I purchased a set of heads that had the combustion chambers set at 58cc. So I had them milled .030 to bring them back to approximately 54cc. Well I wanted to run the impala head gaskets which have a thickness of .029 compared to the stock .049 thickness. The heads have the 2.00 and 1.55 valves. Since I have now removed approximately .050 thickness, (milling, and thinner gaskets) have the larger valves, and will be running the GM hotcam, do you think I will have a clearance problem?

 

That issue is going to be a question of deck height, valve reliefs, and lift. The only way to know for sure is to mock it up and use some clay on the pistons..... all of them, they can vary.

I'm not familiar enough with stock tolerances...........sorry, but you've taken 30 off the head and want another 20 on gasket compression, so I'd be careful :cheers:

 

:iagree:
The only way your going to know, is to check. If I, or someone else, assures you everything is OK, and they hit, who are you going to blame? I think your probably OK. For MY engine, I'm checking it.

RACE ON!!!

 

:iagree: To be extra ****, you'll want to check your valvetrain geometry as well.

 

hey
you should not have any problem with piston to valve clearance issue. 5 yrs a go I ran LPE219 cam with .560 with 1.6rr on my 93LT1 heads. I have no problem. The hot cam is only .525

 

OK, so to check this do I just take modeling clay and push it into the piston reliefs and then bolt the heads on and turn the motor over by hand? Comparing the old heads to the reworked ones, The valve does slightly stick out of the head where the orig. is flat. I may be way off base on this but it seems to me that my clearance problem, if I have one will be when the valves are closed and the piston is at the top. Thanks to everyone for their opinions. :cheers:

 

Yes, that's how you check with clay......that way you'll know exactly where you stand. And, the clearance becomes an issue near and at TDC, when your valves are open or opening, and protruding into your cylinder near the piston.........that's all determined by the lift and timing of your cam (and the multiplication of your rockers).

John brings up a valid point in geometry.....I hadn't thought of that, but you're affecting it 50 thousandths down at the head gasket :yesnod: One other issue that seems to be lost in this discussion is the matter of quench (which has a direct bearing on piston-valve clearance). Like I said earlier, I'm not all that familiar with stock parameters, but it seems to me .060 sounds pretty close. You're affecting this dimension by that 50 thousandths, and I'm wondering if that's a very good idea......? You might rethink the head gasket, I know I would. The amount of compression gained by fooling with oddball gasket thicknesses is minimal when compared to the potential of damage if your valves are kissin' pistons!

As far as the other poster saying you "should" be OK, I dunno. It's kinda like the old Clint Eastwood movie....... "do ya feel lucky today"? :lol: If you're spending your own dough and time to fix it (and have another car to drive), you gotta make that call. In any event, good luck on the project :cheers:

 

No. First of all the valves won't be closed. During the overlap event
the piston will be at TDC and both valves will be open slightly at the same
time. The exhaust valve will be closing, the intake valve will just be starting
to open.

Where you have valve to piston clearance problems though is just before
TDC or just after, depending on your cam timing and such. When the piston
is approaching TDC during overlap it is "chasing" the exhaust valve closed, and on
the other side to TDC the intake valve is opening as the piston is starting
to move down, but the piston speed at that point is much slower than at
mid-stroke, and the intake valve is opening faster than the piston is moving.
The closest point will occur with the piston a few to several degrees before
or after TDC.

Also be careful checking with hydraulic lifters because the plunger could
settle some under spring pressure and you'll get a false reading.

Dan


[Modified by danno85, 2:51 PM 2/29/2004]

 

Do you really think you'll have less clearance with the valves on their seats, than opened?

Bruce says it'll be OK. You can get him to fix it for you it they hit.

RACE ON!!!

 

I was thinking that the valve would be closed on the top of the stroke, and would be opening on the way back down. I didnt realize that the valve would be slightly open on the top.

 

Both valves ARE closed before, during, and after, TDC at the transition of the compression and power strokes. So you are correct for half of the TDCs in the 4 stroke cycle.

 

That's basically the difference between a new valvejob and one that's been run. The old head had some recession on the valve seat, or usually it's a combination of both recession and the way they are setup at the factory.

A good vavlejob will only seat on the top 1/3 of the vavle face, bottom 1/3 of the valve seat. This gets you extra flow and will allow for some recession as it's run.

 

This is from my engine if it helps any:


Dan

 

A picture is worth a thousand words!! :cheers:

 

So what did it measure out to? Looks to be about .070" or so. Can't really tell from that angle.

 

Yes, the picture does help. OK, so I assemble everything,including new gaskets, and hand crank it over. 1. do I need to torque the heads in place? If so, can I reuse the gaskets? 2. How much clearance do I need? I know I will need to allow a little for the lifters to pump up etc.

 

]]

You don't need to torque the heads in place, only need enough bolts to hold them on too.
You must set the valve lash or lifter preload and have the correct lenght pushrods too.

.080 on the intake and .100 on the exhaust is a recommended minumum, but with a hydraulic cam NEVER entering valve float, you can run it tighter than that.

I've run as tight as .030 on the intake and .050 on the exhaust with a relatively mild street-type hydraulic camshaft, but that's really on the ragged edge.

.120 on the exhaust is safer since the piston is always chasing the exhaust valve anyway.

Yes, the head gaskets are re-useable.

Jake