Piston -> Valve clearance checking

[dhardison]

I finally got my 454 short block completely re-assembled and last night I started checking piston to valve clearance. The pistons are factory 11.25:1 pop-ups and the cam I'm using is a hydraulic flat tappet (241 int./249 exh.duration @ 0.050" & 0.572 int./0.590 exh. lift with 110º LSA). Rather than use a hydraulic lifter for clearance testing, I used one of my old solid lifters from the previous cam (so I didn't have to deal with the hydraulic lifter bleeding down). I used a pushrod length checker and with this combo, and the stock pushrods were perfect length.

I mashed some clay in the valve reliefs, slapped on the Fel-Pro head gasket (0.039" thick) and tightly snugged the head in to place. Next I set the valves to zero lash plus 1/2 turn. When I pulled everything apart and measured the clay thickness, the intake was 0.060" and the exhaust was 0.050". I've read and been told that you need a minimum of 0.100" intake and 0.080" exhaust with a hydraulic flat tappet cam. I tried the same thing with only 1/4 turn past zero lash and ended up with 0.080" intake and 0.070" exhaust. :banghead:

Obviously I'd prefer not to pull the bottom end back apart and have the pistons reliefs flycut for extra clearance. I can go to a 0.042" head gasket, but that's not going to gain me enough. The one thing I noticed is the "cup depth" of the solid lifter I was using was much deeper in to the lifter than the hydraulic lifters that will actually be used. This will throw off my pushrod length (they'd be shorter with the hydraulic lifters) and not allow me to use stock length pushrods. Not a big deal, but would that have any impact when checking PV clearance? So what are my options now??

Thanks,
Dan

[Rubes]

Dan...I'm no expert, but I dont think the pushrod length itself will have an effect on your measurement, but the proper positioning of the rocker tip on the valve may have a few thousanths difference if it is way off, but not enough to give your measurments.

Also, why are you going past zero lash with the solid lifter? I think that may be your problem. That will open the valve slightly making your "lift" that much higher.

[dhardison]

Dan...I'm no expertLOL - You and me both!


but I dont think the pushrod length itself will have an effect on your measurement, but the proper positioning of the rocker tip on the valve may have a few thousanths difference if it is way off, but not enough to give your measurments.

Also, why are you going past zero lash with the solid lifter? I think that may be your problem. That will open the valve slightly making your "lift" that much higher.I totally agree. I guess I had hydraulic lifter on the brain when I was going 1/4 & 1/2 past zero. I'm going to re-measure with zero lash using the solid lifter and see if the clearance improves.

Thanks!:twothumbs

[Neil B]

If you are close to minimum clearance, I would recommend checking P-V clearance using the 'valve drop' method. IMO, it's far more accurate than the clay method. Let me know if you would like details on how to do this.

[dhardison]

If you are close to minimum clearance, I would recommend checking P-V clearance using the 'valve drop' method. IMO, it's far more accurate than the clay method. Let me know if you would like details on how to do this. You'll need a dial indicator and a magnetic base to check this way.Yes, details please. I'm thinking this is the method where you use light checking springs and push the valve down at certain degrees before and after TDC. Any info would be great, thanks.

[Neil B]

Yes, details please. I'm thinking this is the method where you use light checking springs and push the valve down at certain degrees before and after TDC. Any info would be great, thanks.

Yep, that's the method. Here are the key steps:

1) Degree the cam and leave the degree wheel in place
2) Install the heads with lightweight springs and without head gaskets
3) Mock up the valvetrain on the cylinders you want to check using the solid lifter.
4) Rotate the piston to 15 degrees BTDC
5) Put a dial indicator on the valve retainer and zero it out
6) Push down on the retainer until the valve contacts the piston
7) Read the dial indicator for P-V clearance
8) Add the compressed head gasket thickness to get 'true' clearance
9) Repeat process every 3-5 degrees up to 15 degrees ATDC

Let me know if you need more details. --Neil

[dhardison]

Thanks Neil, I appreciate the info!

[Turbo Hen]

Neils method is by far the best one for checking P/V clearence i'd say....i think the closest P/V clearence you will have is at 10 ATDC & BTDC......cant remember exactly.

-Carm

[Neil B]

Neils method is by far the best one for checking P/V clearence i'd say....i think the closest P/V clearence you will have is at 10 ATDC & BTDC......cant remember exactly.

-Carm

Yep, minimum clearance on the exhaust valve is normally at 10 degrees BTDC (at the end of the exhaust stroke when the piston is chasing the valve). Minimum clearance on the intake valve is normally at 10 degrees ATDC (at the beginning of the intake stroke when the valve is chasing the piston). So, you really only need to check clearance from 15 degrees BTDC to TDC on the exhaust side and from TDC to 15 degrees ATDC on the intake side.

Also, I like to use the clay method in addition to the valve drop method because the clay method shows radial clearance between the valve and valve relief. It's also the only way to check spark plug to piston clearance (critical with domed pistons).

[Turbo Hen]

You're right Neil......i never used dome pistons anyhow lol :hammer:
-Carm

[jpdeuce]

If I'm not mistaken when using a solid lifter you do not measure piston to valve clearnace with anything but zero lash. Just as you are degreeing in the cam you should measure the clearance with zero lash only. At least thats the method I use.
You can't check cam degree or piston to valve clearance with a hydraulic lifter because of the bleed down. Using a solid lifter eliminates the bleed down but also requires no tightening past zero because it does not have to be preloaded.

[Skip Fix]

I agree a valve drop method is more accurate. I can't eveer not squish the darned clay.

You can still use this method on and assembled motor with "real' valve springs.

As others have said anywhere from 6-10 degrees BTDC and ATDC are the closest areas when you are in overlap.Piston is chasing the exhaust valve closed, and then the intake valve is chasing the piston down the bore.

OK setup:
1) degree wheel on the balancer or crank snout with a crank socket(my Pontiac socket even slides in the timing cover seal!)
2)TDC found
3)dial indicator on valve spring retainer
4) Handle valve spring compressor like for changing valve seals on the motor.
5)Pull the spark plugs

Then at 2 degree intervals from TDC map how much total available clearance there is until the valve hits the piston by using the lever. Unless your valve locks are really loose with no compression in the cylinder it should move the whole valve and not just compress the springs.

Then put the rocker back on, set lash at 0 if a solid. Rotate motor and see actual lift on the dial indicator at those same points. Subtract them and actual valve to piston clearance. No adding in gasket thickness when compressed etc.

If running a hydraulic you won't be able to do this acurrately unless you use a solid lifter(or a shimmed up hydarulic to keep the pushrod cup height the same). The hydraulic collapses unless the oil pump is running. I don't know how you guys using hydraulic rollers degree cams and get correct pushrod length unless you sacrifice a pair to shim internally.

You can also use this to measure lobe lift then actual valve lift to see what ratio your rockers really work out to. It changes as they cylce across the valve tip.

Comp and many other cam makers have the lobe lift at TDC when degreed at different ICs so you can compare your cam to the one you are thinking about.

Depending how good your valve springs are and how bold you are you can run clearances closer than the .120 and do OK. I know alot of "stockers" that can not legally cut their pistons that are 0.040-0.050.

Your lash on solid actually gives you that much more clearance generally.