valve/piston distance at tdc

wgirardot · 02:50 PM

I am in the process of assembling my engine after head stud replacement and seem to be running into an issue. I am timing my cams and not getting acceptable piston to valve minimum clearances on intake 1 and 4. So, just for kicks I took a step back and decided to measure the distance between intake valves and pistons at tdc on 1 and 4 by turning the valve screws at tdc on both 1 and 4. On 1 I am getting 3.25mm before it stops. On 4 I am getting only .75mm before it stops.

Car is a stock '82 SC with original pistons, cylinders, and .25mm cylinder base gaskets.
Heads were flycut all around at .14mm. Why am I seeing this difference as I am more or less just re-assembling an engine that just got the heads worked on ( valve job and fly-cut ), nothing more?

It ran totally fine, and there was no indication of any piston/valve contact before I got into this. Right now I am just concentrating on intake 1 and 4. Would appreciate any insight or comments.

Bill

Zuffenwerker · 10-09-2023, 03:45 PM

Are your pistons installed upside down on the right side bank?

wgirardot · 10-09-2023, 04:28 PM

They are installed correctly on both sides

dannobee · 10-09-2023, 08:05 PM

Recheck the cam timing.

stownsen914 · 10-10-2023, 03:53 AM

I think you had another similar thread about PTV clearance? How did you wind up addressing the chain slack?

wgirardot · 10-10-2023, 04:51 AM

Quote:

Originally Posted by stownsen914

I think you had another similar thread about PTV clearance? How did you wind up addressing the chain slack?

I used mechanical tensioners, and was practicing getting the feel for the process of timing down. That is when I decided to check how far the valves travel at there respective tdc position until they make contact w/ piston. #1 intake went 3.25 mm
And #4 just went .75mm. This where I stopped to re-group

gled49 · 10-10-2023, 07:50 AM

Are you turning the engine 720* = 2 full revolutions to check the timing. Checking at top dead center overlap?

wgirardot · 10-10-2023, 08:01 AM

I have put a pause on timing, until I understand what is going on with the variance between how far I can push the valves in ( off cam at tdc for intake 1 and off cam at tdc for intake 4 )

1= 3.25 mm
4 = .75 mm

'78 SC · '78 SC's Garage

At TDC (and any other crank position) the distance from piston to valve seat is equal for #1 and #4 (you ruled out pistons installed upside down). If your clearances from piston to valve for #1 and #4 on the overlap are not equal, then your valve timing is not equal. Are you sure valve timing was correctly set?

Per gled49's question about overlap, are you comparing measurements obtained after rotating the crank 360*?

r lane · 10-12-2023, 05:02 AM

When you are on 4, where are the lobes on that cam. They have to be away from the rocker swivel. If so, then are the deck clearances the same on 1 and 4. The factory worked all of this out and unless some major mods have been performed or parts installed incorrectly, this can't happen. Bob

wgirardot · 10-12-2023, 10:32 AM

Thank you all for the help on this. My closed, off-cam valve travel to top of piston at tdc 1 and 4 is now the same ( about 3mm ). What is was is the valve screw on #4 didn't want to move much ( didn't like the force/angle of attack ). So I tested moving valve with a clamp, and also wedged a wood wedge under the rocker to keep in place while I measured. Now on to cam timing/testing

Bill