Piston to valve clearance
 

What is the best piston to valve clearance on bbc blower engine with aluminum head?

I am looking it for my engine now i know it is cretical on the intake side.

The best valve to piston clearance is enough to keep the two from hitting... :)

If you're asking what the absolute minimum is, I usually go with .090" as a general rule of thumb, and have run them as close as .070"... but it certainly wasn't on a marine engine. Honestly, any camshaft profile that has any business being in a marine engine, combined with a flat top or inverted dome piston, shouldn't be anywhere even close to the danger zone as far as valve to piston clearance goes.

Do you have your cam specs? Particularly the timing events?

I have isue with p to v i have speed pro dish piston ( L2453F ) and i use the Edelbrock Performer RPM aluminum head with a 2,250'' intake valve.


cam spec:

Intake
valve open 4.7 btdc
center line 110.2 atdc
valve close 43.4 abdc
duration 228.1
lobe lift .365''
valve lift a 1.7 = .621''


Exhaust
valve open 56.1 bbdc
centerline 118.2 btdc
valve close -1.6 atdc
duration 234.4
lobe lift .365''
valve lift a 1.7 = .620''

Have you checked it to see how much you do or dont have? I wouldnt want anything less than .080. Is it a solid roller or hyd roller?

Hyd roller and i have see .056" at intake valve

Also the cam in an sfo custom grind

What lifter did you check it with....reason being you have to make sure the plunger isnt collapsing while you are checking it with the hyd roller lifter. .056 isnt enough, take your pistons to a reputable shop and get them cut. IMO if you are going to have to cut them i would atleast make sure you ended up with around .100 clearence. Make sure you recheck the piston to valve clearence after you get them back. It shouldnt cost you more than 80-100.00 per set

I'd double check your cam timing. That doesn't even seem possible with that cam.

How much exhaust valve clearance do you have? What method did you use to check it?

I wouldn't like to see less than .090, I put a HUGE ultradyne cam in a sbc in the car for a buddy of mine once, we had cam clearence in th.055-.060 range, even though its NOT ideal we changed cam timing 2 degrees and clearence was over .090, there was no way the guy was going to pull his motor and tear it down/get pistons cut and he was set on running that cam. Get your pistons cut IF your clearence is really that tight BUT like mentioned, re-degree cam to be sure you have cam in right, I have ran cams with 250 duration and .650 valve lift on BBC's and had more clearence than you have, Smitty

mx,
I am a chicken, I like to see .100 on both. I would like to make a couple of important points.

1. Make sure that you actually check the clearance with clay on the piston. A lot of times the valve relief in the piston is not big enough around to clear the intake valve and that is where it is actually hitting, especially with a 2.250 valve. You cannot tell that with just a dial indicator, you have to clay it so you can see where the valve is hitting.

2. Make absolutely certain that if you do make the relief larger or deeper, that you are not getting too close to the id of the top ring groove. That is one of the most common places for big block pistons to burn through when flycut.

3. Are you checking it with a head gasket ? I have to agree with the others that that is not all that big of a cam but I dont know how much of a valve relief the TRW pistons have either.

4. If you are going to get them cut, ask your machinis if he wants you to mark one of the pistons. I have an old valve stem that is ground to a point and I put it down through the valve guide and use it like a center punch to mark valve centerline on the top of the piston. On the intake valve you want to do this with the piston at about 10 degrees AFTER TDC. Makes it a lot easier to set up the cut.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

Thanks for all the good reply.

I am sure that the cam timing is right the intake is tight and the exhaust side have almost full lift of clearamce. Now is at 110 intake centerline on cyl #1 i will try to put it at 112 intake centerline and take some measure...

Are you using clay ? If not, there is no way to "see" where your clearance problem really is :(

The cutout might be plenty deep, just not big enough around.

Yes i have use clay. I so already cut the valv e notch on the radial side .080" but i have see now that the cyl #1 and 3 have .095" i think the #4 is not cut at the same place that the other posibly an error.
Before i cut them i only have .021" at the intake. Now the head are bolt on and this sumer time i will try to get enougth clearance with the cam timing i know i will lose some bottom end but this a blower engine that will help for that.

Something just doesnt sound right. My cam has similiar intake opening and closing timing, slightly more lift than yours, 110* install, and I had plenty of PTV clearance when I clayed it. I have flatop JE pistons with a single valve relief.

With a stock deck height, your piston should be .025 in the hole with that piston you have.

Have your blocks been decked alot? What head gasket are you using? How far in the hole is the piston?

Yes the block deck height is at .010'' so the piston is .010 in the hole, and head gasket is felpro mls .040'' compressed thickness.

This measure is at orond 9-10 deg atdc.

I have put my cam now at 112deg intake centerline instead 110 deg and i have recheck the measure, now the closest measure is ,098''.

The speed pro piston dont have a big notch on them and i think that is not desing for a 2.250 intake valve dia.

If memory serves me correct....the valve pockets on those pistons arent machined like a J/E. The pocket is just part of the forging. Like others have stated check and recheck because you dont want to take out more than you have to because that is the weakest area in a BBC piston.

Supermx96

The issue is with the design of the relief on the Speed Pro piston when used in conjunction with the Edelbrock Marine series cylinder head. This series head is a 26.5º valve angle which alters the orientation of the valve in conjunction to the relief. This effect is both radial and to the plane of the valve angle vs. the relief angle. P to V clearance is most influenced by duration rather than lift, therefore your only solution would be to fly-cut the pistons to correct for radial, depth and angular clearance.

Bob

Bob, you had me at hello. Once again, you've provided excellent information!