questions on piston to valve clearance

[Jet City]

I'm trying to use a degree wheel to determine my valve to piston clearance, not sure I understand the correct method. I know what I'm looking for is clearance at intake opening and exhaust closing, since the cams I'm looking at will be 18-23 BTDC Intake opening and 23-27 exhaust closing (@ .050), I figured checking clearance at these points is whats criticle. Here's what I came up with, these were checked w/o head gasket and gasket thickness calculated back in.
Intake Opens (measured @ .050)
25* BTDC = .414 clearance
20* BTDC = .356 clearance
15* BTDC = .299 clearance
Exhaust Closes (measured @ .050)
25* ATDC = .505 clearance
20* ATDC = .422 clearance
15* ATDC = .357 clearance
10* ATDC = .317 clearance
Is there other points I should be measuring?

[Moneypitt]

You need to check the exh as it closes and the intake as it opens. Meaning your above numbers are a little backwards. The tightest would be as the piston nears the top, exhaust, and as the piston starts down, intake. A quick opening valve can catch the piston as it goes down if the intake opening is close to TDC, and the lobe is steep. I usually check the exhaust at 2-3* increments coming up starting about 12-15* before and the same for the intake going away from TDC........Also, what is the recomended minimum, as stated by the cam grinder?.............MP

[malcolm]

You crack me up MP! :D

[Fiat48]

All you care about is the tightest point which is near TDC on overlap..and on my stuff about 8 degrees from TDC on overlap.

[EXTREMEBOATS]

The old fashioned way is to put some play dough on one piston, make it flat and at least 30-40 thousanths. Put your head back on, install push rods and your rocker arms, set lash to cam specs, rotate motor through its cycle. Remove head, cut a slot in the play dough big enough to use a depth gage and measure your clearance. Sounds kinda hokey but it has been done this way for years and you cut out all the guess work. Bare in mind there are different clearances between steel and alluminum rods do to stretch. Check with manufacturer reccomendations. Hope this helps. Degree wheels are used more for degreeing your cam to specs. :wink:

[malcolm]

That's what I was thinking.

[steelcomp]

You're going to need more clay than that. Clay will get you close, but a dial and degree wheel can tell you to the thousandth...if you care.

[Jet City]

The whole idea behind what I'm trying to figure out is what cam will fit into my assembled and balanced shortblock, so this is all happening without a cam. I found this http://www.iskycams.com/votc.php on Isky's site, its exactly what I was thinking, just something to help identify how much lift I can fit, at least close enough to make a reasonable cam selection. I'm familiar with checking V/P using the clay method, and the light spring method using degree wheel, but these both require a cam, a vicious circle.

[Fiat48]

I've never done that. I buy the cam I want and if I have to flycut pistons..so be it.

[Jet City]

I've never done that. I buy the cam I want and if I have to flycut pistons..so be it.
Right now I'm re-camming the engine that came in my runner-bottom, so there are limitations, I don't want to fly-cut and re-balance because I'm sure my simple cam change would take on new dimensions that could cost me time on the water.
I'm going for a fairly lively solid flat tappet deal this season while I gather parts for something more serious, my question then will only be how much lift can I run and get 2 season out of my springs ;)

[EXTREMEBOATS]

Well now that we know you don't have a cam yet, using your degree wheel in an average base line from the cam you were using, you could your degree wheel to cam specs(max lift on intake and exhaust)using the light spring process you can now press the valve down until it touches the piston, take a measurement of how far it traveled, and subtract your assumed lash. This will tell you how much more travel you had with your old cam. Do the math and you will be able tell how much more lift you may be able to use beyond your last set up. Bare in mind different center lines and durations are going to effect this measurement with a new cam. So this is only a starting point consult your cam guy and using these comparisons they should be able to get you what you need. It sounds a little backasswards to me but it's the only way I can think of to get a starting point without doing alot of machine work. Keep us posted on your results it sounds interesting. Good Luck!! :cool:

[EXTREMEBOATS]

Oh and as far as the springs lasting buy premium springs, do your best to use light weight valve train componants and or possibly add a rev kit. Anything you can do to lighten the force to the spring will add life to your springs. One more thing a spring oiling kit(shower)is what we use on some of our motors. The extra oil spraying on the spring keeps the spring cooler which will add to its life. :D

[GofastRacer]

Another way to preserve springs is to back them off when not in use!.

[Jet City]

Another way to preserve springs is to back them off when not in use!.
I think thats a good idea for the higher lift cams, for this state that means over 8 months with the springs parked at the installed height.

