oof440
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posted October 24, 2005 02:00 PM
Piston to valve clearance
How much piston to valve clearance should I have on my 12. I degreed the cams last night, then pulled the head off and clayed the cylinders. It looks like to me I only have 15 thousandths of an inch clearance on the valves but I dont know what I should have. Alot of the car sites are saying 75 thousandths but that seems excessive to me. The cams are set at 105/103 so its not like I'm running some kind of wild valve timing. Any help would be greatly appreciated.
  
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dougmeyer
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posted October 24, 2005 03:13 PM
MINIMUM .060 on the inlet and .040 on the exh.
The difference is because the relative motion between the piston and the inlet valve is "opposing" but the piston is "chasing" the exhaust valve.
If you're running stock cams and you've only got ,015 you're not measuring either the timing or the clearance correctly. You should have plenty at those lobe centers.
D.
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oof440
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posted October 24, 2005 03:32 PM
I had je build me a set of pistons and it looks like I'm probably gonna need to have them cut the pockets out some more. I've got enough clearance on the exhaust but not the intakes. They built me a set of 2 mm over 13 to 1 pistons.
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trenace
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posted October 24, 2005 03:41 PM
Are you using Muzzy cams? (Or other cams with increased lift.)
I had to have the CP pistons made with valve cutouts that took that into account.
Thanks Doug that's great information, I never knew the reason for the difference before, and I'd seen other values that I don't consider as reliable as yours (though as it turns out they were similar) so I was glad to see your post.
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oof440
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posted October 24, 2005 05:22 PM
I'm just using stock cams. This is on an 03 that I was building to run nitrous on. The engine tune is relatively mild but I tryed to build it bulletproof in order to handle a big shot of nitrous. They told me that they already had a blueprint for a 12 piston to go off of so all I had to do was give them the bore and compression ratio I wanted. The only other thing I could think of was that maybe the cams from an 03 are different then the earlier models and they didnt compensate but I dont know.
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oof440
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posted October 24, 2005 05:29 PM
Do you guys think that changing my cam centerlines a degree or 2 would help. I could probably pull the intake back to 104 or 103 if it would make enough of a difference that I wouldnt have to have the pistons cut. I figure I'll be losing compression ratio if I have the pistons machined so either way I'm probably losing performance.
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trenace
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posted October 25, 2005 04:25 PM
No... I don't know exactly how many thousandths per degree the valve motion is at that point but its on the order of a few (maybe five VERY roughly) thou per degree. Even if it was ten thou per degree which I don't at all think is the case, you'd need another 45 thou so just one or two degrees would not do it.
Something is wrong that, if it's presently really at 105 intake and the clearance really is only 0.015", changing the cam a couple of degrees is not going to be the fix on.
Whether it's that the timing is not now at 105 degrees but quite different is one possibility; another is that the clearance really is much better than 0.015", and the last possibility I guess is that the piston maker screwed up bigtime.
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oof440
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posted October 25, 2005 06:01 PM
I went back today and checked everything, valve clearances, cam timing, and piston to valve clearance again and it is still the same. When I was talking to je about this motor before I ever started they told me that they recommended that I run a .040 spacer plate under the jug and that would allow them to add .040 worth of material above the ring pack in order to make it more durable because of the shot of nitrous I wanted it to handle. I guess that they must have miscalculated somewhere but I'm gonna call them tomorrow and see what they have to say.
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Y2KZX12R
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posted October 26, 2005 11:22 AM
 Edited By: Y2KZX12R on 26 Oct 2005 12:25
The piston to valve clearance on the 12r is really tight. I removed the shim under the block (for several reasons) and lowered the cylinders and head. I wanted to run 102 on the intake side so I flycut the intake valve reliefs. The end result was 181 rwho 101 torque and much better performance below 7500 rpm. All the parts in the bike are from the factory except the cam gears. Cranking cylinder pressures are higher than stock.
Its been years sence I did all this and I'm going off memory... If you need specifics I could dig em up.
I'm running very tight clearances on the intake side. I'd have to look but .035" rings a bell.
The piston to head clearance is very tight as well.
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trenace
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posted October 26, 2005 12:06 PM
Edited By: trenace on 26 Oct 2005 13:11
quote:
MINIMUM .060 on the inlet and .040 on the exh.
The difference is because the relative motion between the piston and the inlet valve is "opposing" but the piston is "chasing" the exhaust valve.
Doug, by no means am I disagreeing, I am just trying to understand.
At first I thought I did understand that.
But on further thought, it seems to me that in the case of the intake, precisely because it is opening around TDC, there is no risk of it floating.
Whereas the exhaust closing around TDC could potentially be floating.
So if anything from that aspect I would think (I'm assuming from your post wrongly) that the exhaust would be the one needing more clearance, as it might float near TDC while the intake should not.
Other than that, provided the parts don't touch, I'm failing to understand the importance of direction of motion to clearance.
The only exception I can think of could be that since the intake valve's motion is faster (rapidly opening) around TDC than the exhaust valve's motion (slowly closing) then if there is error in the cam timing due to the cam chain flailing or what have you, more clearance would be needed. But that would really not be a direction of motion but speed of motion issue so I'm not sure that's what you meant.
