engine rebuild connecting rod clearance issues

stang-geoff · May 5, 2011

I'm rebuilding my 302 and I'm having some issues here. I bought new pistons and pressed them on the rods and now that I'm putting the motor back together I can't get 2 rods to fit on one journal.

Rick 91GT · May 5, 2011

Are you reusing the stock rods and crank?

stang-geoff · May 5, 2011

I'm reusing the stock rods. I think what I did is I installed the rods backwards. If I install the pistons on the opposite bank I'll have the rods facing the right direction but the numbered rods wont be with their cylinder they were originally assigned to. The block is bored and honed and I'm using brand new bearings. Is it important that the rods go back to their original cylinders, because I can move them to different cylinders and get the rod and piston to face the right direction. Otherwise I'll have to get hold of a press again and redo the entire thing.

edit:
I'm also reusing the stock crank

84Ttop · May 5, 2011

it sounds more important that you have not balanced this rotating assembly. i would seriously recommend seeking qualified help at this point. you may be in over your head

fox92 · May 5, 2011

IF YOU MIX THE RODS UP FROM THE ORIGINAL HOLES IT WILL THROW THE BALANCE OFF AND THE MOTOR WILL SHAKE AT IDLE. YOU SHOULD NOT HAVE TO REBALANCE THE ENTIRE SET UP FROM JUST SWAPPING PISTONS, REPLACING RODS WITHOUT THE CORRECT WEIGHT OR REPLACING THE CRANK AND YOU WILL NEED A BALANCE.

84Ttop · May 5, 2011

if you change any one part of the internal engine assembly you need to rebalace the assembly. the weight of the pistons affect the balance on the rotating assembly.

capri debris · May 5, 2011

There are two different syles of rod bearings... ones that that are designed for a chamfered crank journal and ones for non chamfered crank journals. If you have a stock crank, you need the chamfered ones. If you look at the crank journal, the area where the shiny finished round journal meets the unfinished vertical part of the crank, it's not a sharp inside corner... it is rounded off. ( see pic below/red arrow)This mandates use of bearings that have a bevel on one side of the bearing... this side must face the rounded inside corner... so when both rod bearings are installed, the bevels on each must face away from each other. You may just have the wrong bearings or you have the right ones and they aren't oriented correctly. Many a new engine builder has not taken this into account and suffers the consequences... don't be that guy.

stang-geoff · May 5, 2011

fox92 said:
IF YOU MIX THE RODS UP FROM THE ORIGINAL HOLES IT WILL THROW THE BALANCE OFF AND THE MOTOR WILL SHAKE AT IDLE. YOU SHOULD NOT HAVE TO REBALANCE THE ENTIRE SET UP FROM JUST SWAPPING PISTONS, REPLACING RODS WITHOUT THE CORRECT WEIGHT OR REPLACING THE CRANK AND YOU WILL NEED A BALANCE.

Um the way they balance the rods/pistons is to get them to all weigh close to the same as each other so I don't see how moving them to other cylinders would throw a single thing off unless you're reusing old bearings. I called my machine shop and told them I was just going to switch the pistons to the opposite bank and he said as long as the numbers on the rod are facing outward and the piston is facing forward it'll work. So I was already planning on doing that.

stang-geoff · May 5, 2011

capri debris said:
There are two different syles of rod bearings... ones that that are designed for a chamfered crank journal and ones for non chamfered crank journals. If you have a stock crank, you need the chamfered ones. If you look at the crank journal, the area where the shiny finished round journal meets the unfinished vertical part of the crank, it's not a sharp inside corner... it is rounded off. ( see pic below/red arrow)This mandates use of bearings that have a bevel on one side of the bearing... this side must face the rounded inside corner... so when both rod bearings are installed, the bevels on each must face away from each other. You may just have the wrong bearings or you have the right ones and they aren't oriented correctly. Many a new engine builder has not taken this into account and suffers the consequences... don't be that guy.

yeah, my crank is definitely chamfered and so are the rods. I didn't take such a great look at my rod bearings though. I'll take the part number off of them and see if my shop got me the right set of bearings. The bearings also have tangs on them so I doubt I could have installed the bearings backwards. I think the rod being on it backwards is more likely to be my issue. I'll play around with the motor later tonight and see what's up. Thanks

stang-geoff · May 7, 2011

Motor seems to be coming together real well now.

Rick 91GT · May 8, 2011

Your right if balancing was done correctly it doesnt matter what cylinder you install them in.

The stock rod bearing is a 2600p in most brands. What do you have...some bearings are also marked top and bottom...bottom is in the rod cap.

engine rebuild connecting rod clearance issues

stang-geoff

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Rick 91GT

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stang-geoff

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84Ttop

They make new pistons every day, so why worry?

fox92

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84Ttop

They make new pistons every day, so why worry?

capri debris

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Rick 91GT

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engine rebuild connecting rod clearance issues

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They make new pistons every day, so why worry?

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