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General Engine Building Discussion

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Hi guys can some one clear up the difference between ovality and out of round for me? Or are they the same thing. Just asking as i am seeing different ways to measure this and the same thing being called both. With a dial bore gauge am i better off taking a live measurement or measuring at a few set points in the bore and calculating it? I feel that a live measurement is better as you can find at what point the biggest difference is. The bore in question is a big end bore on a conrod. Also what sort of out of round/ovality is ok?

I use the term 'out of round' but essentially the same thing. With respect to big end bores in a conrod, you typically will measure them in two planes perpendicular to each other. One plane would be across the parting line of the rod body and cap, and one will be in the direction the bearing will be loaded in operation. You're best to check your workshop manual for a tolerance on out of round as it will vary from engine to engine. Of course less is better but generally I'd want to have no more than around 0.0002" out of round. Beyond this you can have the rod journals resized by your machinist.

Thanks for the reply andre. I am getting a odd shape.

with the rod standing up right with the small end at 12

on the big end in between 12 o'clock and 3 o'clock it gets smaller by 4 tenths but once you get to just above 3 o'clock/ parting line it goes back to the same measurement as at 12 o'clock

Then when you start back at 12 and go through to 10 o'clock it gets 3 tenths smaller then when you get to just above the parting line at 9 o'clock its 4 tenths bigger then at 12 o'clock.

Some weird stuff is going on with this big end they have been closed and honed before they went in to the moter by the looks of the finish in the bore. Just trying to work out if its the hone job or something that has happened in use

Given what you're explaining I'd suggest getting the rods checked and resized by your machinist. Are you 100% sure the rods and caps haven't been mixed up?

No never mixed as the rod and the cap are factory numbered. Thats fine was just curious the rods are not going back in to the motor as i have a stronger rod going in.

Have you ever had a big end closed and honed or main tunnel line honed and found that at the parting line area is wider then vertical. i have been told by a shop that some times on a main tunnel you cant get it back completely round. Have also heard the same with rods Is this a thing? Is this something that boring then honing could over come?

I've only had a couple of sets of rods resized and had no trouble with them. I suppose it's worth mentioning that if the measurement is above spec across the parting lines then this is going to be difficult to rectify. The process of resizing involves taking a small amount of material off the parting face of the rod body or cap which understandably tightens up the bore and provides the potential for the machinist to then hone the bore to a finished size. This process only reduces the bore size in the direction the bearing is loaded. This of course is the more critical direction as it effects the bearing clearance. If the rod is outside of spec across the parting lines then this will affect the amount of crush on the bearing shells.

Thank you for that andre