Exhaust valves hitting pistons

[gyrogearloose]

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.

[gyrogearloose]

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.

[sp2816]

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

[altune]

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

[edvancedengines]

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

[gyrogearloose]

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=browseParts&lvl=2&prt=5&Vehicle_Type=Auto&Cylinders=8&Engine_Make=CHEVROLET&Year=1974&Engine_Size=396-454%20C.I.&partNumber=19332&partType=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/chevy_bb/open_champer_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.

[mcracecars]

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

[sp2816]

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

[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

[gyrogearloose]

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.

[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.


I see so what you are saying is that you used the 31* @ .050 method OK

Another thing you need to do is make sure TDC is verified with your degree wheel using a stop or dial indicator,I prefer the indicator myself

One thing for you to consider is,to do your checking at the #1 intake lobe when doing it this way,,the way you have done it at the intake valve tip you can have some error,I'd reccomend doing it at the intake lobe,then check the clearances with lite springs like someone said previously.

If you can't get proper clearances that way,I'd reccomend setting the cam in with the centerline method and advance it till get sufficent enough exhaust valve clearance.,some where around 110 degree center line would be a close start

[Robert1320]

I am interested in finding out what worked out. I agree with the guys above.

[bjuice]

This is some good stuff here...JUST wanted everyone to know that has participated in this thread ..i am soaking all of this up.....anxious to see what the out come is....

a -lot of varibles thrown in here by some very experience fella's

Brian

[gyrogearloose]

It has been a long day. Ran all over and burned up $100.00 in gas. And talked to a 100 poeple besides everyone up here who is offering good advice. Took the head back to Chapman's to get it repaired, and had them install check springs on the other head so I could mock up the left bank and #3 cylinder again.
Yes I used the dial indicator to find TDC on #1 and it is correct. I also checked #3 TDC for the hell of it, and it is correct. (like the crank is going to be wrong.) Took the other pistons over to H. Doubaugh's shop to have the valve pockets check and realigned to the valve angle if necessary. Tomorrow I will check the clearance of #3 exhaust valve at 10° BTDC on the exhaust/intake stroke. that is the closed the piston will ever be to the piston now that I have the check springs on the valves. Will post tomorrow with results and any further developments.
Thanks for all the help todate,
gyrogearloose

[gyrogearloose]

Forgot to say that I asked if the valves were set back to the correct depth and the chambers were re-clearanced after the resurface and was told that they were.

[edvancedengines]

Hi again,
By no means was I saying that Chapman does not know what they are doing. I was meaning that it does not matter who sets up your valve springs including me, if they are not set up for your exact cam needs and no more extra clearance from coil bind than is the minimum necessary they could be done wrong. Also ask them about your exact springs with the cam profile and lift and rpm range you want and see if they are recommending this same spring.

How much valve to piston clearance do you have to the intake valves? You can possibly get some additional wiggle room there too like I said earlier, by changing your camshaft lobe Intake Centerline.

Ed

[gyrogearloose]

I didn't get anything done yesterday. (Some days I could just strangle some employees.) I checked the clearance of #3 exhaust valve at 10° BTDC on the exhaust/intake stroke of #3. With the dial indicator I have .765" clearance between the exhaust valve and the piston.* I again checked the cam timing at #1 and it is just where I set it. 31° BTDC @ .050. I again checked the cam timing at #3 It is the same as #1 as far as the intake and exhaust opening and closing.

Ed, I know, Not sure that I said my answer correct. Anyway sorry for any miss communitactions. The valve springs I used are Crane Part # 96848 and I also used the Crane rtoller lifters Part # 13552-16. I like keeping everything matched if possible. With the clearance I have between the valves and pistons, I don't think it is a clearance problem. Will look at the valve train componets tomorrow. Still not sure what I am looking for.

* I had to check back in my notes to see why I put so much clearance in the combustion chambers, but when I had the combustion chambers layed back, I was using 50cc domes. I am now using 42cc domes, which accounts for all the clearance between valves and pistons.
Still looking for any ideas,
thanks,
gyrogearloose

[sp2816]

That number doesn't sound correct. With your domes, duration on the cam, and valve lift, if you get numbers higher than .180" you had better measure again.
To check the clearance, you need to set the valve lash to .000" with the pushrods and the rockershaft on. Then you rotate the engine over to 10*BTDC on that cylinder (#3 would be 100* on the balancer). The camshaft will actually open the valve and start allowing it to close. Then the exhaust valve is depressed additionally by hand until it touches the piston. The extra distance the valve will depress down from the rocker is your clearance.
When you come up with your number this way, you can always add your valve lash to the number and come up with your running piston to valve clearance. Say you come up with it being .075" and your normal valve lash is .025", your piston to valve clearance is .100".

