When my 350 was bored out, I also had the block zero decked. I will be making huge engine mods soon, one of them being a cam swap. How much lift can I go before I will run into problems with the valves hitting the pistons? Thanks in advance.

 

#1. What size (diameter) valevs are you planning?

#2. What heads are you planning?

#3. Back to the demo-derby again? Station wagons are great for that. The long, straight roof section seems to add a lot of body rigidity, and the frames are sometimes (frequently) heavier on a wagon. Just be aware of the fuel tank location in the quarter panel on some of the early '70s wagons. Wagons also frequently had a higher axle ratio and large engines/transmissions. A '75 Olds Custom Cruiser with a 455/TH400 and that huge friggin' steel bumper on each end would be a tank.

 

Vader,

In: 2.202"
Ex: 1.600"
These are for the AFR 195cc heads.

Yup, back for more demo derby this year. I am hoping to get a 72 Buick that someone has for sale, but I'm waiting to hear word from a local restaurant as to whether or not they will sponsor me. If they do, the car's mine. If not, I have to settle for an 85 Olds. Thanks again.

 

The lift figure is not the best way to determine if the cam will cause a valve to piston interference problem. When the valve is at full lift, the piston is more than 2/3's the way down in the bore.
The duration and overlap have more to do with this. Or how much the valve is open at or near TDC.
That being said , bigger lift cams usually have more duration
and overlap.
No one can tell you defenaitly what cam will or won't fit in your
motor without causing a problem because they can't see inside your motor.
Comp cams gives these generallized recomendations.
If you are using a comp cams 292 magnum (244@.050& .501")
or larger you should check valve to piston clearance before runnig the motor...
If your plans are for a roller I'd defeniatly check. Some rollers althou not that big, really yank the valve open quick.
I've put agressive solid racing cams (260+/-@.050&.555")
in decked blocks with shaved heads with good clearance, but
you should check for yourself.

 

i've not seen anything close to a problem untill over .6 and that's with out a head gasket in place. always best to dummy one side up and check reguardless of the cam

 

There ya go. Never assume anything when it comes to engine clearances.

 

Ok, I guess what I should be asking then is how do you check to see if you have clearance or not. The engine will be in the car, by the way. Thanks.

 

Get a can of modelling clay (Play Doh) or Silly Putty in your favorite color. The clay is better, since it is more viscous and won't sag after an impression is made. Clean a piston top on a cylinder that is easy to work around, like #1 or #3, then place the Play Doh in an elongated oval across the valve depressions in a mound about 1/2" thick.

Set the head on the case dowels (in position) without a new head gasket. If you have an old, CLEAN, compressed gasket you can use that, but it's best without a gasket. Install a few head bolts and snug them in place - about 20 ft/lb should do fine.

Assemble the valve train on the cylinder that has the clay on the piston. Take up the rocker lash, then rotate the crankshaft by hand at least two full revolutions - you want both valves to make an impression in the clay.

Disassemble the valve train and remove the head. Rotate the piston to TDC and check the thickness of the clay at the deepest valve impression. This will be the valve clearance. Cross your fingers and hope that you have at least 0.050" of Play Doh left there.

 

Here's another way.
do as Vader says except don't use putty on the piston.
Instead rotate the motor till the valve is closed, lifter is down and on the backside of the cam. Now adjust the valve for 0 lash. Now mount a dial indicator on the valve retainer and now tighten the rocker nut till the valve opens .100" Do the same for the other valve. Now slowly rotate the motor, if you get any resistance other than spring tension, STOP. If it rotates ok, you have more than .100" valve to piston clearance.
Note :: You must use either a solid lifter to do this test or
modify a hydraulic lifter to eliminate the plunger travel reguardless
of whoos method you use.
Some other possible points of interference in the valve train
are spring coil bind, rocker slot travel, pushrod binding in the guideplate or cylinder head slot, valve guide or valve seal to retainer, rocker body hitting retainer, rocker body hitting poly lock, rocker running off the valve tip.
If you check using this method with no head gasket,if you are ok @.060"
clearance you will have .100 clearance with a .040" gasket.
installed. .100" valve to piston clearance is recommended by most of the Cam manufactures, to ensure the valve won't smack the piston at high rpm.

 

i like the indicator method best. it's more accurate, the play doh never seems to be as definate and neat as i'd like.

 

0.10" clearance for the exhaust, 0.07" clearance for the intake.

Here is a good description on how to with a dial indicator.

Piston to valve clearance

 

Does having the block zero decked have an effect on any of this? Thanks for the replies.

 

the deck clearance or lack of doesn't matter with te indicator or the clay method

 

Actually it may. Since the block from the factory is probably +/-0.002" from having the the correct deck heigth, and the machinist wants a true, flat, square surface on the deck, he would have to machine away some of the block. That could bring down the deck heigth below zero.

My point is if the desk was machined, the valves will be closer to the pistons.