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-   -   Piston to valve clearence help. (http://www.hotrodders.com/forum/piston-valve-clearence-help-230341.html)

1fast62 03-05-2013 01:06 AM

Piston to valve clearence help.
 
hey guys,
i checked my piston to valve clearence on my bbc.. it has kb pistons with a .250 dome height.
i have a set of 781 heads and i checked the clearence and it was 58 thous. i was reading you need a min of 80 thous. is there anything i can do besides fly cutting the pistons. and im running a 40 thous gasket already.
thanks for the help as usual.
Brandon

F-BIRD'88 03-05-2013 02:26 AM

The absolute minimum needed on the intake valve is .060" net with lash accounted for. (set the running valve lash)
This assumes that you will be using a valve spring that will not allow the valve to float or bounce at high rpm.

You can move the cam a bit to get more intake VTP clearance.

The depth does not tell you about the valve radius clearacne in the piston notch.
If the radius clearance is also tight the valve may kiss the edge of the piston knotch.
Its not that big a deal to take it apart and make the valve relief a little deeper and or make the radius a
bit bigger. (you will only need to do it once as compared to the big mess if the valves do kiss the piston at rpm)
This is a common common issue on most all BBC pistons. ESpecially the factory crate motor pistons. like the 502-502HP motor.

you need more clearance on the exhaust side as the piston chases the exhaust valve.

Whenusing a KB piston with a oval port head or any non factory head you must also verify piston dome to chamber wall edge clearance @ TDC. Sometimes minor chamber wall edge clearance grinding is needed too.
You need to check this and the valve relief radius clearance around the open valve too.

you cannot do any of this using a hyd lifter.

vinniekq2 03-05-2013 05:12 PM

retard the cam
you should properly adjuct clarance. if the cam is wrong ,change it.
if you dont fix this,what Bout the other mistakes?

Mr. P-Body 03-05-2013 06:20 PM

BBCs are very tempermental regarding valve train. We've had PROPERLY prepared high revvers touch the pistons with all 8 exhaust valves. I always give them .120" on the exhaust if going over 7,000. It's a good "habit" to keep, in any case. The tiny loss of compression is far offset by the less perilous position of the valve.

Fly-cutting may be inconvenient, but it's worth the effort. A good automotive machine shop has the capability or knows who does. If the pistons are pressed and not "floaters", this complicates matters. A REALLY good machinist with a REALLY good vertical mill can do it with the rods still attached. Shops like ours use a "piston vise", that "keys" on the pin bores, and therefore can't have a rod attached. My mill is fine for the piston vise, but I don't have a good mill vise (not my "department")...

Vinnie takes a little different "route" but is essentially saying the same thing. Fix it, don't gamble. You have too much money tied up in it to allow a "short cut" to ruin it.

Two questions: Did you have "light" springs on the head during the measurements? Did you degree the cam prior to taking the measurements? If the answer to either of these is "No.", start over and do it with these as part of the procedure.

Jim

BOBCRMAN@aol.com 03-05-2013 06:28 PM

BBCs are very tempermental regarding valve train. We've had PROPERLY prepared high revvers touch the pistons with all 8 exhaust valves. I always give them .120" on the exhaust if going over 7,000. It's a good "habit" to keep, in any case. The tiny loss of compression is far offset by the less perilous position of the valve.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Listen good!! Many BBC's become history because of this problem. I always used the .100" on the exhaust but not on a BBC, they will at least clean the carbon off the piston pad on the exhaust side..

F-BIRD'88 03-05-2013 06:49 PM

This is so true. If there ever was a engine that may get into trouble with valve piston contact at rpm its a BBC.
The valves are big and heavy, making them much more harder to control at rpm.
Even if you have to address the piston issue by mnakeing the reliefs bigger and deeper, by hand, it is well worth the time and effort. Another engine you really got to watch for VTP clearance is the Ford 429-460.

The piston chases the exhaust valve.

tresi 03-05-2013 10:47 PM

Isky used to make something like a fly cutter that you powerd with a drill. You installed it in the head in stead of a valve and set the head on the engine then chuck the drill on the stem. It had stops on it that you set so you didn't cut too much. Check on Isky's site if you're not wanting to go to the machine shop.

OLNOLAN 03-06-2013 09:53 AM

Cut your losses
 
Quote:

Originally Posted by 1fast62 (Post 1653634)
hey guys,
i checked my piston to valve clearence on my bbc.. it has kb pistons with a .250 dome height.
i have a set of 781 heads and i checked the clearence and it was 58 thous. i was reading you need a min of 80 thous. is there anything i can do besides fly cutting the pistons. and im running a 40 thous gasket already.
thanks for the help as usual.
Brandon

Brandon, I know this is not what you want to hear, but its the best common sense advise I can give you as a young hotrodder.

While I admire your tenacity at trying to do this yourself, maybe its time to reconsider everything about the build,cut your losses and pay a pro engine builder to assemble it for you. Usually assembly costs are not that high if all the parts are there. I mean that with all the expensive parts you've destroyed in multiple attempts to DIY, get a pro to put it together and if you ask him, he may let you come hang around while he's doing it and teach you a thing or two.

Also I'd like to point out that if you're still trying to use that big butt bertha mutha trucking thumper bumper cam(or whatever they call them) (249/255@.050-.663/683) ? Its wayyy too much for what you're trying to accomplish especially with the 781 heads.

If you just have to have a big dog cam, consider one a bit smaller than the Lunati, perhaps a Comp 292, but get it with a 112 LSA. This way you're not likely to have PTV clearance issues. With the wider lobe separation the piston is farther down in the bore when the valves are at full lift eliminating most PTV clearance issues. The little bit of low end torque you loose with the 112 LSA versus a 110 or 108 will help it hook out of the hole and you'll gain it back in the midrange and top end.

Nolan


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