What is the best way to measure a rods length

Ted Hamilton

Design Drafter / Racer
Pull head and run a dial guage through complete stroke subtracting TDC from BDC (or read the BDC once TDC is zero'd.) Fixture needs to be pretty solid to prevent flex.
 

NMS

New member
Ok. Let me ask it this way. What is the best way to do it with it out of the motor. I have use my calipers and im not getting the same numbers. A stock is supposed to be 3.303. So from what point to what point do I go by that will give me the correct number of rods length.
 
Ted, that's how you measure stroke, to measure rod length, measure the holes on each end, then measure rod inside from big end to small end, then add 1/2 of the individual hole measurements.
 

alvin l nunley

Premium User
Not the best way, but if you measure, with calipers, measure the distance between the edge of the holes, measurer each bore and divide each by two and add that to the first measurement.
If you had gauge pins that fit the two bores, you could measure between them and then add half the diameter of each.
Thing is, calipers are not the most exact way to measure anything.
If you had a height gauge, and a 1 tenth dial gauge, you could get real close. And if you had expanding bore mics even closer.
If you had a CNC and a gage for finding the center of holes, even closer. (I had one of those but can’t remember the name. Expensive) You could find the center of one hole, dial over and find the center of the other and the CNC read out would tell you the exact center to center. Of course you would have to be sure both hole centers where exactly on the same X axes. Of course with a little trig you could compensate for that.
And you thought this was going to be easy. I have this idea that people, who sell these long rods, know few people have the tools to measure if they got what they paid for.
 

flattop1

Dawg 89
Like they said it is a center to center measurement. if you know what you have you can compare to the others by measuring top of rod jounal to bottom of pin hole as a comparison only.
 

sCREamnClones

New member
Not the best way, but if you measure, with calipers, measure the distance between the edge of the holes, measurer each bore and divide each by two and add that to the first measurement.
If you had gauge pins that fit the two bores, you could measure between them and then add half the diameter of each.
Thing is, calipers are not the most exact way to measure anything.
If you had a height gauge, and a 1 tenth dial gauge, you could get real close. And if you had expanding bore mics even closer.
If you had a CNC and a gage for finding the center of holes, even closer. (I had one of those but can’t remember the name. Expensive) You could find the center of one hole, dial over and find the center of the other and the CNC read out would tell you the exact center to center. Of course you would have to be sure both hole centers where exactly on the same X axes. Of course with a little trig you could compensate for that.
And you thought this was going to be easy. I have this idea that people, who sell these long rods, know few people have the tools to measure if they got what they paid for.
AL...are you saying 'somewhere near the 3 5/16th mark on my yard stick' ain't close-enough?
 

C&L_MOTORSPORTS

New member
Bill stated in the rules that you are to measure at the top of the crank pin hole and the bottom of the wrist pin hole and it can,t exceed 3.375. this gives you a + .015 rod length. I wouldn't worry about length in center line but rather how the rule instructs you to do it as it will be done in tech.
JMO
 

Bob Evans

Grumpy Old Admin
Staff member
Measure from the bottom of the rod hole to the top of the wrist pin hole, then measure from the top of the rod hole to the bottom of the wrist pin hole. Add them together and divide by 2.
Aml Racing Dad makes a good point though, if all you're concerned about is a stock clone rod.
 

C&L_MOTORSPORTS

New member
you can figure it any way you want but i'll check mine like the rule states just in case I get teched. and don't depend too much on a set of $19.95 digital calipers from harbor freight. I can make them read what I want.
 

flattop1

Dawg 89
Measure from the bottom of the rod hole to the top of the wrist pin hole, then measure from the top of the rod hole to the bottom of the wrist pin hole. Add them together and divide by 2.
Aml Racing Dad makes a good point though.
I like this I always had trouble deciding what the actual rod length was when I got into the rod and piston bag , even though they was flatty's
 
Thing is, calipers are not the most exact way to measure anything.

That's true, but how accurate do you need for measuring rod length in a mower engine? Decent dial calipers will get within a 0.001" window and splitting the lines is possible and fairly repeatable with good ones. That's ignoring the necessary feel and finesse involved...

and don't depend too much on a set of $19.95 digital calipers from harbor freight.

Because nobody should.
 

DynoDon

Moderator
Just so you guys know, AKRA has defined the procedure for measuring the rod in Box Stock. The way it will be teched is by measuring the rod journal to the pin journal.
 

Bruce Fowler

New member
We have had a Rod Length rule for ever up here in the GREAT WHITE north and it has not changed in many years. The rule states the measurement from inside of Rod end to inside of Pin end must be 2.350" minimum to 2.370" maximum with 90% of the rods measuring in the 2.360" to 2.365" range. The 2.350" is an old GX140 measurement but still on the books. Just for your thinking I own a Princess Auto caliper and it cost me $12.00 to have it calibrated and it is accurate.(Princess Auto is your Harbor Freight) I use it when doing tech at the track.

Bruce
 

DynoDon

Moderator
If I were you I would make sure that there is no slop in that caliper from out at the very tip to the deepest part of the jaws. I purchased one from H.F. and it had .002 slop from that distance I quoted.
 

jg299c

New member
When you go into most machine shops, you'll usually see one of these three brands of calipers, Starrett, Brown & Sharpe, or Mitutoyo. And there's a reason for that, it's because they're very accurate and very repeatable. In quality machining tools, you usually get what you pay for, JMHO.
 
Tesa also makes a good set.

How long do you expect the HF calipers to hold their calibration? Or last at all? I've tried them and they have a coarse feel and sharp edges.
 
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