buckeye79

iTrader: (1)

I have built several gen1 / 2 small blocks but this is the first experience I have had with an ls series.

I have a low mileage lm7 that I took to the machine shop to just have cleaned up and checked. They said everything was great and I would be able to get by with a .002" hone. So I went ahead and had them do the work. I got the block back and it appears that they did a good job. I have measured all of the cylinders and they really are all within .002" of the factory spec.

I also had a low mileage l33 aluminum block with the flat top pistons, floating pins and rods, and the 799 heads. this engine had taken on water after being pulled for a lifter problem, so the bores on this block will need a complete re-bore to salvage.

My plans for this first build are to use the lm7 block that I already have honed and use the stock l33 pistons and rods (new rings and bearings of course)

My question is this since this is my first ls build, I am not completely up to speed on the clearances: Will I be ok doing the stock l33 pistons in the lm7 block even though the block has been honed .002"?
The pistons measure good and the piston skirts look nice. I plan on just file fitting the rings and running it, but figured I would try to get some input first.

truckdoug

iTrader: (26)

i think it's best to talk in piston to wall clearance rather than bore sizes or skirt diameters, GM used different size slugs in different size holes due to manufacturing variances, but they generally used some crazy tight numbers like .0005 to .0013 or something from the factory (with the coating)


i shoot for .0025 with my junkyard crap. remember that stock cast pistons don't "grow" as much as a forged piston.

the 5.3 in my daily/truck is at .0035 and slaps like an 8-armed pimp until it's warmed up.

moekluse

iTrader: (2)

When you say that the l33 took on water, was it running and hydro locked? If it did, I'd check and make sure you're rods aren't bent.

buckeye79

iTrader: (1)

So .0025 no issues but .0035 slaps? Is this true even with the floating pistons / pins

buckeye79

iTrader: (1)

no the motor had failed lifters and the guy that pulled it left it sitting outside uncovered. Why he didn't just repair the motor is beyond me. He didn't want dod but pretty sure he could have fixed all of that without pulling the motor

KCS

iTrader: (20)

If they've been honed .002", then they're probably well beyond GM's tolerance for piston-to-wall clearance. For the 3.898" bore LS1/6 engines, the maxi is .00212".

https://ls1tech.com/forums/generatio...learances.html

truckdoug

iTrader: (26)

the .0025 motors (actually a .0026 and a .0022) I have may slap a little but it's a 3.5" dump about 2' back from the turbine outlet so I cant hear it if it does.

yeah the .0035 motor slaps like crazy on cold start, but once it comes to temp it runs fine. cheapo DNJ cast rings.

I had to ball hone the **** out of the block since it sat with water in the bores. but whatever. it hauls parts around, doesnt smoke and didnt cost ****.

as far as slap goes...my fathers 2000 5.3 sierra slapped like a ************ from day one off the lot. 2xx,xxx miles later and it still gets 14mpg towing a tandem axle car trailer.

and i'm not sure cold slap has anything to do with pin type, more of a rod angle thing i think.

grain of salt though...i'm long gone from the auto machinist work. now i just slap stuff together then best I can and run it until i get bored with it and build something else.

99 Black Bird T/A

iTrader: (4)

This information alone would convince me to replace the stock pistons with slightly larger forged pistons.

My understanding is the noisey piston slap issue from the factory was more of a piston wall clearance issue...it was a little too loose until the piston warmed up and expanded.

Correctly set - tight pistion to wall clearances allows the piston to transfer more heat to the cylinder and the piston actually runs cooler.

Pistons really should not be reused...

http://www.enginebuildermag.com/2001...ston-problems/Piston Replacement
Pistons should almost never be reused. Many pistons that appear to be in good condition and show no signs of scuffing or damage often have worn upper ring lands which can cause ring problems if reused, so be sure to measure carefully. Also, check the fit of the wrist pin carefully as looseness here can cause problems and noise.

Pistons also have to be replaced if the cylinders are worn and require boring and honing to oversize.

Other reasons to replace the pistons would be to change the stock compression ratio or to increase the durability of the engine if it is being modified for more power.

buckeye79

iTrader: (1)

I have been able to find the bore and clearances for the lm7
but not the l33. I would assume they are the same but I keep seeing that the l33 is built with less clearance so not sure if that would be from a smaller bore or a bigger piston. I guess I just need to quit talking and get my bore gauge and micrometers out.

buckeye79

iTrader: (1)

Well finally got to measure my clearances. Setting the bore gauge to the smallest piston, the worst cylinder is less than .0035" and the smallest cylinder is slightly less than .002" clearance. Guy at my local gm service desk says that that gm says maximum .0028 service limit clearance. Since I only have one cylinder that is slight bigger than this limit, I am going to go ahead with the assembly and see how it does.
I have built gen 1 and 2 SBC with similar or even more clearance without issue.
I know the ls series are a totally different animal but will see how it goes.

Wish me luck! Thanks for all of your input!