worth some work to reduce the variation IMHO. My son and I worked his engine down to .002" variation, max/min.

BTW, are the heights you listed in order of the cylinder numbers, or in order of the 4 odd cylinder numbers first, and then the 4 even cylinder numbers? If the latter, then there is a pattern on the deck work.
 
Yes, they are in order. I noticed a pattern as well. I've checked the deck with a 18" Starrett straight edge and I can't get a .002 feeler under it. They seem flat. There definitely isn't .010 bow in them. I am going to verify that tonight with the cast iron straight edge I use to scrape milling machine and lathe ways flat. I think it is either the middle throws on the crank being a little longer, or longer rods on those cylinders. I'm going to see if I can measure any difference in piston travel on those cylinders that would indicate the crank being the culprit.

I should probably check the rod bearings anyway. I can pull the pistons out, check the bearings, maybe measure or compare the rods to each other. .010 difference should be easy to see. If it all checks out I will button it up and re-torque the rods. I did find out that I have a bigger mix of rods than I thought. Remember, six are copper coated and two are bare. I found out that one of the bare ones is a later 645 part from a post 73 318-360. This thing just keeps getting better and better...:mad:
 
If it ran good why mess with it?

Those rods will be fine. Lots of theories about the copper color. Never heard about "dipping" before. They sure as hell aren't plated... I have two sets of resized and balanced 340 rods and they are a mix. I don't lose sleep over this stuff.

I'd be more concerned about those heads being machined properly, having the stems equal length...stuff like that.

But I've only been working in these things for 60 years...
 
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