What is the deal with Duramax diesel head gasket thickness grades?

Posted by Little Power Shop on 30th Jan 2014

While the GM Duramax 6.6 diesel is a great engine, if you keep it long enough you will probably be replacing the head gaskets at some point.With aluminum heads and an iron block, sooner or later a bunch of power and/or 100,000+ miles on a multi-layer steel design, you will encounter head gaskets issues.The quick and easy way to tell you have a head gasket issue once you notice coolant being pushed out of the overflow tank is by squeezing the top radiator hose after you shut the truck off.If the hose stays rock hard several minutes after then engine is turned off, you most likely have leaking head gaskets.So now you go to order parts and you are confronted with a bunch of choices when it comes to head gaskets for the Duramax 6.6 engine.The left and right gaskets are different and there are three different thickness grades.So how do you know what gaskets to pick? Read on.

For some reason that isn’t exactly clear, the engineers at the factory used three different thicknesses of gaskets when assembling these engines.So not only are there left and right gaskets, but there are also Grade A, B, and C gaskets.(Btw, the same head gasket is used for all engines including the LB7, LLY, LBZ, and LMM engines)I’m assuming there was some sort of manufacturing variances that they were trying to accommodate.When you tear one of these down you will sometimes find an A on one side and a B on the other.Or sometimes B gaskets on both sides.There doesn’t seem to be a real rhyme or reason to it usually.So how do you know which gasket to get when doing a head gasket job?

The first thing we need to remember is that any time you are doing head gaskets on a Duramax, you need to have the heads surfaced.Do not pass go.Do not get extra bonus points for finishing the job quicker.Send the heads directly
a machine shop to be surfaced.They almost always need four to six thousandths shaved off to true them up.So, by simple logic alone, we need to use a thicker head gasket when putting the engine back together.So that is why we only send the C grade head gaskets in all of our Black Diamond Duramax Head Gasket Kits.This really isn’t a compression ratio issue at all.If you look at the bottom of the Duramax cylinder head, it is perfectly flat.You can take any amount you want off the bottom of the head and it will not change the compression ratio.The only reason to use a thicker gasket is to keep piston to valve clearance proper.So by using C gaskets in both sides, you are covered.You have the best possible combination of parts to put your engine back together with.We have had a few people question the effects on the pushrod and valves, but they forget that the valves are adjustable on the 6.6 Duramax Diesel.Therefore any time the heads are done, the valves should be adjusted.

One last thing, do yourself a favor and reinstall ARP head studs when putting the engine back together.The higher clamp load that the ARP studs provide over stock head bolts will keep your head gaskets from ever leaking again under most applications.They really don’t cost that much more and are a great piece of insurance against future issues.

So in closing, whether you buy the gaskets from us or not, request C grade gaskets for both sides.It is the only way to be sure you have the right gaskets for your engine.There is absolutely no reason that a regular engine should go back together with A or B grade.