95 gs 160,xxx mi

I decided to rebuild my bottom end with forged internals(je8.8, eagle rods). Of course I began by droppin the block off at a local machine shop to have it tanked, bored, honed, decked, crankshaft polished and have the rotating assembly balanced. I thought that I had enough background info to know exactly what I needed to take into the shop and what I needed to provide them to do the job correctly. I have since read that people have taken their flexplates in as part of the rotating assembly. As I said, I didn't. Will this make a difference? The machinist already balanced the crank and told me that he had to add alot of heavy metal to it due to it being so out of balance?? Enough metal added to up the price of the job 50-75 dollars. Could this be due to it not having the flexplate attached? There was nothing major wrong with the engine when I deciede to rebuild except that the comp. #'s were low(150-170) which I attributed to worn rings(came to this conclusion after doing a wet comp. test in which the #'s shot way up into the 220's). My second question is about the piston to wall clearance. I included the je spec sheet that came with the pistons that indicated a wall clearance of @.0035". When I showed the sheet to the guy, he explained that the pistons have this clearance already built into them. I am now hesitant to allow him to continue the work. Does anyone have any responses to this? Thanks in advance as without this community Eclipse wouldn't be part of my vocabulary. :thumbsup

The machinest has to balence the crank to the pistons/rods. So when you change rods or pistons, the weight changes and balencing is required. I am kinda surprised he had to add weight but okay, i dont do that for a living.

As far as the clearance built into the piston...I am not positive but i think that is inaccurate. Different build motors will use a different clearance. Turbo or forced induction uses more clearance than a N/a motor. How can JE speculate the clearace best for your motor? I could be wrong but as far as i know, the piston is 20 over bore the standard spec then the machinst bores the correct size and takes into account the clearance target.

If he is going to stand by his work, then let him finish. After all if its wrong, he can tear it apart and fix it.

Terry

Terry, thanks for the response. I bought the je pistons off a friend who never used them for his neon, but for some reason they came with two spec sheets, one is blue and one is green. The green one has a minimum wall clearance of .0031"-.0036" if I remember correctly, and that is the sheet that I gave him as a reference. I have the blue on here in front of me and it reads "Clearance is built into piston based upon finished bore for normal operating conditions". I have no idea what pistons this blue spec sheet is for, but the minimum wall clearance for our bore is listed at .0022"-.0028"?? It seems to me that I should just call him up and remind him that I plan to turbo, so we can decide on the proper wall clearance.

Thats odd, i have had new JE pistons and new Weisco's and never recieved those sheets. Those clearances are typically for a N/A motor. Now perhaps the 20 over piston is machined for clearance of a n/a aspirated motor. Now the machinest will factor in additional clearace for turbo which is typically .0036 - .0040.

Terry

The flexplate/clutch assembly/flywheel etc. has a history of arguements on wether to balance with it or not. IMO you don't need it. Those parts should be built/sold as center balanced parts any way. Think about it; if you bought a new clutch, you would have to dissasemble your engine to maintain your balance. It's a really old arguement that just doesn't make sense.

As for the turbo/non turbo, you won't have to worry about the piston to wall clearance, BUT you will want to take that into effect when you do the rings. I don't have the numbers off hand but you will want to go to a bigger gap when you boost (the larger side of the gap limits that they spec).

I am a little disappointed for you that he chose to add a great deal of weight. Generally, we always had the shop take off the weight when balancing to keep the rotating mass down. But it should be fine if done properly.

MB

The flexplate arguement i think stem back from Ford motors where the flexplate is not balenced on its own. Same with some antique Mopar motors.

Overbore is simply a larger piston and cylinder to account for boring out any damage from the stock pistons.
Standard piston: 3.444", .020 over is 3.464. Think of this as the "zoom" setting on your camera.

Piston to bore clearance is the extra space all the way around the piston to allow for metal expansion. So a .0038 clearance, if you had exactly 3.464 pistons, would mean the bores would need to be 3.4678". Think of this as the "focus" setting on your camera.

Notes:
You need all 4 pistons to clearance an engine. JE tolerances are so good that machinists often will only ask for one. Insist on 4, and the pistons should come back numbered.

You can check clearance when you get it back from the machinist, by using 2 feeler gauges on each side of the piston.

A clearance can only be calculated by knowing the metal expansion rates of the piston AND the block, and the temperatures each are expected to reach (hence turbo/nitrous/na question). This is tricky and best to leave to experienced builders. The number JE prints on their spec sheet now is based on input from AEM, Howell, etc. It wasn't on there when the piston first came out.

My personal clearance target for a JE piston, 420a DSM block, and Turbo is .0038 to .004. I had a piston ring break from overexpansion in a .0035 clearanced engine that overheated.

When balancing, adding weight to one piston takes less time that removing it from the 3 others. I see this as being lazy, but whatever. If the added weight ever comes loose, you'll be unhappy.

Ring end gaps are just as important or more important than clearancing and balancing. Take your time and do it right.

Thanks again all. Good stuff. I will be checkin the clearance as soon as I get the parts back form the shop. And I WILL take my time to get the ring end gaps correct. Thanks again.