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Old 03-05-2013, 12:06 AM
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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

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Old 03-05-2013, 04:12 PM
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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?
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Old 03-05-2013, 05:20 PM
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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
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Old 03-05-2013, 05:28 PM
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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..
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Old 03-05-2013, 09:47 PM
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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.
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Old 03-06-2013, 08:53 AM
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Cut your losses

Quote:
Originally Posted by 1fast62 View Post
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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