Piston/Head interference??? - Team Camaro Tech
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post #1 of 6 (permalink) Old Apr 24th, 00, 09:12 AM Thread Starter
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I just built a new 400 SB over the recent Christmas break. I've had a "miss" right around 3 - 4k rpm that feels almost like the rev limiter kicked in - sometimes it pulls through it and other times it loses speed. It's worse in first gear than in the others. I also have this odd rattle sound from about the #2 cylinder head location - particularly if I just rev it a few hundred rpm's off idle and let it settle back down - at least that's where I can hear it the most easily.

My brother-in-law (lifetime mechanic and racer) believes it is a piston hitting the head occasionally as a result of a problem with either the wrist pin or rod bearing. One possible cause - I had one piston (hyperuetectic) and rod (5.7")assembled so I could fit check the longer rods in the 400 block for block rail and cam interference. Then I had them pulled apart so they could balance the assembly. The machine shop told me at the time that the wrist pins are not always happy about being yanked out and re-installed. Could this have caused a crack in the pin bore that expands under a load and allows the piston to move enough to just "kiss" the head?

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1
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post #2 of 6 (permalink) Old Apr 24th, 00, 07:02 PM
 
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Hmmm. This is an interesting one!

I guess it's possible to damage the pin bore(s) of the piston during assembly. If this were true, it may also be possible for the pin to partially seize, chewing up the aluminum bores, resulting in excessive clearance...yeeek.

Assuming you have spent the time to isolate a connecting rod-related problem, you may want to consider doing the following: remove the oil pan while the engine is warm, then separate the rod cap from the suspect rod assembly as well as on a "known good" rod assembly, then compare the amount play at the small (pin) ends. By doing this with the engine warm, as uncomfortable as it may be, the pin bore may have expanded enough to exaggerate a clearance issue. Also, you will know right away if there is a bearing problem.

Don't know what else to think right now. Anyone else have any insight on this one?




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post #3 of 6 (permalink) Old Apr 25th, 00, 07:24 AM
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Assuming a stock deck height of about .025 and another .040 for the gasket, not likely the piston is hitting the head. If the pin is this loose, make big racket. I'd take a look at the valve gear in this area first.
The hesitation might be the timing cent. advance. Might try backing off the intial advance about 4 degrees and see if this helps this.
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post #4 of 6 (permalink) Old Apr 25th, 00, 07:02 PM
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Take a look under the valve cover, look for a broken valve spring near the end. Check for any valve train problems, loose studs, pushrod wear etc.
If none, try pulling spark plug wires at idle, one at a time, pull from the cap and be careful not to get shocked.
See if there is any change.

DONT pull the plug wires if you have an HEI distributer. They can develop enough spark energy to arc down to the module and blow it.

Listen to your brother in law!
Good luck, David

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The older I get, the faster I was!

[This message has been edited by davidpozzi (edited 04-25-2000).]
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post #5 of 6 (permalink) Old Apr 26th, 00, 04:32 AM Thread Starter
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Thanks guys. While working thru the possibilities I considered either a bad valve spring or even a rocker arm. My brother-in-law claims to have heard a number of pistons whacking the head over the years and feels pretty strong about his diagnosis. I'm hoping he's wrong of course!

The car is in the body shop this week but as soon as I get it back I'll pop off a valve cover and start there - then move on if necessary. I'll let you know what I find.

------------------
Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1
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post #6 of 6 (permalink) Old Apr 26th, 00, 05:07 AM
 
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I've got a nice big Holley 850 that will probably fix the problem... Just so Happens it's for sale....

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Shawn Peterson
1969 Z28, 406SB
http://www.geocities.com/corsican69
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