Measuring runout on front disks -- '68 SB

[james Kekeisen]

Very basic question but here goes. If I'm measuring runout with a dial indicator is the amount of runout HALF of what is the total measurement of movement or is runout equal to the total measurement. I'm measuring the runout on my front rotors and the dial indicator moves .010". Haynes says the limit is .005". Am I in spec? Do I need to have the rotors turned or replaced? Thanks.

Jim

[Wayne W.]

Re: Measuring runout on front disks -- '68 SB

.010 is marginal. With new seals etc. it probably would work OK.

[Wayne W.]

Re: Measuring runout on front disks -- '68 SB

.010 is marginal. With new seals etc. it probably would work OK.

[Michael W.]

Re: Measuring runout on front disks -- '68 SB

The runout is equal to the difference between the lowest reading and the highest reading. If you set the guage to read 0 at the low spot and it reads +.010 at the highest, the runout is .010". If your low spot reads -.005 and your high spot reads +.005 the runout is still .010"

Whether this needs to be corrected or not I'm sure will be of some debate. If you keep a firm pedal after a long drive, and feel no pulsing in the pedal while braking, I think you will be OK. Probably half the Corvettes on the road have runout (or play in the wheel bearings) that is equal to .010 or more and don't have a problem.

Turning the rotors by themselves will most likely not cure the problem. You need to have the hub and rotor machined as an assembly.

[Michael W.]

Re: Measuring runout on front disks -- '68 SB

The runout is equal to the difference between the lowest reading and the highest reading. If you set the guage to read 0 at the low spot and it reads +.010 at the highest, the runout is .010". If your low spot reads -.005 and your high spot reads +.005 the runout is still .010"

Whether this needs to be corrected or not I'm sure will be of some debate. If you keep a firm pedal after a long drive, and feel no pulsing in the pedal while braking, I think you will be OK. Probably half the Corvettes on the road have runout (or play in the wheel bearings) that is equal to .010 or more and don't have a problem.

Turning the rotors by themselves will most likely not cure the problem. You need to have the hub and rotor machined as an assembly.

[Joe L.]

Re: Measuring runout on front disks -- '68 SB

James------

It's as Mike says with respect to runout measurement. If the dial indicator needle MOVES a total of .010", then you have .010" TOTAL indicated runout or TIR. However, be sure that you have the spindle nut turned down to ZERO end play if you take the TIR measurement on the car. You CANNOT accurately make this measurement if there is end-play in the bearing assembly.

Since your question involves the FRONT rotors, correction of this runout is really not a big problem. Simply have the rotors AND attached hub assemblies trued on a precision brake lathe by someone that knows what they're doing. Have them take the MINIMUM cut necessary to get them into spec. Make sure, however, that if your rotors are no longer attached to the hubs via rivets, that you secure the hubs to the rotors with reveresed lug nuts torqued to lug nut specs when you have the truing performed. Then, you must always index the rotor to the same orientation to the hub if you ever seperate them for any reason.

Another thing to keep in mind: make sure that the inside surface of the rotor and the surface of the hub which mates to the rotor are ABSOLUTELY clean and free of any rust, dirt, burrs, etc. BEFORE you dial indicate the rotors and BEFORE you have any truing performed.

[Joe L.]

Re: Measuring runout on front disks -- '68 SB

James------

It's as Mike says with respect to runout measurement. If the dial indicator needle MOVES a total of .010", then you have .010" TOTAL indicated runout or TIR. However, be sure that you have the spindle nut turned down to ZERO end play if you take the TIR measurement on the car. You CANNOT accurately make this measurement if there is end-play in the bearing assembly.

Since your question involves the FRONT rotors, correction of this runout is really not a big problem. Simply have the rotors AND attached hub assemblies trued on a precision brake lathe by someone that knows what they're doing. Have them take the MINIMUM cut necessary to get them into spec. Make sure, however, that if your rotors are no longer attached to the hubs via rivets, that you secure the hubs to the rotors with reveresed lug nuts torqued to lug nut specs when you have the truing performed. Then, you must always index the rotor to the same orientation to the hub if you ever seperate them for any reason.

Another thing to keep in mind: make sure that the inside surface of the rotor and the surface of the hub which mates to the rotor are ABSOLUTELY clean and free of any rust, dirt, burrs, etc. BEFORE you dial indicate the rotors and BEFORE you have any truing performed.