Valve reliefs vs shaving tops or both????

S

SmokeSmacked02

New member
Joined
Sep 2, 2011
Messages
79
I want to run this by everyone and get an opinion and why one might feel one way or another about which to do and when and why. The motor I am building is for competition use only, it's a 96 12 valve block, factory head being ported with runner intake, 234/254 Schied cam, 13mm ag pump, factory shot peened USA rods, marine bowl Pistons,arp bolts and studs on the bottom end, contemplating girdling it right now. It's for a 2.6 truck and just really building this engine to get started out knowing it may not be perfect for the first season or two but a work in progress. My main question is I have a guy I have got to do a lot of work for me and trust his knowledge but I'm stuck between Scheid saying .120 valve reliefs and then he is saying he doesn't see a problem cutting .080-.100 off the top of the piston and when I ran that by Scheid he says "no it will run like sh*t!!!" I would like to lower compression to the 13:1-14:1 area, right now my average piston protrusion is around .040" so I'm going to need to cut some off but would like to know which is the best route and why. Sorry for such a long post but I want to give as much info as possible and if I have missed some I will do my best to answer. Thanks everyone hopefully this will help others as well.
 
Use valve reliefs, keep the compression up. You'll be happier in the long run.
 
Listen to Scheid , unless you run a really large turbo ... more to the fly cutting than looks spend the extra money fly cut them ..
 
Last edited:
Do it right the first time and do the reliefs. I would do reliefs and open the bowls a little if it was me.
 
Ok I figured for the amount of engines they build they would be the ones to listen to on that but he just didn't give me any reasons why to. Is it necessary to actually have .120 cut? I know the last engine I built the cam called for .080 and I had quite a bit of clearance when I measured everything out. Seems like that is quite a bit but may be best to have a safe zone there.
 
With 0.040" piston protrusion, you're going to have to cut at least 0.020" off all the pistons to maintain adequate piston to head clearance, unless you plan to run an oversized gasket.
 
I would put on the girdle and find a good set of used billet rods. Bore the block out to put cam bushings in for every journal. As for valve reliefs and piston protrusion, you want to get this right. If you cut off too much and get your compression down, it is not going to run very well.
 
Cut reliefs, but if you want to lose a touch more compression, make it quicker and cheaper, do "straight cuts". Think of it as putting them in the mill and making a straight sided slot back and forth where the valves sit, versus making round cuts right where the valves sit. It's quick and easy, and can be done in any mill in less than a hour.

I would probably cut the pistons down to about .020 protrusion also.
 
Has this block been decked a lot? That's a lot of protrusion. Also when they check the protrusion did they check each piston down the center of the block or on opposite sides of the block. Pistons can rock in the cylinder and give you a false reading. One side will be low and one will be high if done on sides of the block.
 
jedeka said:
I would put on the girdle and find a good set of used billet rods. Bore the block out to put cam bushings in for every journal. As for valve reliefs and piston protrusion, you want to get this right. If you cut off too much and get your compression down, it is not going to run very well.
Click to expand...

I may end up changing rods out eventually before this is all said and done, I am hoping to have it out by next season early on which here would be early/mid March so I am not in a huge hurry but I am doing this on somewhat of a budget and all though 3k more isn't a whole lot for a build like this there are many other things that need to be addressed besides the engine and eventually I will have to get started on that as well. I have talked to several people and have heard anywhere from .005"-.015" on the piston protrusion is okay for an engine like this?





DISTURBED said:
Has this block been decked a lot? That's a lot of protrusion. Also when they check the protrusion did they check each piston down the center of the block or on opposite sides of the block. Pistons can rock in the cylinder and give you a false reading. One side will be low and one will be high if done on sides of the block.
Click to expand...


The block has been decked and if I remember correctly it has about .006"-.008" off but I will have to make sure on that, it may have been more but I did this one with a batch of others so my numbers may be off. And i checked protrusion and I also did that off the sides of the piston so that may be where I got off.
 
9724VF350 said:
Cut reliefs, but if you want to lose a touch more compression, make it quicker and cheaper, do "straight cuts". Think of it as putting them in the mill and making a straight sided slot back and forth where the valves sit, versus making round cuts right where the valves sit. It's quick and easy, and can be done in any mill in less than a hour.



I would probably cut the pistons down to about .020 protrusion also.
Click to expand...



Just curious , what kinda clearance should there be with head and Pistons for growth with heat?
Fortunately my engine measured well and nothing has to be cut
 
97rada said:
Just curious , what kinda clearance should there be with head and Pistons for growth with heat?
Fortunately my engine measured well and nothing has to be cut
Click to expand...

Normally, .030 is a good place to be. With lots of heat and big RPM up to .050 can be needed.
 
97rada said:
And normal crush on a gasket is around .056 right? I didn't know they swelled up that much.
Click to expand...

I have also been wondering this as well and would the crush on the gasket change at all with fire rings or does it still pull down and have the full crush of a factory gasket?
 
97rada said:
And normal crush on a gasket is around .056 right? I didn't know they swelled up that much.
Click to expand...

I've seen farm tractor engines with .022 protrusion and a .045 MLS gasket touch when they get hot.
 
I didn't expect it would grow that much. I threw a .020 over gasket in mine since .008 was taking of the deck. That for good measure if I wanna mess with cams so hopefully no issue
 
I always keep a good crushed gasket around just for checking piston to head and PTV clearance. Might be a good thing for guys to hang on to instead of tossing them in the trash
 
So if I have this correct for instance after I get another measure on my protrusion and say it comes out to actually be .040" and the gasket crushed (.061") leaves me with .021" PTH clearance which isn't enough, so I can get away with shaving roughly .010" off the top and running my valve reliefs .120" and be in good shape to give me roughly .030" of PTH clearance? I know this isn't a time for rough measurements I just want to get a good idea in theory.
 
You must log in or register to reply here.
Back
Top