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Busy building a 4 bolt main 351C. Crank has been machined to -20 on both mains and rod journals. Using Clevite bearings on both and the mains bolted up perfect.

However we have some issues with the stock rods. New ARP con rod bolts fitted by the machine shop.

When we torqued up the caps on the rods (40 to 45 ft/lb) 4 of the rods are so tight we cant move them. Bearings are correct. Undid them again and removed all the rods/pistons from the bores.

Bolted them onto the crank again individually without fitting into the bores, 4 of the pistons are still tight so they will definitely be going back to the machine shop.

However the issue is with the other 4 rods that seemed to be OK. When torqued on the journal should there be any sideways play, in other words, if we hold the piston upright then rock it side to side should there be any play at all. Obviously it will slide sideways on the journal but holding the piston at the top and rocking it side to side should there be any play at the big end/bearing end.

I read one of Georges stickys about having the big ends re sized after fitting new rod bolts, whats the reason for that.

I would hope the machine shop would have done that as they are usually very good. However I am not confident now of fitting any of these rods. Any suggestions

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what's up Horace? sounds as if you may have the caps on the rods flipped around or possibly mismatched on different rods?

the rod caps are located by the shank of the rod bolts, they should be a snug / tight fit. then the large end bearing bore is machined with the caps torqued in place, they're a perfect fit in the position they were in when machined. if you were to flip the cap around the other way the round would not match at the parting line and the thickness could be off side to side also. for sure the beveled chamfer will not match and the cap will ride the edge of the crank and do nasty things

Yes there needs to be a certain amount of free play with the rods torqued in place on the crank journal, this is called connecting rod side clearance

did you find out anything?
yea sorry should have updated the forum.

The problem was with the machining of the rod journals on the crank, especially on the two journals that were tight. The crank had been machined elsewhere and they had not taken it all the way to the radius, 4 of the rods were therefore two tight.

Got my machine shop to check all the big end journals and they were all spot on. They had sized them when they fitted the new ARP rod bolts but we had them re check.

Your right 4v there has to be clearance and holding the rod upright and rocking it tends to magnify that clearance and that what was confusing me.


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