Moparts
Deck and head surface finish.
Posted By: Stanton
Deck and head surface finish. - 05/02/10 09:57 PM
I'm looking at Cometic gaskets and they recommend a surface finish of "AR50" (average roughness 50). What the hell does this mean ?!?!? What type of machine provides that finish ?
Posted By: Kern Dog
Re: Deck and head surface finish. - 05/02/10 10:03 PM
I just used a red Scotch Brite and some Lacquer thinner. Zero problems. I am not a hillbilly.
Posted By: moparman89
Re: Deck and head surface finish. - 05/02/10 10:52 PM
RA- roughness average. It the distance between the peaks and valleys of the finish. Most machine shops dont even have the tool to measure it. The shop I'm at is the only one in Houston to use it. A nice machine finish and scotch brite should get you close enough.
Posted By: RapidRobert
Re: Deck and head surface finish. - 05/02/10 11:23 PM
Is 50 on the smooth side or the rough end of the scale?
Posted By: DAYCLONA
Re: Deck and head surface finish. - 05/02/10 11:32 PM
Quote:
Is 50 on the smooth side or the rough end of the scale?
50 is on the "smooth" side,...generaly most heads are finished at 70 micro inches, that's just enough to barely grab your fingernail as you drag it across the head
Mike
Posted By: moparman89
Re: Deck and head surface finish. - 05/02/10 11:37 PM
A 0 would be perfect, 4 is basically a mirror finish. I dont know if there is a scale per say, the higher number, the rougher it is.
Posted By: Stanton
Re: Deck and head surface finish. - 05/02/10 11:47 PM
The shop I'm using uses a machine with cutters as opposed to grinding. The machinist doesn't think its finished fine enough for "MLS" head gaskets.
Can the Cometics be coated prior to installation to make up for the rougher finish?
What about using 2 gaskets - a Felpro on the block and then a standard steel next to the head (aluminum).
The issue is the deck height is too low and I need to gain a bit to make sure I've got enough clearance. A .051 is still leaving me real tight - .033 quench. If I could use a .039 Felpro and a .019 steel that would yeild a vertually perfect .040 quench.
The alternative is to mill a bit of the top of the piston. The assembly has already been balanced. Since they're partially dished the weight removal would be minimal BUT would a gram or so affect balance that much ??
Posted By: DAYCLONA
Re: Deck and head surface finish. - 05/02/10 11:48 PM
Quote:
A 0 would be perfect, 4 is basically a mirror finish. I dont know if there is a scale per say, the higher number, the rougher it is.
but keep in mind the smoother the micro finish, the more chance of blowing a head gasket, esp. if the gasket compression is in the .020-.030 thickness range, the RA fiqure required by some gasket manufacturers, is what they find offers the best "bite" at retaining the gasket..
Posted By: 6PakBee
Re: Deck and head surface finish. - 05/02/10 11:53 PM
The only MLS head gasket I've ever worked with is the 2.0 Neon engine. On these Chrysler has a sealer they want you to coat the gasket with, both sides. On the Cometic, I thought they could custom build almost any thickness you wanted?
Posted By: DAYCLONA
Re: Deck and head surface finish. - 05/02/10 11:59 PM
Quote:
The shop I'm using uses a machine with cutters as opposed to grinding. The machinist doesn't think its finished fine enough for "MLS" head gaskets.
Can the Cometics be coated prior to installation to make up for the rougher finish?
What about using 2 gaskets - a Felpro on the block and then a standard steel next to the head (aluminum).
The issue is the deck height is too low and I need to gain a bit to make sure I've got enough clearance. A .051 is still leaving me real tight - .033 quench. If I could use a .039 Felpro and a .019 steel that would yeild a vertually perfect .040 quench.
The alternative is to mill a bit of the top of the piston. The assembly has already been balanced. Since they're partially dished the weight removal would be minimal BUT would a gram or so affect balance that much ??
I wouldn't touch the balanced assy, and I definetly wouldn't stack gaskets....I don't know if it's still avaliable, but Victor had a Teflon coated composition head gasket for both big and small block mopars,...installed height was .068, compressed/torqued height was .045.....I used a set few years back (about 10 yrs ago) to drop the compression on my 11:1 SM Blk Mopar,...still on there, also used a set on a customers BigBlk where the heads were "cut" too much,....I don't recall any numbers, but they were Victors....
Mike
Posted By: dmerc
Re: Deck and head surface finish. - 05/03/10 01:25 AM
I'm running .030 quench on my tightest cylinders and have had no problems. Are you going to race it? What kind of RPM do you turn? I had ordered the mopar performance thick headgaskets just in case but didn't need them. I just looked at them and they are Felpro # 8553PT. They mic out at .053.. I wonder if these are the ones that compress to .039?
Posted By: MLR426
Re: Deck and head surface finish. - 05/03/10 01:28 AM
MLS gasket finishes are best done with a CBN OR PCD button on a high speed mill. CBN (cubic boron nitrite) is for Cast Iron, PCD (poly crystalite diamond) is for aluminum. Most mills will turn 1,250 rpm at 36 inches a minute. Aluminum will be a mirror finish with a good grade aluminum head, cast iron will be mirror like as well. Many shops out there still use a Storm Vulcan 85B with a carbide 60 degree cutter at low speeds with success. And no it's not about gripping the gasket with a rough finished head, it's about clamping pressure of two flat smooth surfaces together. Many shops if they could afford it would change to a CBN / PCD mill in a new york minute if they could afford it. They just keep using what they have to get by.
Logan426 Performance Plus Cylinder Head
Posted By: RapidRobert
Re: Deck and head surface finish. - 05/03/10 01:38 AM
As said I'd leave the short block alone. Get some MLS Cometics custom made in the thickness you need to get your quench where you want it as depending on the app .033" is scary and with the piston rock measure twice (at least) and on my next one I am going to place a piece of lead shot in a cutout on the head gasket and check exactly how much quench I have as I am a firm believer in it and I want it exactly right. One guy said he could not get his cometics to seal until he sprayed each side of each (3) one with permatex "high tack", the red stuff which may increase the thickness a bit also affecting the quench dimention. If I waste a set of gaskets I can live w that. In a MP bulletin from 20 years ago they said to check it by eng upside down on a stand & taking off a rod cap then carefully get the crank all the way down (TDC) then push the rod/bearing down from the crank & see how far it will go further down before the piston hits the head.
Posted By: moparman89
Re: Deck and head surface finish. - 05/03/10 01:47 AM
The machinist doesnt "think" the finish will be good enough because he doesnt want the liability. If your going to stack gaskets. I'd do a felpro/steel shim/felpro combination. That would give you even more room to work with. Cometic tells you not to coat their gaskets with anything, but I've never used them without coating with copper coat beforehand.
Posted By: Stanton
Re: Deck and head surface finish. - 05/03/10 03:43 AM
Quote:
The machinist doesnt "think" the finish will be good enough because he doesnt want the liability
This guy is a long time friend and he does a good number of the local race engines including the blocks for a successful nitro funnycar team. So I trust what he says about the finish not being suitable for the MLS "out of the box". I was wondering if there was an alternative - like a sealer or something.
Quote:
d do a felpro/steel shim/felpro combination
That's way too much. The Felpros are .039 and the steel shim is .019. That would put my quench at .079 !!
What about the Copper "Head Savers" ????
Posted By: moparman89
Re: Deck and head surface finish. - 05/03/10 06:49 PM
Yeah no kidding its too much. If your going to run all three use thinner ones to make up the desired quench. I understand he's a competent machinist but how are you doing funny car blocks and can't get a finish for an MLS gasket? Interesting. Copper coat the MLS and run with it.
Posted By: MLR426
Re: Deck and head surface finish. - 05/04/10 02:07 AM
Quote:
Yeah no kidding its too much. If your going to run all three use thinner ones to make up the desired quench. I understand he's a competent machinist but how are you doing funny car blocks and can't get a finish for an MLS gasket? Interesting. Copper coat the MLS and run with it.
Alcohol and nitro funny cars use copper head gaskets and they use them dry and with copper coat.
I'm not saying shops that use Broach mills for good surface finishes aren't competent but high quality is only done on a cbn / pcd high speed mill.
logan426
Posted By: Stanton
Re: Deck and head surface finish. - 05/04/10 02:14 AM
Quote:
how are you doing funny car blocks and can't get a finish for an MLS
The blocks are sleeved and the decks rarely need to be touched.
Posted By: dOc !
Re: Deck and head surface finish. - 05/04/10 02:33 AM
What is the typical finish a regular machine shop will give you? .... is a Rottler better for the job ?
Posted By: MLR426
Re: Deck and head surface finish. - 05/05/10 01:48 AM
A rottler is a better high end cbn/pcd mill
logan426
Posted By: moper
Re: Deck and head surface finish. - 05/05/10 12:15 PM
The leaking/seaping issues with Cometics are directly the result of the wrong finish in most cases. If you look at a brand new edelbrock intake or head, that machining is not good enough IMO either. The more "stockish" the gasket the rougher the surface can be. Felpro blue wants something like 85, Cometics like something closer to 30-35 and as said, the right equipment to measure roughness average is expensive and pretty rare.