[Moneypitt]

Hey, if I can crack you up, so be it. Life is to short not to see humor once in a while. However, I use my "cracks you up" method, with a degree wheel and a travel indicator and my valves don't hit my pistons........(Well at least as long as the rods are attatched to the crank). Beware of the Isky grinds, great camshafts, but they grind some of them "up", and if you kick em, they're way up........The clay/puddy method was used for years before the average home mechanic type had the necessary tools to check to a Knats ass. To say clay is taking the guess work out of it "cracks me up".........MP

[Fiat48]

I check the same way Moneypitt does. Usually find tightest is 8 to 10 from TDC on overlap. Me and clay never got along.

[Jet City]

Hey, if I can crack you up, so be it. Life is to short not to see humor once in a while. However, I use my "cracks you up" method, with a degree wheel and a travel indicator and my valves don't hit my pistons........(Well at least as long as the rods are attatched to the crank). Beware of the Isky grinds, great camshafts, but they grind some of them "up", and if you kick em, they're way up........The clay/puddy method was used for years before the average home mechanic type had the necessary tools to check to a Knats ass. To say clay is taking the guess work out of it "cracks me up".........MP
What do you mean about Isky's grinds, "but they grind some of them "up", and if you kick em, they're way up"?
As far as the clay vs. degree wheel thing, I think it has more to do with a higher level of understanding than tool cost, I think the average gearhead has an accelerated learning curve because of forums like this one.

[Jeremy1Kanobe]

I agree you should find the closest piston to valve at around 8 to 10 from TDC. It is recommended that the intake valve have a min of .080" clearance and the exhaust to have around .100" piston to valve clearance. This is how the custom piston manufacturer sets the valve relief depths up. In an BBC head there is usually no issue with exhaust valve clearance. The exhaust valve has alot of freedrop and you typically won't have an exhaust "Pocket". the exhaust cut would be more of a "Clip" cut.

[Moneypitt]

Just a little clearance problem due to someone that had not degreed an Isky cam, and bumped it out of habit. It was a little up already, like 3 degrees, and the additional 4 was just to much. After degreeing, and installing straight up it was fine and is still running today. (12 years) no problems.....MP

[Jet City]

Just a little clearance problem due to someone that had not degreed an Isky cam, and bumped it out of habit. It was a little up already, like 3 degrees, and the additional 4 was just to much. After degreeing, and installing straight up it was fine and is still running today. (12 years) no problems.....MP
Oh I understand, already had advance ground into it. It seems most of the other makers are doing that as well from what I see.

[Fiat48]

Oh I understand, already had advance ground into it. It seems most of the other makers are doing that as well from what I see.
Their "cure" for people buying too big a camshafts and losing low end.

[Fiat48]

What do you mean about Isky's grinds, "but they grind some of them "up", and if you kick em, they're way up"?
As far as the clay vs. degree wheel thing, I think it has more to do with a higher level of understanding than tool cost, I think the average gearhead has an accelerated learning curve because of forums like this one.
I hope I can explain this.
The "I don't have a degree wheel or dial indicator P/v clearance check:
Install light tension test springs on the valves of #1.
Install the head with the gasket.
Install the pushrods and rockers.
Adjust the valves to 0 lash.
Get a feeler gauge. Stack enough together to get around .080 total.
Put that feeler gauge stack of .080 between the rocker arm and the valve tip.
Use that gauge as a slight prying tool to open the valve a little more (which the light tension spring will let you do.)
Now keeping it spread a little..roll the motor over near overlap point and "feel" the tightness...find the tightest spot and measure that clearance.
Add your valve lash and that's your P/v clearance. Do the same thing on the other valve.
Caution...if you have less than .080 (or whatever stack you chose to feel with).
Hope I didn't lose anybody with that explanation.

[steelcomp]

I hope I can explain this.
The "I don't have a degree wheel or dial indicator P/v clearance check:
Install light tension test springs on the valves of #1.
Install the head with the gasket.
Install the pushrods and rockers.
Adjust the valves to 0 lash.
Get a feeler gauge. Stack enough together to get around .080 total.
Put that feeler gauge stack of .080 between the rocker arm and the valve tip.
Use that gauge as a slight prying tool to open the valve a little more (which the light tension spring will let you do.)
Now keeping it spread a little..roll the motor over near overlap point and "feel" the tightness...find the tightest spot and measure that clearance.
Add your valve lash and that's your P/v clearance. Do the same thing on the other valve.
Caution...if you have less than .080 (or whatever stack you chose to feel with).
Hope I didn't lose anybody with that explanation.
Hell, I'm throwin my indicator away! Damn PSI piece of crap sticks anyway! :rolleyes:

[Jet City]

Their "cure" for people buying too big a camshafts and losing low end.
Thats what I suspected, if DD2000 is right, choosing the correct (smaller) cam and running straight up will yeild a much flatter torque curve with a higher peak and virtually no peak HP loss.

[Fiat48]

Got another method that doesn't require even test springs. Requires 2 rubber bands. And I am not going to try to explain that one. You figure it out. :D
Can even be done with one rubber band. Or even valve seals.