Quick question for those using MLS gaskets. Did you have to have your heads machined for them in order to seal or did you just put them on nd le it ride?

 

Block and heads both need to be smooth for MLS gaskets to seal well. If you can run a fingernail over it and catch anything, it isn't going to seal right.

 

if you check with the company you are buying from they will state the RA that they require. most common machine shops will not the ability to get such a fine RA. so make sure you ask first.
actually here is a copied example.
What surface finish is required to us an MLS head gasket?
A surface finish of 50 RA (roughness average) or finer, is recommended for a proper gasket seal. Anything rougher may conflict with the gasket design.


http://www.cometic.com/faq.aspx#q4

 

I was going to answer with an RA # (45) but, nobody has the equipment to measure RA. Use the fingernail. If you are shopping for a machine shop, have them show you an aluminum head they just decked. If you can see or feel lines in the deck surface, move along to the next guy. I have done a ton of engines this way and never had a head gasket issue (I exclusively use MLS gaskets on every engine I build).