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Tunnel size

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Hi guys just wondering what you guys do for bearing tunnel size on a performance build.

Do you normally shoot for bottom size for the crush or top size and dont really worry?

the engine i am building is a rb30 and there is only 0.0005" difference from top size to bottom size of the main bearing bore and 0.0004" difference from top size to bottom size on the rod big ends. The machinest said he will shoot for top size when he line hones the block.

Also one of the brand new rods is 0.0004" over top size the rest are at top size is this something to worry about?


Anyone want to chime in on this?

I noticed Manley make thier conrods at the tightest end of allowable tolerance, and infact even a tenth or two tighter on some of thier rods.

I would trust the bearing manufacturer and aim for the middle of thier allowble tolerance. They specialise in the making of bearings, and would have a fair idea about the desired tunnel size for optimum crush. So targetting the middle of thier recommended tolerance would make sense to me.

It's a little hard to answer as it's really going to depend on exactly where your clearances are now. Essentially the tunnel size sets the crush on the bearing shells which is essential for proper bearing retention. Provided your inside the manufacturers tolerance then this is a given and you're good to go. The other aspect however is that the tunnel size will also affect the bearing clearance. In general with a performance build you're likely to want to shoot for slightly looser bearing clearances than stock so aiming for the top size may help achieve this but it all depends how everything measures up.

If you've got a rod that's outside spec then I'd recommend having it resized.