Hope that this helps,
Bill M

[gyrogearloose]

It has not been a good week. I don't know why I posted this number this way, but, .765=.0765. Please don't ask, because i don't have an answer for this. Anyway with the 50cc domes, it wasn't that much, but ok.
Now for the update. I talked to JE today to get replacement pistons for #3 & 7. It is a discontinued part. The replacement piston (#258236) is 35 grams lighter than the pistons I am using. They said They would machine two pistons for me, but it would be a minimum of 2 weeks. Which in realility is more like 1 to 2 months.
So we have decided to put the backup long block in the car and start over with the current engine.
So until I find the problem, I will not be posting to this thread anymore.
The race season is too short to be sitting out for 3/4 of it. Thanks for all the help and advise from everyone.

gyrogearloose

[edvancedengines]

I could probably get you a set of pistons custom made in 3 weeks. If not custom made maybe even quicker if it is a standard style of dome or combustion chamber configuration.

Another thought is that possibly Mike Panetta at Diamond could get you some together quickly.

I personally hate the JE pistons that they call nitrous pistons. Just a personal opinion. I also hate what looks to me like mountains sitting on top of a piston. I have been making mole hills out of mountains on pistons for a long long time. Even if someone does bring me an out of the box JE nitrous piston, I chop chop until I am happy.

Look at your burn pattern on your piston and in your combustion chamber. Look good at all the soot and crap. Now clean the piston good and wire brush the top hard with a brass brush. If you see any color where that color is , is your actualy burn pattern. I think you will discover that most of the previous colorations just washed off the piston, particuarily the piston dome and the quench pad of the piston will possibly look like it has never even seen any heat at all. If that is what you see or anything similar you can gain power with a different dome profile that works better with your chamber.

Wo cares if you lose a little compression if you are improving the flame probagation of the joining of the many different flame fronts to get a better burn. Actually I have done some pretty high compression stuff with domes that were not as larrge, particuarily with large swept volume engines.

You did have some very important measurements missing from your posts though. You never posted what the LSA was or what Intake Centerline you had it set up for. Even if you are degreeing in a cam using the .050 opening and closing numbers it is still advisable to check the Intake and the Exhaust centerlines.

Maybe your v/p clearance was enough, Maybe other guys would like an .0765 exhaust clearance. I don't.
Not to me it wasn't. I do not do exhausts less than .100 and I prefer .125. Intake is not so big of an issue and I do get them closer.

Ed

[billhendren]

We run the int clearance down to .040 on all our high compression stuff to keep from loosing c.c. and reduce the dome size. on the ex with .0765 you will get into trouble.As Ed said .100 is bare minimum with .125 preferred.
We are a W.D. for J-E and they are 3 weeks out but they have an expedited service if $$$ don't count. 7 working days is 25% more 5 working days is 40% more or 3 working days is 50% more.Bill

[gyrogearloose]

Thanks for the offer on the pistons Ed. We disucssed the the options of replacing just the two pistons and talked it over with Wade who does the balanceing for us, Harry over at HD Machine who was going do the valve pocket cutting and Mike at Chapmans. Wade said that it would not be a good idea to just replace the pistons and not rebalance the rotating assembly. And when I told Harry about the clearance I had, he told me just what Ed and Bill are saying. Made comment that I have been dodgeing the bullet for some time now. Harry said he set his clearances to .090" min.
So I have bought a new set of pistons and will start over and set all the clearances back to where they belong.
When the head gasket blew on the left bank, we had the heads resurfaced in Boise Id, (we were racing at Firebird in the Ignitor and didn't want to come home, so we repaired it in the trailer at the track) at a machine shop that a local racer suggested. The only thing I can think happened is that they machined more that the .009" that they claimed. And I have no one to blame other than myself for not checking the clearances after they machined the heads.
That is what happens when you get in a hurry. That is why I am using the backup engine and take my time with the main engine.
But HEY, thanks for the advice everyone. I have learned something again. This problem won't happen to me again. :)

[edvancedengines]

HEY, thanks for the advice everyone. I have learned something again. This problem won't happen to me again

That is what we are here for. Glad that now you have a better idea of things to be aware about.

Ed