Can you explain? Thanks!
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dougmeyer
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posted October 26, 2005 05:09 PM
Well, I think you answered your own question. But it's because IF the parts do touch you only succeed in having the piston shove the exh. valve closed whereas if you tag the inlet there's nothing that can give way that doesn't cause expense. So the greater margin for error comes in handy. Those dimensions are generous. On Bonneville stuff, where an over rev or missed shift is not likely, I've run the V to P and squish much closer than what's normally recommended.
It's really a long standing theory from the days when valves really did "float" with some regularity, unlike today where it is a rarity.
Doug
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trenace
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posted October 26, 2005 06:19 PM
Thanks Doug!
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oof440
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posted October 27, 2005 03:30 AM
So if the .060 and .040 is on the safe side then what do you think that I could get away with. All of the valvetrain is new and I'm running some stout valve springs so I dont think that I'll have to worry about float. Anything that'll make this mess easier to clean up I'm open to.
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Y2KZX12R
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posted October 27, 2005 06:30 AM
oof440, its all about what level of risk you feal is acceptable.
I went back and looked and i had .015" before flycutting the intake valve reliefs on the stock pistons. I took off .020" I now have .035" and its been fine for 3 years 20,000 miles. (missing a redline shift here and there) and stock cams and springs.
Carbon buildup hasnt been an issue so far.
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VincentHill
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posted October 27, 2005 01:41 PM
Just to pile on, make sure you measure every piston and valve clearance because the 2 I measures were correct but the others were not and I touched a "FEW" Valves. I no longer have JE Pistons
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oof440
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posted October 27, 2005 05:47 PM
Vincent do you know what yours were at before you hurt the valves. Does this seem to be a constant theme with je pistons, I thought when I ordered them I was getting some good stuff but now I am less then happy. I did make sure and check all the pistons and they were all between about .018 and .015. By the way I really appreciate all the help guys. I havent had a chance to talk to je yet but I have had a chance to talk to a machine shop locally that does really nice work and I'm thinking about just haveing the valve releifs cut another .025 to bring me out to .040 of clearance. I but the less material I have to remove the better.
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dougmeyer
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posted October 27, 2005 07:25 PM
I'm confused by all of this. Aftermarket pistons (I dare say ANY aftermarket pistons) have better quality control than stockers and all have deeper valve reliefs to accomodate the expected performance cams. Why you guys are having v to p problems is beyond me. Of course an over-riding consideration is squish which changes everything equally. But, if you have properly designed cams (and I don't think I know what sticks oof440 is using), good pistons (and I consider JE one of the best), any problems lie in the setup and measurement of the valve train. There is no excuse for bending parts.
Here's an opinion you won't find to be universal- Compression is the priority. you get more power by bringing the compression to the highest you can get, even if you need to alter cam timing slightly to make it work.
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psycho1122
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posted October 27, 2005 08:24 PM
That has worked very nicely for me 
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psycho1122
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posted October 28, 2005 09:33 AM
Doug;
However, As you know, my 1270 has become quite the Torque Monster since the rebuild. (Third Gear Blew). I have 245 psi cold at the current config.(The old config. was 230 psi per hole)and have Over 100 ft. lbs. by 3,800 rpm's and a 111 ft. lb. peak at 7,400.
The Horsepower peak comes on earlier (10,300 vs. 11,300) than the previous engine but falls off 1,000 rpm's early. The only hardware change when the engine was redone was rings, bearings, CAM CHAIN, seals and billet gears. The cams were not re-degreed and I feel that I lost some overlap.
In your opinion Doug; with the same stock gearing, which engine config. would be a better at accelerating? I currently shift it at 10,800 and it is STRONG!
Does TOO much psi sacrafice top end Horsepower??
Could I benefit acceleration wise from a smaller rear sprocket?
I hate to say this....My 1270 is very Eye Abusa Like.
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oof440
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posted October 28, 2005 01:26 PM
It seems crazy to me that I would have to worry about this stuff with aftermarket pistons too. I figured that buying the je's would pretty well keep me away from any quality issues. But I've checked and rechecked and today I had one of the guys that races an AMA funnybike come over and double check my work because you guys had me thinking that this had to be my fault. But he said that it looked like I had everything covered and I was just going to have to have the pistons cut. I know it sounded ridiculous to talk about changing cam timing in order to clear up the problem I know that any compression issues would be solved with the nitrous but I was just desperate and really wanted to have the bike together for this weekend so I was just throwing anything out there to see what you guys thought. I know that you guys know alot more about these engines than I do so if it wasnt feasible- which in my head I really knew it wasn't- you guys would let me know. I'm going to send the pistons to the machine shop and have them cut. Thanks again for all of you guys help.
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dougmeyer
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posted October 28, 2005 03:05 PM
IMHO
If you're SURE you're doing everything right. Set the cams to give you the minimum V to P you're comfortable with, note the timing to see of it's within the 101 to 107 range (which would mean that you are probably doing the job correctly) and run it.
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psycho1122
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posted October 29, 2005 05:29 AM
Doug?
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