Exhaust valves hitting pistons

Old 05-28-2007, 07:54 PM
  #1  
gyrogearloose
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Default Exhaust valves hitting pistons

We have a 598ci that the pistons hit the exhaust valves only in cylinders number 3 and 7. All the other piston and vlaves are OK.
Brief history:
The heads are Dart Big M 410s. Chapman racing heads did all the work.
We use TD rocker shaft assemblies. Smith Bros. push rods, .100 walls.
We used these heads on the 565CI we had before we built the 598. NO Problems.
About a year ago the engine blew the head gaskets into the valley on
cylinders number 3 and 7. At that time, you could see that the pistons and exhaust valves where touching. The head needed resurfacing and the chambers touched up to clearance the pistons again after the resurface. We thought that this would clean up the problem. We ran it thinking all
was ok to the end of the season in November. When we disassembled
it to go through it for this year we noticed that the pistons and exhaust valves in cylinders 3 and 7 had been contacting each other again. As we had approx 100 runs on the engine, we thought that the valve springs were getting weak and did not think anymore about it because we change the valve springs, exhaust valves, pistons and bearings at about 100 runs. Again Chapman Racing Heads did the work on the heads. We mocked
it up and checked all the clearances as always.
We took it out for test and tune about 3 weeks ago and put 2 runs on it, it ran fine, so we put it away until the first T/S race at our local track. In the ninth run, it ate number 7 exhaust valve. After we took the head off to inspect the damage, we seen that number 3 exhaust valve has been touch the piston again. We again measured everything and we can see no reason for this to be happening. We are lost for a explanination for this to keep happening. And it is only on cylinders 3 and 7.

Spec info:
Crane cam sold roller/Crane roller lifters .810 lift int, .800 lift exh. timing 31°BTDC @.050".
Triple springs rated at 908 pounds at full open.
TD rocker shaft assemblies with 1.7 to 1 ratio
JE Pistons/46cc domes.
.005 deck clearance
Dart heads with 118cc chambers.

Any ideas or help would be appreciated on what is wrong. We can't afford to keep rebuilding this engine and
until we find out what is causing this, there is no sense trying to run this engine.
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Old 05-28-2007, 08:08 PM
  #2  
gyrogearloose
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I forgot to say that when we reassembled the engine in the past, not sure that the heads or rocker assemblies were put back on the same side of the engine. So it is possible that this is a problem with numbers 3 and 7 only. Just don't know.
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Old 05-28-2007, 09:18 PM
  #3  
sp2816
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There is some more information that is needed. Are you running steel rods or aluminum? Timing chain or timing belt? Are the piston valve reliefs at the same angle as valve angle? How many RPM's are you turning?

When you are checking the piston to valve clearance are you checking on #1 only or on all cylinders and what value are you coming up with? I try to have .100" for the exhaust valve as a minimum. 10 degrees BTDC of the cylinder checking is the critical timing for the exhaust valve because the piston is "chasing" the exhaust valve closing. It is 10 degrees ATDC for the intake valve. I usually like to advance and retard the camshaft 8-10 degrees (timing belt makes this easy) to check this clearance, also. That way if I want to advance the cam 4 degrees, I already know that there won't be interference problems.

The margins on the exhaust valves for #3 and #7 could be thicker which could put those valves closer to the pistons to begin with. When you had the heads shaved you would actually make the problem worse, because the valves are getting closer to the piston by the amount you have the heads resurfaced. The timing chain could be stretching some or the camshaft timing could be off some on those cylinders or it could be twisting some. The deck height is stated as .005", has that been verified on every cylinder, because they can sometimes vary a few thousands from cylinder to cylinder.

Since you are having recurring problems, I would check the piston to valve clearance with the rocker shaft numbered for each cylinder head and verify the rocker ratio for each. If all of the numbers still measure up, then I would flycut the pistons and increase the piston to valve clearance by .040"-.050" as long as the piston decks could handle it.

This is just some things that I would check and I am sure that there will be some other good inputs on this, but we will need to know what is the exact value that you are coming up with for the clearance.

Bill M
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Old 05-28-2007, 10:25 PM
  #4  
altune
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It seems strange only 3 and 7 touch.
one of the deals may be, the head guy resessed some of the valves up into the head and not others. did you cc the heads ?
the only other explanation would be the camshaft is ground wrong, have you used a dial indicator to see how much of a lift at full open you get?
hmmmm
another thing, what crank is in it? Could crankshaft deflection be an issue?
maybe crank is slighly twisted.
Al
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Old 05-29-2007, 03:24 AM
  #5  
edvancedengines
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Two very good responses already, both from Altune, and from sp2816.

I concur completely that you need to check each cylinder for V/P clearance and for deck ht. IN today's world it is difficult for me to think that any good shop, much less a shop of high reputation would possibly not be sinking each valve to be identical. Now with the newer Sunnene and the Srdi attachments and the Serdi machines it is almost a given that each valve seat will be identical in cut and in depth. Still it does not hurt to check them.

Possibly if you are running a tight V/P clearance tolerance you could be getting a tolerance stack with the piston being jst a wee bit taller in the deck and the vlave margin being jst a wee bit thicker and they collide. Not likely but possible.

Unlikely but possible is that the crank rod journals are not in complete index and the piston is in a different position when the valves are in overlap on the affected cylinders. That is not likely but is possible.

Like was said earlier check the camshaft to se if a mistake was made and the lobes are not all the same. If your T&D Rocers have the same number stamped on them, I can pretty much assure you they are right on.

Does your engine require different length pushrods in the different cylinders? If it does and you got them mixed up, that could change the rocker geometry and perhaps the lift in some cylinders. Yep' I know that is stretching it, but we are guessing at possibilities.

Like was already mentioned it is unlikely that your .005 deck is ecatly a .005 in every cylinder. So the affected cylinders could be just a little higher. Everyone does not check piston deck ht the same either.

Last but not least are the valve springs.
I would be using a better spring with that much lift. You did not mention what your on the seat pressure was but by your open pressure I am guessing they may be low also. If perchance you are have some vlave control issues but not bad ones, and for whatever reason those valves are closer to the piston, then possibly those are touching or hitting and the others could be very close to doing the same thing.

If your cam is a mainline camshaft company cam, I can tell you that their recommended spring pressures are at the best marginal of what is needed. If you have a speciality custom ground cam, then you should be able to follow the cam grinder's recommendations for it.

Along the subject also of valve springs is valve spring set-up. If the springs are set-up too far from coil bind you may be experiencing spring surge and when the springs suge there is nothing holding the valve closed for a few brief split seconds. I don't care if Chapman set them up, or any other head ghru they can still be setup wrong for your requirements. If you are getting anyone to set up your valve springs give them all the info about the cam and the rockers you can give. At times I am limited in valve spring set-up becuase of inferior springs.

PS;
You do know that you can increase your EX Valve to piston clearance by changing the way your cam is degreed, don't you?

Ed
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Old 05-29-2007, 06:48 AM
  #6  
gyrogearloose
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To answer some of the questions ask above.
The crankshaft is a Lunati 4.500" stroke. Rods are Lunati billits 6.7".
As stated above, the cam is Crane, Part Number: 19332 Grind Number: 328-358-16R
http://www.cranecams.com/?show=brows...tType=camshaft
Comp Cam Times belt. New belt
Chapman Racing heads is a very reputiable head shop.
http://www.chapmanracingheads.com/p_aboutus.asp
The push rods are all the same length in respect to intake and exhaust.
I have checked the cam timing using #3 cylinder, both intake and exhaust.
Yes, the deck height is .005 across and there is no sign of piston to head contact. .039 headgaskets.
The pistons are JE #131448 and the dome is only 42cc and not as I first stated.
http://www.jepistons.com/cat/je/auto...per_dome.shtml
We shift this engine at 7800 rpms and the car goes through the trap at about 7300 to 7500 depending on the track and weather conditions.
We use a MSD AL7 ignition system and we take a d/l after each run, so I know the rmps are as stated.
I can turn this engine over by hand on the stand and there is no contact.
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Old 05-29-2007, 07:23 AM
  #7  
mcracecars
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I can turn this engine over by hand on the stand and there is no contact.
well, no they will not contact on the stand, only when everything is heated up and expanded and moving around.....
interesting problem though, I tend to suspect valve float, or the bad dynamics that Ed stated. Howevery why only 3and 7. Are the spring hights the same as the others, seat pressures.? Is the valve lash on these cylinders changeing ?
Have you tried swapping the heads from side to side to see if the problem follows the head?
Just a few random thoughts here, as I am sure you have checked everything you can think of. I know how you can run around a chase your tail over these types of problems that dont make any sense, But I have usually found there are simple solutions to major problems.
good luck with it
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Old 05-29-2007, 07:39 AM
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sp2816
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Those are some nice pieces and the timing belt will not allow the timing to change like a chain can. But, you must replace the valve spring with a lightweight checking spring and check the piston to valve clearance with a dial indicator or with feeler gauges to ensure the clearance. By turning it over on the stand, you do not know if it is .000" or .200" or anywhere in between. It will rotate for both, and you could possibly still be touching the valves and still be able to rotate it. The clearance decreases with increased rpm's and heat, so that it why it is necessary for the .100" number.


Bill M
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Old 05-29-2007, 07:42 AM
  #9  
topsportsman1
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I am kinda curious to see if this cam was actually degreed in and clearances checked?

What is this cam degreed in at?
What method was used,CL method?

I am taking it that this is a normally asperated engine,and not being sprayed

Oh and whats the LSA on this cam I must have missed it
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Old 05-29-2007, 08:37 AM
  #10  
gyrogearloose
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Originally Posted by topsportsman1
I am kinda curious to see if this cam was actually degreed in and clearances checked?

What is this cam degreed in at?
What method was used,CL method?

I am taking it that this is a normally asperated engine,and not being sprayed

Oh and whats the LSA on this cam I must have missed it
The cam is degreed in at 31° BTDC @.050 lift at the valve stem. I use a dial indicator and degree wheel.
Normally asperated. Ron's Fuel Duel terminators burning methnol.
See link to cam spec above.
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