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Unhappy Piston Valve Clearance problem

I had my 83SC motor rebuilt by a reputable shop two years ago. Last year at RA I mis-shifted and bent a valve. I am now rebuilding the motor myself. I am not upgrading the motor this time. While doing the valve - piston clearance check I found that the intake valve has 0.5mm clearance and the exhaust valve has about 0.7mm clearance. The book says 1.5mm and 2.0mm. I talked to the owner of the shop and he said that is conservative but agreed that the numbers I found were too tight. How conservative are those numbers? My mechanic says they use 1mm for both as a minimum. I figure since the motor has to come apart AGAIN I might as well have the pistons taken down to get near the 1.5 and 2.0 numbers even though I might not be able to quite get there with the exhaust.

Also I am thinking about asking the shop to take the motor, fix this problem and return it to me in the stage it is in.

Any opinions?

Thanks
TK
Old 07-15-2004, 08:31 AM
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What you have is way too tight for piston to valve. If you are running the motor at high revs, or tracking it, the more the better since you are up higher in the rev range. Shoot for the factory specs .060 on the intake and .080 on the exhaust, any less gets risky. FOur valve heads, and smaller motors can get away with less. You have to take it apart and correct it, but make sure that there is enough meat in the dome to drop them down the amount needed. Also realize that by dropping the valve pockets, you will lose compression. I am curious as to why, if you are running a stock motor, your clearance is so tight to begin with?

You definetely need to get there with the exhaust, of the two, that is the more critical, since the piston is chasing the exhaust valve closed and if you blow a shift or something like that, the exhaust valve usually gets tagged first.

ALso, how are you checking the clearance, clay or dial indicator?
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Old 07-15-2004, 09:05 AM
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Also Shaved the heads

I forgot to say that they also shaved the heads when they did the motor two years ago.

I am checking the clearance by using Wayne's method of turning the screw in until the valve touches the piston.

TK
Old 07-15-2004, 09:10 AM
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Do you know haow much the shop took off the heads? It acn be compensated for somewhat by using a thicker (.5mm) cylinder base gasket or by stacking the gaskets. I personally only stack a maximum of two gaskets, but I know of people who won't stack them at all.

Jeff
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Old 07-15-2004, 09:16 AM
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How much head shaving

I don't know how much they took off other than it appears to be too much.

The shop owner mentioned using a thicker gasket but recommended "fly cutting the pistons"

The really odd thing I just found out is the #4 exhaust valve has about 2.3mm of clearance. The #4 intake is at about 0.4mm.
Old 07-15-2004, 09:20 AM
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It is very important that all the heads had the same amount of material taken off, or at least the ones on the same side of the motor are the same. There is not a lot of material on cis piston to remove, some , but not much. Did the shop do any machining to the cylinders? This could of got messed up too.
If this shop is "reputable" like you say, I think they should fix the clearance problem. Henry from Supertec had a good point on a recent thread when he said everyone will make a mistake, but what is important is how they deal with it. Well that is close to what he said anyway.

Jeff
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Old 07-15-2004, 09:31 AM
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Explain to me excactly how you are checking the clearance? You need to run the crank with the cams timed and the lash set properly, and check the piston to valve through the range. You can make your life easier by installing light springs and using a dial indicator. Or some people like to lay strips of clay across the dome, time the cams and turn the motor over, then remove the head and measure the clay at its thinnest point. I am not familiar with turning the screw until the valve touches the piston, explain that to me.
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Old 07-15-2004, 10:29 AM
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method of checking clearance

In Wayne's book about rebuilding 911 engines he describes how to check the valve to piston clearance using the valve adjustment screw. After paying the aformentioned shop to do my cam timing I turned the valve adjustment screw in until it just touched the valve. Next I turned the motor over with a wrench until you see that the valve and piston are getting close together. By turning the screw until the valve just touches the piston you can get a good estimate of how close the valve is to the piston by counting the turns on the screw. 1 turn =1mm. You have to do this for every point in the rotation that you want to check the clearance on. I put little marks on the crank pully at each postion that represented roughly how much clearance I had at each point. That way I could see a crude profile of what the valve clearance was like during the rotation so I could go back and double check it later by going directly to the point I had determined to be the closest.
Old 07-15-2004, 10:42 AM
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Christian, the idea is that for each turn of the screw, you are opening the valve by 1mm. If you can turn it twice without hitting the piston, then you'd have 2mm clearance.

