C69A exhaust valve to spark plug clearance

Z

Ziggster

Well-known member
Posted this back on the HAMB on Saturday, and received no responses.

As I look into performance aspects of the flathead, I’m puzzled at what seems to be an impossibility.
While assembling the long block a few years back, I checked the gap between the spark plug tip and exh valve. The heads were the stk cast Ford C7RA-A, plugs were NGK 3212, Felpro gasket (0.055” uncompressed), pistons were Egge “stk style dome” (0.060”). The distance between the exh valve (closed) and plug tip was measured to be 0.128”.
So, with the Isky 1007B profile on the original camshaft, cam lift is listed at 0.358” for both intake and exhaust. How is it that the exhaust valve is not hitting the plug tip. I’ve run the engine for about 30 - 40 mins in total so far and checked plug gap, and there is no sign of contact between the two.
It’s a Sat morning, and I’m still not 100% awake after a 14 hr door to door drive yesterday, but shouldn’t there be a massive problem?
 

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Sorry to say but your numbers or at least the .128 number is for sure incorrect. You can check it again with head on and valve open and closed. Measure down from top of head where spark plug seats to valve and then subtract the spark plug thread area from sealing gasket surface to end of tip. That will give you an approximate number. I hope I have not confused you.
 
Ok. Thanks, will do. I read somewhere that the clay can get distorted during the process of removing the head. Looking at the pic again, it seems the valve is not fully seated, but hard to tell for sure. I did verify the piston was at TDC, but not knowing enough about piston/cam relationship, maybe the exhaust valve was already beginning to open before TDC, which I think is normal.
 
when measuring just make sure the ex valve is seated all the way, TDC is irrelevant in this context. I would say .025" is enough clearance between spark plug and valve, unless you are revving the thing where valve float becomes an issue, but even then the valve shouldn't open more, it just bounces off the seat. You can also 'side-gap' the plug if it's too close.
 
At TDC both valves will be closed but it may be either #1 or #6.

Rotate the engine so you are at TDC, power strike for #1.
Pop the distributor cap, which is probably marked with cylinder numbers. #1 is at about 10:00 position or second plug wire socket from the top. At this position, both valves should be closed.
 
I measured TDC on number 1 cyl. I’m using a helmet style distributor, so using it as a reference I think is not possible.
This does have me wondering though what are “typical” maximum lift values on the cam lobes. I understand there are many variables, but being a novice, I never gave the cam lift specs a second thought as it related to possible clearance issues with the spark plug. I’m using NGK 3212 plugs, and they seem to be correct in terms of the thread length for the heads I used as the tip just barely protrudes.
 
If you follow my earlier post you will have your answer as far as lift. The difference from closed to max open will give you your actual lift number.
 
According to the Isky 1007B spec sheet, cam lift is 0.328. Not sure what it actually is as cam is the original that was re-ground by “Pete” who has since retired, but I’m sure it’s spot on base on his reputation.
Will try to measure it for sure.
 

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Ziggster said:
I measured TDC on number 1 cyl. I’m using a helmet style distributor, so using it as a reference I think is not possible.
Click to expand...

Trace #1 plug wire back to your distributor cap, noting the "clock" position. Pull the cap and then rotate the motor so the rotor is pointing to the "clock" position for #1.

Suggest you check your numbers again. Below is a spreadsheet I did a few years ago with some common cams.

589589
 
Ziggster said:
According to the Isky 1007B spec sheet, cam lift is 0.328. Not sure what it actually is as cam is the original that was re-ground by “Pete” who has since retired, but I’m sure it’s spot on base on his reputation.
Will try to measure it for sure.
Click to expand...
589587
 
glennpm said:
Trace #1 plug wire back to your distributor cap, noting the "clock" position. Pull the cap and then rotate the motor so the rotor is pointing to the "clock" position for #1.

Suggest you check your numbers again. Below is a spreadsheet I did a few years ago with some common cams.

589589
Click to expand...

I mistyped. The spec sheet shows a valve lift of 0.358”.
 
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