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Old 12-02-2004, 04:58 PM
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Angry Piston/valve interference problem

I have built a few small blocks in my short years on this earth and have recently assembled a 355 with H345NP pisons, and a cam with intake duration of 218 @ .050 on the intake, lift at the valve is .462" Heads are 993's with 2.02 and 1.6 stainless valves...

The cam was installed straight up, but I didn't use a degree wheel...
This engine was spun by hand after assembly, no interference problems. Made it through break-in with absolutely zero problems. No compression problems.
No "scary" noises. The engine pulls hard to 6000rpm like it should...

Now the problem...

I pulled the heads (yes both) for a reason I won't discuss here. (That reason being bolt ingestion. At least she wasn't running at the time! ) Anyway, when I pull the heads, I find that EVERY piston has light marks where the intake valves have contacted each piston. The marks are not consistant between each piston, some have full eyebrows, others only have a VERY light mark at one corner of the relief...The notches in these pistons are not machined, but "as-cast". Keep in mind that this engine would have never been tore into thinking there was a problem like this...I have filled each intake port with water with the heads assembled. No leaks. I do not feel that the valves are bent.

Now, I know for a fact that the block has not been decked, it was virgin. The heads, however, have been "trued"... I bought these second hand and do not know to what degree they were trued/milled...

To what extent with this mild of a cam would the heads have to be milled in order to cause a problem like this? Can I use a burette to cc the chambers and find the problem that way? These heads as cast should be 75cc....Any other ideas as to the cause of this?

Thank you gentlemen...

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Old 12-02-2004, 05:23 PM
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I hope you used fel pro blue head gaskets.

You need to clay your pistons and check valve clearance.

It should be fine, even without a head gasket.

With it running like you said it is unlikely the cam was timed or drilled wrong.

The most likely answer is that it was floating the valves.
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Old 12-02-2004, 05:33 PM
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Yep, Fel-pro blue stripe gaskets....Dang glad it wasn't a MLS type now!!!

Would it be floating the valves? I thought that the intake followed the piston downward as it opened?

BTW, after studying the cam card, the intake opens at 23 deg BTDC, is that abnormal?
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Old 12-02-2004, 05:40 PM
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23 degrees BTDC? Sounds like that is very early? What brand is that? It doesn't sound right for a cam this size?
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Old 12-02-2004, 05:47 PM
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unless you have valves that are longer than stock and many people do for different reason such as this,your heads must have been milled quite a bit.i have a 014 block bored 60 over with those pistons and a bigger cam and had no problem.you can get a head gasket from GM .051"'s for about 17.00 ea.that would help you and shouldn't hurt the power if the heads have been bottomed out.check the valve stem lenght against your old ones if you have them.
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Old 12-02-2004, 06:18 PM
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Thank you for the replies....

It is a Comp Nitrous HP cam. pn#12-556-4

I'm starting to think that the heads are milled excessively. How would I go about checking the amount they were milled? Could I approximate the amount by cc'ing the chambers? I don't want to try another set of heads unless I know they are the problem...The thicker head gaskets would be an option...

Thank you for all the help/ideas...

Does anyone know what amount of milling would cause valve contact with these pistons? If so, what reduction would you see in volume of the chamber, approximately?

THANKS!
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Old 12-02-2004, 06:32 PM
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Are your valve springs old? Maybe you are floating them just enough to kiss the piston tops.
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Old 12-02-2004, 06:45 PM
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They are new "Z28" style springs, and are very comparable to the spring specs that Comp calls for use with that cam...

I thought that since it is only the intake valve making this mark, that it could not be valve float because the intake valve follows the piston downward in its stroke as it opens...

I'm gonna borrow a friends cc'ing gear this weekend and see what the chambers size up to...

If it turns out to be the heads, can I still use the pistons? Any chance of detonation from the rougher channels the valves pounded into the piston troughs? As you guys can tell, I really don't want to go through the motor again, it went to gether and ran flawlessly...If you consider smacking the pistons with the valves "going together good"!!!

Any other thoughts/suggestions? Thanks!
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Old 12-02-2004, 10:54 PM
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993 have low plug location. You can't mill them much over .005 without gasket retention nightmares.
You say you know the block is never been milled? have you checked the deck height? Even stock stuff can range from 9.0 to 9.30. What's the compression height of the pistons? How far are the pistons in the hole?

Duration has a bigger affect on interference than lift. 23 BTDC is about right. The valve begins to open 23 degrees of crank rotation before the piston reaches the top. At this time the valve is almost all the way open and the piston is at it's upper most travel. With 2.02 valves in the heads, the extra valve area might cause a interference problem. Two way's to fix it. Fly cut the pistons or use a thicker gasket if you can before quench becomes too big. You need .080 on the intake and .100 on the exhaust unless you intend to degree the cam. Then you an get away with as little .040 and .070 if you time the cam for it.
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Old 12-02-2004, 11:28 PM
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You know, I haven't payed much attention in the past to when the intake opens...but you bet I will now.

23 degrees is like what you find on all out race cams turning 7500 or more RPM in motors where everything is checked and double checked for clearence before assembly, not on a .460" lift 5500 rpm engine.

At least not in MY experince.

Personally I would say to swap cams ...or at least advance your cam a few degrees!
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Old 12-03-2004, 06:08 AM
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Thank you for the info Johnsongrass. In your experience, if these heads were milled say .010" what would the chamber volume be at roughly? What amount of milling equals around 1 cc on a 75cc chamber? What I'm wanting to do is cc the chamber and try to see roughly how much these heads have been cut...

If the heads check o.k. I may try the thicker head gasket, but if I was making contact earlier, then the thicker head gasket is only gonna net me around .010" clearance, is that correct? And you were saying that I need at least .080? I think the compressed thickness on the old "blue-stripe" fel-pro is at .041" and the thick GM gasket is .051"

Man, what a headache...Ignorance is bliss...And to think I would have never known unless I dropped that bolt down the wrong hole!!!!

Any other ideas/suggestions?
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Old 12-03-2004, 07:43 AM
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http://www.edelbrock.com/automotive/cams_chev.html

compare your opening point to those cams.
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Old 12-03-2004, 04:14 PM
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Thank you NXS....With that information I'm leaning more towards the cam timing now...I'm gonna pull the engine and do it right, degree the cam, and then CC the heads...

I built this motor to be a "throw together" cheap street mill, but I guess the cam I have chosen has a little more setup required than what I am used to. I am lucky that nothing was severly damaged and that I can re-use all my parts...

Thank you everyone for your input and advice!
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