LT4 Piston Deck Height Discrepancy? Uh oh...
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
LT4 Piston Deck Height Discrepancy? Uh oh...
I'm in the process of installing a set of ported heads on my stock 82,000 mile LT4 shortblock. I figured I would calculate the compression ratio. I used a micrometer and bridge to find out the average piston deck height...well everything looked hunky dory until #7... .054? That is about .020 off what it should be. The engine ran fine when disassembled, but it has been run hard. I checked and rechecked the measurement, trying different locations, ensuring no dirt particles were causing an error, etc. Even visually, the piston looks further down in the hole than the other 7.
What could cause this...assume the piston and rod are within spec for length? Only thing I could come up with is a failing rod bearing...
Anybody have any thoughts on this? I did not plan to touch the shortblock other than a new oil pan... If it is .020 in the hole from rod clearance then that would mean it is has a rod bearing clearance of ~.050! Does that makes sense? Again oil pressure was fine, no misfires...only question I have is I had recently dragged the car and it was down about 4 mph from the beginning of the year...I was chocking it up to different weather conditions and worn valvesprings...
1 0.024 2 0.029
3 0.027 4 0.027
5 0.03 6 0.027
7 0.054 8 0.026
Thanks
:(
What could cause this...assume the piston and rod are within spec for length? Only thing I could come up with is a failing rod bearing...
Anybody have any thoughts on this? I did not plan to touch the shortblock other than a new oil pan... If it is .020 in the hole from rod clearance then that would mean it is has a rod bearing clearance of ~.050! Does that makes sense? Again oil pressure was fine, no misfires...only question I have is I had recently dragged the car and it was down about 4 mph from the beginning of the year...I was chocking it up to different weather conditions and worn valvesprings...
1 0.024 2 0.029
3 0.027 4 0.027
5 0.03 6 0.027
7 0.054 8 0.026
Thanks
:(
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
I'd think it would be a bent rod before it would a bearing. The rod can be slightly bent and make it run like crap without making you hear anything. If there was that much slop in the bearings you would certainly have had knock.
[Modified by Nathan Plemons, 8:13 AM 3/9/2003]
[Modified by Nathan Plemons, 8:13 AM 3/9/2003]
Burning Brakes
Joined: Oct 2002
Posts: 1,152
Likes: 1
From: Wichita KS
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
I'd suggest you go back & double check the deck for flatness using a straight edge & feeler gauges, paying close attention to the area around #7...though I can't imagine that much warp in the deck. Once I had that accomplished I would make sure the pistons are setting squarely in the bore using feeler gauges, as piston rock can affect your measurements. Also make sure you are measuring straight down to the center of the piston, this will minimize the deviation caused by the piston not setting absolutely square in its bore (Piston rock). I have good pics and specs on all of those measurements; I'll go take a look at what all I’ve got...
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (Nathan Plemons)
Bent rod!
How could that have occured? I'm quite sure the shortblock is original (bolts, gaskets, etc). I have never filled the engine with coolant (hydrolock, etc) and the valves looked fine as did the cylinder head.
For gee whiz, I computed a CR of 10.5 for that cylinder compared to the others at 11.24 average...
How susceptable is a bent rod to failing...I would like this shortblock to last me another year (383 planned for next winter)...I know that sounds half arsed, but
hmmm....
How could that have occured? I'm quite sure the shortblock is original (bolts, gaskets, etc). I have never filled the engine with coolant (hydrolock, etc) and the valves looked fine as did the cylinder head.
For gee whiz, I computed a CR of 10.5 for that cylinder compared to the others at 11.24 average...
How susceptable is a bent rod to failing...I would like this shortblock to last me another year (383 planned for next winter)...I know that sounds half arsed, but
hmmm....
Burning Brakes
Joined: Oct 2002
Posts: 1,152
Likes: 1
From: Wichita KS
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (MSR)
I agree with MSR on the TDC issue, make sure you find TDC with a travel indicator & again make sure you position the travel indicator in the exact center of each piston so each measurement is consistent & minimizes the piston rock deviation which could result if you are measuring out on the circumference...
[Modified by superlund, 11:41 AM 3/9/2003]
[Modified by superlund, 11:41 AM 3/9/2003]
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (MSR)
I never located TDC per se...I held the bridge over the piston with the micrometer set at a negative .3 and rotated the engine over slowly and watched the maximum number the indicator read.
In other words, I just rotated the engine over and watched the piston top out by reading the highest piston depth...ie: .030, .029, .028, then back down to .029, .030, etc. So .028 is the depth.
