Hitch

Good afternoon guys, I have been searching parts for my up and coming changes to my engine. I currently have FelPro 1003 head gaskets that have a 4.166 bore and .041" compressed thickness. When I built this motor I noted that if I ever wanted to increase the compression a little that it could easily be done by going to GM head gaskets that were listed with a .029" compressed thickness.

Well now that I go and look for them again I can not find the part number in my notes and there does not appear to be anyone offering a .029" compressed gasket. Now Cometic offers almost any thickness but my concern is the requirement of the surface preparation. Does any one know the actual RA required and how smooth should this be?

I am leaning away from the Cometic gasket because of this but thought I would ask the group. I do have other options like the Mr. Gasket 1134G with a 4.130 bore and .028" compressed thickness. The other option is a Victor Reinz head gasket with a 4.125 bore and .026" compressed but I worry that will be to thin and make the quench to small and with being at .009-.011" piston to deck height it will cause problems.

When I built the motor I had plenty of piston to valve clearance but I don't want my compression to get to high. Right now with .010 the hole, .041 head gasket, -5cc piston reliefs and 68cc heads I calculate I am right at 9.8:1 with the change to the .028 that would make me right at 10.3:1. With the aluminum heads I think that will keep me safe on pump gas. Thanks guys..

David

MrPbody

My experience with Cometic is in relation to use on the Cadillac Northstar engine only, but I assume they are similar construction for other engines as well. the ones I use on the Northstar are a sandwich gasket, so I don't know how they could make one as thin as you want. they are a great sealing gasket, but not thin. Are you sure you want to run thin gaskets with the aluminum heads anyway? might be better to have the heads milled a few thou and use a more forgiving gasket IMHO.
I run mid grade fuel in an OT car of mine with 11:1 compression and aluminum heads with no problem at all BTW. 377 ci SBC.

Russ

Hitch

Well not as thin as I want but with the Cometic gaskets I have more options. I'm trying to avoid milling the heads then having to mill the intake too..

David

MrPbody

The intake surface on the heads is normally milled along with the hd surface.

Scott Marzahl

David, check out Mr Gasket 1134G thats what I using, .028", 4.134 bore, I think its 6.8cc and my deck is .010"

Fordracer9

http://www.cometic.com/technical.aspx

Surface finish of 50 RA is recomended.

Hitch

I have read that before and was intentionally vague because every (5) machinists I spoke to could not qualify or quantify what that meant...

Fordracer9

Easier for me to give abother link than try to type all this out. This will explain it all very well. RA or RMA is explained about 2/3 of the way down.

http://rlengines.com/Web_Pages/Cylin...L_Engines.html

roger newman

My block has a RA average of 28. I am using Cometic .040 bore and .035 thickness. Compression ratio of 10.86.

Mr. Wizzard

Over the last 12 years, we haven't used anything except Cometic head gaskets on the racing engines we build. For best results, make sure you clean the block and head deck surfaces with acetone before bolting the heads on. We also never use any gasket sealants with the Cometic's.

Westlotorn

Modern OEM engines use an RA finish in the 16 range.
FEL PRO MLS head gaskets allow up to 60 RA.
If the shop is using a CBN cutter with a slow feed they can get you under 20RA.

If they are using older tooling, very common today the finish may be closer to 50 RA or more.
Tip: If you have a block or heads machined on older equipment and wish to upgrad to MLS gaskets it can be done.

Take a good piece of machine steel and wrap it in 600 grit wet and dry sand paper.

It only takes about 10 to 12 strokes accross the surface to smooth it well below the desired 50 RA finish. MLS works better with a mirror smooth surface but the special finishes used by Cometic and Fel Pro on their gaskets allow a rougher finish.
When MLS first came out and only OEM gaskets were available we used to teach shops this technique to keep them in business. It added no more than 10 minutes total to block or head prep and allowed the work to move forward even when older tooling was all they had.
Fel Pro 1142-026 is MLS and .026 thick, allows the 60RA finish 4.100 bore size

427Hotrod

I've had great luck with the Fel Pro MLS gaskets without havign to get too worried about the finish. In fact I just installed a set today on my 555" that I'm converting to a twin turbo application.

If you were to end up at .035" or so total quench..I wouldn't worry about it. It will be fine. Mine has the pistons .009" OUT of the hole and I've run it as close as .028" total at 7500+ RPM with no issues.

JIM

Westlotorn

The beauty of MLS is the ability to seal while the engine is moving, the Head is bouncing on the Block. This only happens in old Iron engines when the pressures are extreme, Superchargers, Turbo's, Nitros or Detonation.
The constant movement of the Head on block happens in these engines with very high cylinder pressures, the head bolts strech on each combustion and the head can be measured to bounce up and down, this flexing and constant movement will wear out a MLS unless the RA is smooth, below 60 RA with Fel Pro. They will live a while with a rougher surface but will last a very long time on a poper surface.
I know they tested a Small block salt flat engine at 2200 HP with MLS, this design works.

Scott Marzahl

The old Chevy Power Manual states the minimum piston to head clearance with gasket is .035" with a .005-.0055 piston to wall clearance.