Piston to Head Clearance advice. ????

[OldRunner]

I have lots of clearance for Piston to Valve, ( 130 ) but the side or the squish area of the piston is aprox .030 with 060 Cometic Head gasket , is this enough . I had a head gasket failure and I am wondering if this may be the cause. I know with out the head gasket on , the pistons were hitting, could not roll it over, install gaskets and it rolls, but shows the .030 clearance . Not sure what other variable come into play.

Thanks

 

[Jackpot50]

What kind of compression are you running.
Does the piston show signs of hitting head.

 

[lewtot184]

usually .040" with steel rods, tight bearing clearances and moderate rpm is considered the minimum. some people will claim running tighter successfully but i'm a skeptic. aluminum rods need .055".

 

[rumblefish360]

Total and actual clearance is .030?
If so, I'd probably say to close unless a serious rod was used.
The tightest I have run is .035 with stock rods. Lucky for me, I didn't have an issue.
Inlike to run my small blocks Pistons at a zero deck height and use a .039 gasket with the Edelbrock closed chambered heads. If I need a half point less, I can use the .055 head gaskets. Close enuff.

 

[OldRunner]

Compression is about 10.1, ?? , No signs of piston hitting, Manley Pro Rods, this is an INDY 528 Hemi, had the heads deck about .03 last year, then had this gasket failure, Find it hard , that Indy would build it that close, but I am thinking thats it. Was running a .062 Cometic, guess I should go .070 or .075 ??

 

[lewtot184]

being that's a hemi i'll assume the .030" clearance is at the side of the dome,.....correct?

 

[OldRunner]

yes ,correct.

- - - Updated - - -

If I go with a .070 vs .075 Gasket, is there much difference in compression loss between the two ? Is there a big enough difference to worry about .

Thanks

 

[lewtot184]

i think your ok. think about your measurement that may be 45 degrees to the direction of piston travel due to the semi-circular chamber. the piston travels in the vertical but the measurement isn't perpendicular to the travel. i believe the piston will have to travel more than .030" in the vertical to close that gap up, maybe .042". quench distance and piston to head clearance is usually thought in terms of the piston being at right angles to the head surface, typical wedge stuff. the hemi has to be thought of differently and measured differently. did you get your measurement by claying in the piston and head?

 

[OldRunner]

Yes, clay on the piston

 

[68 HEMI GTS]

thats a little tight, IMO. on a hemi i usually get some real soft lead solder. tape it to the piston and roll the motor. that will give you a more accurate gauge of clearance than the clay

 

[Cranky]

I don't have any experience with Hemi engines but have gone as tight as .020 (accidentally) on a stroker 400 using aftermarket rods. It ran a 10.0 right off the trailer and made several more passes until the NOS was turned on. It blew out a cylinder on it's first pass on the gas so I know .020 is too tight . I've seen many dirt track small block engines running .030 with success and plan on building an engine for myself with quench set at .030. You also have to consider how much piston/cylinder clearance you have when going tight. If you measure using clay or solder you need to measure how much rock the piston has in the bore then consider how much or less it'll have at operating temps.

 

[lewtot184]

piston rock can really mess up measurements.

i think you guys need to apply some high school geometry here, specifically the pythagorean theory. the measure taken is not in the same plane as piston travel.

 

[OldRunner]

Hopefully the picture tells the story . it is the side of the dome I am asking about,

 

[lewtot184]

do you understand what i'm saying?

 

[OldRunner]

To a degree, yes. In your first comment you suggested .055 clearance. I do not have that, I am ready to pull it apart and order a new set of gaskets .070 or .075 to be sure. I was hoping I would get a definite answer , that what I have is sufficient clearance. I guess it is best to be sure then always wondering if I am good. The same time I do not want to disrupt or create other issues by having to much clearance, if thats possible in this situation.

Thanks for all the comments.

 

[moper]

lewtot is right - the space you have there is not "the distance" you think it is. It's more because the sides of the piston are tapered, as is the chamber, and the piston is approaching on an angle in relation to the chamber wall. It's like sloping armor on a tank... the actual thickness is not measured perpendicular to the surfaces, but in the direction of travel.
Basically - aside from indy's quality - you have plenty of room there - and tighter is better. The best race hemis use pistons hand fitted to the chamber for that reason.

 

[Meep-Meep]

I have no hemi experience either but looking at where the clay is I can make a few observations. Your clearance issue is along the pin CL and the piston does have some room to slide back and forth in that direction. Not sure how much a piston moves in that direction while actually in operation, or if it moves at all, but something to consider if you push it all the way over and find you have like .015" on one side. The rocking can be a real life issue since the piston is much more likely to move that way so I'd check the sides of the dome near the intake and exhaust reliefs. I'm thinking too tight won't allow for carbon build up and maybe a slight knock might develop as a result.

Perhaps a way to force the piston to move in an unfavorable direction, in order to check clearance on one side of the dome, is to use the solder as suggested on one side and clay on the other. The harder material might initially force the piston to one side as you roll it over giving a more correct minimum clearance reading on the clay side. Also do this to check the rocking clearance. Solder on intake valve pockets and clay on exhaust to rock the piston and see what you get then flip to check the other side.

 

[lewtot184]

i was going to draw a picture of this and post it but can't find my compass and protractor,..darn it! think right angle triangles and hypotenuse. the sum of the square of the right angle legs is the square root of the hypotenuse (the true piston to head dimension). of course i'm assuming a 45 degree measurement and i think your measurement is taken less than 45 degrees to the bore centerline. less is more in this case.

 

[68 HEMI GTS]

i find this post interesting. so to me a small block chevy is something you may take internet advice from. a 20,000 hemi engine probably not. there are several things that play into how much clearance you should have. bore size, a smaller bore like a ford mod motor at 3.550 can run a tighter clearance. i have ran them down to .025 no issues. piston to wall, a loose piston requires more clearance. also rod type, and bearing clearances, loose motors require looser clearances. granted i've only built one mopar hemi (my own), done several 814ci for IHRA ford hemi's. when i bought my valve gear off Dean Nicopolis, a guy whom has build more hemi's than anyone on this board, he stressed to me the importance of having enough dome to head clearance. he is the one that told be about the solder for checking purposes. granted if your motor is just a cruise night type of deal, then your probably just fine. if your tell tale is more likely to end up of 7800 then i would add a little wiggle room. Why don't you call indy and ask them what the intended clearance should be (they should know, they build enough). to me it seems tight, i don't know how you run the motor, or what type of head gaskets you had on it. how its built, etc. sometimes if the piston is hitting the head it can be hard to tell, obviously if there was no carbon on the side of the domes and they were clean, then its to tight. i error on the side of caution, that way im not working on the damn thing constantly. the Pythagorean theorem is correct though when you go up on head gasket thickness, adding .010 will not get you the same on the side of the dome.

 

[OldRunner]

Thanks again for all the opinions, they have all been taken into consideration. The only answer I would like , is the Pythagorean theory , if you go up .010, what are you adding to side angle of the dome, more or less then .010