Cory1970

OK durring an inspection of my short block, I fount that my number one piston is not square with the block. On the TOP Left, it is .009 inches out of the bore. On the BOTTOM Right, it is .009 inches in the bore..... My head porter says this is too far out. My motor is bored 40 over ad has been decked a couple of times, if that makes a difference. Could the machine shop have over bored it too much?? Does anyone have any suggestions, experience....ect....

P.S. I am going to get a mic on it tomorrow and get all the bore specs. I only had time to look at number one the other day

Corvette Kid

You should only check height on the pin axis. You're going to get variations from rock where you're checking it. I'm not sure if that is too much, never checked it that way.

Cory1970

How do I check height on the pin axis? I know I will get variations from rock, but should it even be rocking this much (.oo9)? Plus the variation is not vertically like I would assume. The variation is cocked sideways at about 10 o'clock and 4 o'clock, how could this be?????

The short block had about 3000 miles on it, and it ran great before I took off the heads. No piston rattle or anything...it might have had a very slight vibration, but it was not vary noticable. There was oil on the spark plugs, but my valve seals were blown out (shop put in cheap umbrella seals after I gave them the good ones). We assumed all the oil was coming from the valves because the seals and guides were so badly warn out.

zwede

Could you have a bent rod?

Cory1970

I had not thought of that before... It is possible if the piston is cocked in the bore, .009 is not much, but bent is bent. I will measure the rest of the pistons tomorrow.

Corvette Kid

Check the height only at either end of the pin, in other words at 3 or 9 o'clock. No other area will give you an accurate height measurement.

Cory1970

ok I see your point. I see why that would be best for checking deck heigth, but it is not the deck height I am so concerned with. I am concerned with the piston being highest at 10 and lowest at 4 o'clock. The pin axis should cause the highest point to be at 12 and lowest at 6 or vis versa. My deck height is close to zero at the pin axis, but why is there variation right and left. 9 and 2 o'clock should be at the same height but they are not.....

Corvette Kid

Yes, I see what you're saying too. Further checking is definitely in order.

Cory1970

well I will get back to you tomorrow evening... thanks for the advice

Solid LT1

You said it is 0.009" above the deck? Even if you are talking about the "rock" of the piston, 0.009" (lets just round it up to 0.10") is pretty far above. I hope your machine shop used a BHJ "Block-Tru" fixture on the motor when they decked the block surfaces to true the deck surfaces to the cranckshaft centerline. The pistons should all be close to 0.010" above the deck if the block is properly machined. You will need to go to a think "MLS' type head gasket at least 0.050" thick to give yourself a proper clearance to the cylinder head. I would seriously conside a new machine shop if all the deck heigths are different or even more than a 0.005" variation.

Cory1970

Well actually the shope was just bought out...... so I need a new shop anyways.... I am in Atlanta... does anyone have a recomendation

biggestjohn

Wow, are all the pistons like that? Sounds like the deck has been milled at the wrong angle.

MotorHead

Check to see if the rod is in the center of the piston on the pin, also check the sideplay on the rod big end on the crank, I forget the measurement but somewhere around .020" is max I think

69ttop502

Cory, there is a great engine guy in Atlanta. His name is Gary Grimes and Grimes automotive. Not cheap but one of the best around. Sorry I don't have a phone number for you.

Caboboy

This is a real good point. I don't know how much rock a forged piston has, but if you have a short compression height with a 4 thou clearance +/-, you'd get some rock/slap till it warmed up and tolerances tightened up. That should only be inboard and outboard though, not at the locations you mention. Are the rods bushed or pressed? Pressed rods that are off center might explain this kind of variance.....

71coupe

If it has floating pins, make sure the rod isn't sliding back & forth on the pin while rotating the engine.