Piston protrusion - head gasket selection

[Z71]

I´m currently rebuilding my 6,5TD for my K1500 and I´m ready to bolt on the cylinder heads. The block has been shaved .010" so I installed Mahle .010" reduced compression height pistons.

The pistons protrude around .007" (some a few thousands less) out of the block, which I suppose is borderline in order to obtain .035" of piston to head clearance using a STD thickness Fel-Pro head gasket.

I have both the STD thickness 9701PT Fel-Pro headgaskets and the .010" thicker Fel-Pro 9521PT gaskets here in my shop. I´m leaning towards the thicker gaskets to increase piston to head & valve clearance, but I´m a little worried about the reliability of a thicker gasket. Also, of course, it will lower compression. Which ones would you install? - Any comments?

I need the engine to be dependable and produce decent power. Towing will be limited to pulling a trailer with my race car once in a while.

Engine is stock except Heath Max-E-Tork Eprom, Heath HO injectors, Heath Turbo-Master, main cap stud girdle and a Fluidampr balancer. Most engine components are new, except block(929), crank, rods and heads.

You´re all a great help here at TDP. Usually a quick search will answer my questions.

Thank You!
Anders

[john8662]

.007" is fine.

The spec is really .006" so I think you'll be just fine with the standard thickness .045" gasket. Are you using your bridge and dial guage at the center of the piston?

The varying you're seing is most likely the rod length, getting the rods all the same length is quite a process on these engines and it's very crucial.

J

[Z71]

Yes, I used a bridge and a dial gauge at the center of the piston. When I installed the pistons I used a few rods from a spare engine I have (cracked block) to end up with .004" - .007", hope that´s within "limits"?
The rods that were in the engine would have given me .001" - .007" if I remember correctly.

Thanks!
Anders

[Robyn]

I agree, the .007" will be fine.

Now there is absolutely nothing wrong with going to the +.010" felpro gaskets on this either.

The rule on these engines is "Piston protrusion" to be FLUSH to no more the .005" out of the hole.

Either gasket will work fine.

Myself, I just might opt for the +.010"

Just remember, as the engine gets time on it, carbon will deposit on the tops of the pistons and this reduces the clearance.

Valve clearance on these is not an issue, as the cam timing, lift and all that stuff are set so the valves are all neatly out of the way when "MA Piston" comes screaming up to TDC.

Good luck.

Missy

[Kennedy]

Back when we toured GEP I was told that through the course of the years they have actually been creeping the protusion up to put the piston closer to the head as this helped them with their emissions.

At .007" I'd say you are right where you want to be. The only thing better would be reduced compression slugs (volume in the middle same protrusion) and blueprinted protrusion so all are the same.

I thought the modern 9701 PT was .049" compressed?

[Robyn]

I got ahold of a pair of gaskets from a new P400 that came in with the wrong heads (had wrong intake style 90 degree instead of the 60 degree)

These gaskets measured .041 after having been squished once and not ran.

Missy

[Z71]

Great expert advices.

I was looking all over for the compressed thickness of a STD Fel-Pro gasket - thanks Robyn for that info.

So this brings my piston-to-head clearance, using the 9701 PT´s, to .041" minus .007" = .034"

I haven´t been able to find a minimum piston-to-head clearance in my books regarding the 6,5. I just have an idea (maybe read somewhere?) that I would like to see at least .035" Anyone have that data?

Thanks

[More Power]

I have the 1998 GM service manuals, a 6.5TD service training manual, and reams of other data. Piston protrusion specs could not be located in any of those.

However, an AERA Engine Specifications data sheet for the 6.5L I have on file, dated 7/93, specifies "Piston Position +.0039-.0059" above" in its cylinder block specifications subsection. I assume this is the piston protrusion specification.

Assuming the AERA specs are accurate (and I believe they are), thicker gaskets are required on all milled block decks. The decks on the block I'm working on now were both milled .005" (at least they were stamped .005 on the freshly machined surfaces). I'll be using the +.010 Fel-Pro head gaskets to get the protrusion requirements back into line. My measured protrusions were all above the high-limit AERA specs by a few thousandths. Jim

[Z71]

I also ended up using the +.010 Fel-Pro head gaskets to be on the safe side. I´m really satisfied with the engine. It has been in the truck for about a year now with 11000 trouble free miles.

Anders

[More Power]

I also ended up using the +.010 Fel-Pro head gaskets to be on the safe side. I´m really satisfied with the engine. It has been in the truck for about a year now with 11000 trouble free miles.

Anders

That's good to hear!

Incidentally, general engine rebuild information suggests that downward pressure needs to be placed on the piston crown for some period of time before measuring protrusion, which will help reduce the effects of an oil/assembly lube film on the bearings. Makes sense. The rod bearing clearance and piston pin bearing clearance combined could produce a variability between piston protrusion measurements that's due to a difference in oil film thickness.

As was pointed out earlier in this thread, the recommended bridge/dial gauge measurement should be at the piston's center. I saw/measured a difference in side-to-side protrusion on the piston pin centerline. I didn't use a bridge. I used a straight edge and feeler gauges. Takes more time to do it this way, but no special tools are required. Jim

[Z71]

Good point about applying downward pressure on the piston crown before measuring. Haven´t done it before but I have a Chevy 350 that is due for a rebuild, so I´ll try it and record the difference before and after pressure is placed on the piston just to see the effect it has.

Anders