Problem installing Pistons

Burgundystang · Apr 12, 2004

This is my first time to rebuild a motor and ive had the crank turned, cylinders bored and honed, and rods machined. We were putting the pistons on this weekend and everything went fine untill we got to the seventh one. We put it on and then when we tried to rotate the crank it wouldnt budge. We pulled the seventh piston out and tried a different piston; same thing. We pulled it out and put it in a different hole; same thing. I cant imagine what the problem is. Any advise?

Route666 · Apr 12, 2004

Maybe it is just cumulative effort, if you can, try putting number 7 in, and unbolt another one, and see if it is free again.

That's about all I can reccomend, other than the hole is too small, ie wasn't bored to the right size.

allcarfan · Apr 12, 2004

route666....the cylinder could not have been too small because he tried the number 7 piston in another cylinder and it would not budge.

Have you checked the 'roundness' of your piston? Is the ring installed properly? What are your clearances on your bearings?

Burgundystang · Apr 12, 2004

well its a new piston and i also tried to use a different piston and still had the same problem so i dont think that the roundness is and issue. I believe that we installed the rings properly, we didnt have problems iwth the others and tried the seventh quite a few times. the people that turned the crank told me to get .010 under bearings and thats what i got.

fastcoupe68 · Apr 12, 2004

You still need to check your clearances on the crank rod journals. Dont rely on everything being okay, this does not always work out. Get some Plasti gauge or mic the crank to make sure you have the proper clearances. If it's to tight it wont turn. You should have a min of .001 and thats a little on the tight side, optimum would be .0015 to .002 on your rods. Also check your piston to cyl wall clearance.

1970 slantroof · Apr 12, 2004

A long distance guess would be that the rod journal for number 7 cylinder is slightly oversize and is being clamped by the rod bearings. Try just barely tightening the rod cap down and see if the engine will turn over. Unfortunatly you cannot in most cases get by on assuming everything was done correctly.
Like has been said, you probably need to mike the crank first to verify that, check the inserts you got to be sure they are correct, etc. Did the machine shop have the pistons when they bored and honed the cylinders?
Howard

Fomocohipo · Apr 13, 2004

your using a breaker bar to turn it right? Its near impossible with anything shorter.

Route666 · Apr 13, 2004

allcarfan said:
route666....the cylinder could not have been too small because he tried the number 7 piston in another cylinder and it would not budge.

Have you checked the 'roundness' of your piston? Is the ring installed properly? What are your clearances on your bearings?

Oh I thought he meant he tried another piston in no 7 bore.

67coupe351w · Apr 13, 2004

Something else you may want check is that you got the rod caps bolted on in the oreintation that you took them apart when you got them back from the machine shop. I dont know if this holds true in small blocks casue when i built mine i went ahead and made sure I did that but I know on 460's the main caps must go in in the right direction or the 2 rods will pinch against each other and by the time you get all your pistons in the crank WONT MOVE. (its a bad experiance dont ask )

D.Hearne · Apr 13, 2004

Sounds like he's got something on backwards. All the numbered sides of the rods face outward on the bank they're on. Main caps go with the arrows facing the front, but occasionally they do NOT go in the order the numbers indicate. Had a 390 like this, the #'s 2 & 4 were swapped before it was line bored at the factory.

hindle · Apr 13, 2004

Check the ring gaps as well...

Burgundystang · Apr 13, 2004

Wow thanks a lot for all the help. Im going to try what yall have suggested this weekend. Lets see... I have been checking to make sure that the caps are on in the correct direction, I checked the ring gap on one of the rings before i started but will go back and check the rings on the pistons im having problems with, i am using a crecent wrech(maybe a little over a foot long) to turn it (but it is definately not moving if i tried harder i would be afraid that i would break something), and finally im going to check the machining on the rod and the crank with a mic. what should they be? Thanks again.

Burgundystang · Apr 16, 2004

OK, I just used a mic to measure the insides of the to rods and the diameter of the crank on both places. Everything measured ok. Do yall have any idea what could possibly be causing this problem?

slapper · Apr 16, 2004

D.Hearne said:
Sounds like he's got something on backwards. All the numbered sides of the rods face outward on the bank they're on. Main caps go with the arrows facing the front, but occasionally they do NOT go in the order the numbers indicate. Had a 390 like this, the #'s 2 & 4 were swapped before it was line bored at the factory.

That would be my long distance guess also.

Burgundystang · Apr 16, 2004

Ive checked the caps and they are on correctly. I put the main caps back on in the same order that i got them back from the shop in (1.2.3.4.5). And i cant imagine (though i may be wrong) that it could be that anyways because the crank turns quite freely until i put on the seventh piston.

67bluestang · Apr 16, 2004

Burgundystang said:
OK, I just used a mic to measure the insides of the to rods and the diameter of the crank on both places. Everything measured ok. Do yall have any idea what could possibly be causing this problem?

When you checked the rod, did you have the bearing in place and torqued to proper specs? If not, it won't be accurate. I would also check the crank journal size on that journal to the one next to it. Have you tried installing #8 without #7 in to see if it turns? This problem sounds like something is not sized correctly.

Burgundystang · Apr 16, 2004

I checked two rods without bearings in them and had both of them torqued. And if i put in #8 the same problem occurs.

Route666 · Apr 16, 2004

What happens if you put in 7 and/or 8 and take out a couple of the other ones? It might be a cumulative tightness maybe?

67bluestang · Apr 16, 2004

Burgundystang said:
I checked two rods without bearings in them and had both of them torqued. And if i put in #8 the same problem occurs.

One thing i would try. If the engine is on the stand and upside down, without putting the pistons in the cylinder, try putting the rod (with the bearing) on the crank and torque it down. Then see if you can rotate the piston/rod assembly around the crank. If it doesn't move, then you know the problem is not cumulative. I would also get some plasti-gage and torque it then check it.
Also, are the bearings stamped .010? Maybe you got a .020 in there by mistake.
I'm just assuming its been cut .010.

D.Hearne · Apr 16, 2004

That would be my guess too. Either you have the wrong size bearings or some how that crank journal wasn't turned.

Problem installing Pistons

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