Using this approach on, say, every 10 degree around the crank should give pretty reliable results.
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Old 07-15-2004, 10:48 AM
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I guess that would work if you didnt have any other tools, and that will give you a rough idea. But a degree wheel is cheap, a light spring is cheap, pick up an inexpensive dial indicator, position is on the valve stem and take a reading at least every two degrees, and this way, you can get a definite, and rock solid number, not a ballpark. Do your self a favor, spend the extra time and do it that way, that way you will know EXCACTLY what you have, and where you have it. A few degrees on the crank can make a HUGE difference in the clearance.
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Old 07-15-2004, 11:03 AM
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OK - I'm confused.

I can see how using a dial indicator can give you the valve lift for any position of the crank / cam you wish to measure, but how does doing that that give you actual valve to piston clearance at those selected points?

Pistons and heads go on next, so I am very interested in your responses.

Thanks!
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Old 07-15-2004, 02:11 PM
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Are you running the same base gaskets as the shop installed? If so, what thickness?
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Old 07-15-2004, 03:30 PM
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Old 07-15-2004, 03:52 PM
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Very funny Henry.
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Old 07-15-2004, 03:58 PM
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The dial indicator method will give you the best actual bottom line number. By dummy assembling the head before final assembly, using light springs on the valves, not the actual running springs, just a light spring that is long enough to pull the valve closed, you install them in the head, assemble the top end of the motor, time the cams, and install the degree wheel. You then begin to rotate the motor over by hand until you reach the point where the piston and valves are closest- usually within 30 degree of tdc on the exhaust stroke. You then record the crank degree angle, position the dial indicator on the valve stem, or retainer, with enough travel and zeroed, and press the valve open by hand until you contact the piston. Record the travel, and then rotate the crank another 1 or 2 degrees, rezero the indicator, and open the valve to contact, again noting the number, and on , and on , and on. Then switch over and do the same on the intake. This way you plot out the entire critical piston to valve clearance range, but most importantly, take the lowest reading, which is the point where the piston is clossest to the valve, and see if you fall within a respectable limit. Its easy, but a little time consuming, but in the end you know- excactly what you have.

Also, it is best to use the degree wheel again when final assembing and checking the cam timing, relying on the pulley notch is not the most accurate all the time, they are often close, but not dead on. So to use a degree wheel, and a dial indicator again the find true TDC and then time the cams is most accurate.

But then again, the accuracy pretty much goes by the wayside once the engine is running since you are driving cams from a chain as long as your leg, which is flopping around and stretching all over the place. I'm going to make a gear drive for the 911, with some straight cuts in it, that way the timing stays dead on, and it will make noise that will make people say what the hell is in that? See how much power you pick up when your cam is running where it should be!!!!!!!! It would be evil.
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Old 07-15-2004, 04:16 PM
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Old 07-15-2004, 08:23 PM
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Christian,

Didn't the Aero 911 engine have gear driven cams? Are you coping that?

JP
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Old 07-16-2004, 04:51 AM
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Yes, it may have, as did the 962 race engines, but they did it by stacking up gears with idlers, so there still was a stack up up play due to the backlash needed for each gear,we were thinking of trying something different to minimize that.
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Old 07-16-2004, 06:35 AM
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Christian,
I always enjoy your posts...good info! I'll buy a set of those gear-driven cams as soon as they are ready...

I'm not a fan of the "screw in the adjusters" method, as detailed in Waynes book.
So, when I re-did my 3.0, I first checked clearances with solder strips (as per Walt Watson). I was still a little unsure as to the accuracy, so I used the method as you described. Most accurate. I'd read about the same procedure in another book on rebuilding VW engines.
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Old 07-16-2004, 07:27 AM
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Thank you! Like I said, the dial indicator is most accurate, since you are getting a true number. Some may find it intimidating, but it is quite easy once you inderstand the method to doing it. I am not saying the the clay method, or the method descibed in Waynes book is wrong, or dont do it, but this way you see a real number, the ohter ways will get you in the ballpark. I just finished up a motor where when I checked it, I had .039 piston to valve on the exhaust, so I had to take it all back apart and drop the reliefs down to get the .100 I wanted for this application. So dont feel bad if you find you have to take it all apart again, sometimes thats the way it goes, but if you do it right at first, then you dont worry about it down the road.
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Old 07-16-2004, 07:45 AM
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