The piston is assembled with rings so it doesn't move much...but I did purposely position the micrometer point in the center of the piston (definitely from left to right, but not so much front to back-car direction) as well as carefully scraped off any carbon along with some brakecleaner from piston crown. All the other cylinders were fairly consistent so don't think I did something wrong or different...even checked some other cylinders and rechecked it again...I'm quite sure the piston is lower than it is supposed to be...
Would normal (maybe hard) running bend a connecting rod? Again stock OEM rod-probably a Powdered Metal with a cracked cap...
I'm stumped. This coming weekend I maybe able to examine the lower end...
Thanks for the help folks.
Also I will also check the deck for flatness...I will have to buy another tool for that...:)
[Modified by No Go, 3:20 PM 3/9/2003]
In other words, I just rotated the engine over and watched the piston top out by reading the highest piston depth...ie: .030, .029, .028, then back down to .029, .030, etc. So .028 is the depth.
The piston is assembled with rings so it doesn't move much...but I did purposely position the micrometer point in the center of the piston (definitely from left to right, but not so much front to back-car direction) as well as carefully scraped off any carbon along with some brakecleaner from piston crown. All the other cylinders were fairly consistent so don't think I did something wrong or different...even checked some other cylinders and rechecked it again...I'm quite sure the piston is lower than it is supposed to be...
Would normal (maybe hard) running bend a connecting rod? Again stock OEM rod-probably a Powdered Metal with a cracked cap...
I'm stumped. This coming weekend I maybe able to examine the lower end...
Thanks for the help folks.
Also I will also check the deck for flatness...I will have to buy another tool for that...:)
[Modified by No Go, 3:20 PM 3/9/2003]
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Burning Brakes
Joined: Nov 2001
Posts: 1,241
Likes: 9
From: Houston TX
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
no go, i don't think you're doing anything wrong in your measurements. your one piston is obviously more in the hole. if it were a deck issue, the head gaskets would never seal.
it might be a rod out of spec, but i still think it's bent. i've seen a lot of bent stock rods, in both fords and chevies, without good explanations. ymmv, of course, but i wouldn't worry too much about it. (especially since you're getting a stroker in the mid-term future.)
side-note: given the deck clearance (which is very typical for these engines), you're using an impala head gasket, correct?
it might be a rod out of spec, but i still think it's bent. i've seen a lot of bent stock rods, in both fords and chevies, without good explanations. ymmv, of course, but i wouldn't worry too much about it. (especially since you're getting a stroker in the mid-term future.)
side-note: given the deck clearance (which is very typical for these engines), you're using an impala head gasket, correct?
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
I guess it COULD be a bent rod, but I would bet on an unfortunate tolerence stack up. We have all heard of the factory freak, that runs better than anything esle? Engines like this, make those possible. Until you tear it down and measure everything, it's all speculation. Good luck, and...
RACE ON!!!
RACE ON!!!
Melting Slicks
Joined: Jan 2000
Posts: 3,418
Likes: 1
From: NW Chicago Suburbs
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
Bent rod!...
Eric
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (silver & red CE)
I don't know...I hardly hear of bending connecting rods...breaking them yep.
More to follow this weekend...if the bearing checks out ok...maybe the bet will be how high will it rev before it blows!
Just kidding...
:)
More to follow this weekend...if the bearing checks out ok...maybe the bet will be how high will it rev before it blows!
Just kidding...
:)
Burning Brakes
Joined: Nov 2001
Posts: 1,241
Likes: 9
From: Houston TX
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (silver & red CE)
yes, i've seen bent powdered metal rods, in both chevies and fords. it doesn't have to bend much to get the difference we're talking about, it would barely be visible to the naked eye with lots of comparing.
but ya, it could also be a tolerance stack-up. i've not seen the factory stack things up .030" off, though. that's a huge amount out of tolerance, even if you split it equally between the piston pin location and center to center rod length.
do all the pistons look identical? like, you're sure that one is an OEM piston?
but ya, it could also be a tolerance stack-up. i've not seen the factory stack things up .030" off, though. that's a huge amount out of tolerance, even if you split it equally between the piston pin location and center to center rod length.
do all the pistons look identical? like, you're sure that one is an OEM piston?
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (MSR)
Upon re-examination...the piston looks identical to the other 7 and OEM style casting.
Will be pulling the pan tomorrow to see what the bottom end has to tell.
I've installed the heads already...:) I got anxious...!
Will be pulling the pan tomorrow to see what the bottom end has to tell.
I've installed the heads already...:) I got anxious...!
Safety Car
Joined: Feb 2000
Posts: 4,230
Likes: 0
From: VA
Cruise-In II Veteran
Re: LT4 Piston Deck Height Discrepancy? Uh oh... (No Go)
Don't know but for some reason most of the LT4 i have heard of blowing pistons or rods is usually #7. I find that so strange.
:skep:
:skep:
