Connecting rod ovality spec?

['78 SC]

I'm using ARP connecting rod bolts and upon the advice here checking the big ends for distortion due to the increased clamping force. The spec book shows 56.000-56.019mm for the connecting rod big end bore, but no specific limits for ovality. Is there a published value for this?

Taking multiple readings, I'm getting a range (max - min) of 0.005mm. That seems good, especially considering permissible range for the bore is 0.019, but is there an actual limit?

[Flat6pac]

A machine shop can measure the ovality by spinning the rod in a inside mic to measure runout and correct.
Bruce

[Discseven]

0.005mm is indeed not a whole lot! Am not commenting on that. Am going to suggest checking the inside of the rods with the bolts in place. Check the interior surface of the bearing area being flat near the bolt areas. I've seen cases where the center "bite" area of rod bolts will push the interior of the rod journal area out to ruin ID tolerances.

[Mark Henry]

I'm assuming your rods are torqued to spec, right?

Then .005mm equals about .0002" (2 /10,000) is a pretty small amount.
I look for a .0002" - .0005" out of round tolerance.
If you are doing you measurements correctly I'd say you're good to go.

['78 SC]

Right Mark, I torqued the bolts on one rod and used a bore gauge (direct read to 0.001mm) to check different diameters to see how much ovality existed. That's when I realized I didn't have a numeric limit to check against. Granted, 0.005 is down near the limit of what I can resolve in a garage shop with regular instruments.

Discseven has a good point about checking near the parting line. I was intentionally not trying to measure the bore right on the line, but I can see how checking for local distortion is a good idea.

I'll go through the whole set this weekend. I'm measuring bolt stretch to confirm correct tightness, then checking multiple diameters.

Thanks all for the comments.

[chris_seven]

I would think that you should aim for the following:

Ovality of big end bore should not exceed 0.015mm
Taper should not exceed 0.009mm

Squareness of gudgeon pin max 0.025mm per 25mm

It may be worth 'blueing' the faces of the cap to rod joint and looking at the contact pattern.

I think you need a minimum of 80% contact. The fretting that causes ovality can reduce this figure quite significantly.

['78 SC]

Thanks for the comments. Sorry for the slow response, been busy.

I torqued the rod bolts and checked bore dimensions in two directions. Fortunately all are within Chris' guidelines.




Since I'm using ARP hardware, I also checked the bolt stretch. I torqued all the bolts to 50 ft-lb (ARP's recommendation), then checked stretch.




I then estimated the torque needed to hit the desired stretch (0.0095 - 0.010 inch). When installing the rods, I used this calculated torque and checked the stretch for each bolt. Some required a bit more adjustment. I ended up with bolt stretch in a narrow range.