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Old 04-03-2009, 06:48 PM   #1
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Default Needs help checking rod/crank.

Hey guys, I'm rebuilding an engine and need to know to to check that the crank and the rod are still in good condition.

How do I judge this? How much play should the rod have where it mounts to the crank?

Thanks for the help.

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Old 04-03-2009, 07:47 PM   #2
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Expect a few different answers.

-Some of us will measure the .000" of wear delta from original.
-Some of us measure concentrisity (roundness) of the crank pin
-Some of us will simply say "it feels too loose."

I like to measure concentrisity of the pin. Once you reach a differance of about 5% I consider the pin ready for replacement. Any greater than 5% and you risk pin failure.

What to look for... 1) scaring on the pins. a good pin will have a nice burnished finish. 2) gaulling of the rod bearing surface 3) cracks & or voids (missing pieces) of rod bearing surface. 4) segment of rod bushing completely worn away. **some rods do not have inset bushings, but inspection remains the same**

caviots- 1) you need a min of .0015"-.002" play between the pin & rod bushing so proper lubrication can occure. 2) the crank pins are ALWAYS under load 360-degrees of crank rotation. Trust me... you won't eat a pistion because of a loose fit. 3) as the rod and pins wear the port timings slightly change and CR gradually falls (5%-10%). So don't be surprised if you need to change nitro %, plug temp & pipe as the gallons flow through the engine.

Just because a rod and pin are loose, it does not mean they are ready for a change. This just means they are good and borken-in. The rods and pins wear together as a set, the moment you replace the crank or rod, they have to wear (cut) new bearing surfaces. This mis-match of new and old will cause the two parts to wear excessively fast until the bearing surfaces match; by this point the wear will probably be great enough the "good old part" is now a "bad old part", out of tolerance and in need of replacing.

I guess my short answer is don't replace unless you have good reason. and when you do replace... replace everything; the rod, crank and wrist pin.
chuck "chunk" thomas
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