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Crank, Con-rods, and Pistons

The standard equipment these days is of a pretty high quality, and most probably the only part you'll need to change is the pistons (maybe) and perhaps con-rod bolts. The crank is often best left 'as is', with the only modifications really needed are a general cleaning up of casting/forging dags, etc, and a polishing of the bearing surfaces. While it's out of the engine, however, it's often worth looking to see if any weight can be removed safely and to get it shot peened after a careful balancing.
By the way, it's a common myth that shot peening increases the strength of a component. Wrong! All it does is give the surface of the component a tough skin and tends to remove any stress points on the surface that could possibly be a crack starter. So - If two identical cranks, with one shot peened and the other not, were revved until they broke, the untreated one would break first, thus giving the illusion of lesser strength than the treated one, when in fact the untreated one simply broke from a unnecessary stress crack, the metal in it being every bit as good as the other treated crank. Perhaps a better view of shot-peening is to think that it stops the crank or whatever from being weak.
For really high revs, the counterweights on the crank should be knife-edged to reduce wind drag and drag when the crank hits the sump oil. It's also well worth checking the crank for straightness, though you may get an unpleasant surprise, and have to get the crank straightened.
The same process for con-rods, but they must be balanced end-for-end and also made equal weights to each other. The con-rod bolts are much better these days than in past, but if there's any doubt go out and get a set of ARP (or equivalent) rod bolts for your engine. If the correct bolts aren't available, you might consider a set that's got a slightly larger diameter and reaming the con-rods to suit. The important thing with con-rod bolts are snugness of fit, ie, no matter which end the bolt goes through (Some come up through the cap), it must have a slight interference fit (0.0005" or so) and that interference must continue through to the other side of the con-rod so the cap (or main part) fits very accurately on the other half. The head of the bolt must fill the available area as much as possible, and the threaded part of the bolt must only be just long enough to cover the retaining nut. Every time you strip down the engine, replace the bolts - DO NOT REUSE THEM!! (Put them to good use - Give them to someone you want to beat!)  Shot peen after balancing, etc

This is an aftermarket titanium Toyota 4AGE con-rod, made by Cunningham Rods, of California. It's absolutely beautiful! They're amazingly light, and when I held it, it was like holding a gold plated bit of air. They're a good thing for reducing reciporocating mass, though they need to be well protected, as you can see by the oxide coating they have. Titanium is very reactive with many substances, and so must be coated in such a manner to be used successfully.

Remember - Most engines fail spectacularly because of the con-rod bolts failing!
The pistons are also quite important, and if you have the sort with a slot milled into them in the oil ring groove, them keep them as ashtrays. The only good pistons have holes drilled in the oil groove, and perhaps a few more holes just below the ring groove. (Don't try this at home though) Good pistons also have thin rings, eg, the #1 compression ring may only be 1mm (0.040") thick, the #2 slightly thicker, and the oil control ring not much thicker again. Good gudgeon pins are tapered so they weigh less than parallel pins. Ceramic coating is also a good thing to keep the heat where it should be instead of the engine oil, etc. Remove all sharp edges and balance carefully, matching weights as a set. Do NOT shot peen.

This is the sort of thing you can expect if you over-rev the engine once too often. This is a 4AGE that has been revved way past the sensible limit, as it was revved to 9500rpm odd when it really should only go to about 8500rpm. One rod let go, then out of sympathy the one next to it followed ...

4AGE con-rods. Note that in the photo on the left the rod on the left has a groove cut into the side to squirt oil up onto the bottom of the piston to cool it, while the one on the right has the conventional small hole. That notch is sometimes called the 'Cosworth Notch'. They both end up doing the same job, though the one with the slot would tend to help cool the piston more and also make the rod a little stronger. Both rods have a small hole in the little end to also help oil lubricate the gudgeon pin. The rods on the left are known as an 'H-beam' type rod, whereas the rod on the right is known as an 'I-beam' type. That's because of the shape that the strengthening ribs make on the long section of the rod. There's no huge advantage of either design, but a well-made i-beam rod will weight slightly less and be slightly stronger than the equivalent h-beam.

Not all engines fail because of the con-rod bolts letting go. What can happen with frightening speed in a turbo engine is the pistons cop the brunt of mixtures being too lean, and so the temperatures go through the roof. That funny rattling noise can be the result of things like the piston on the right .... So please, when tuning a turbo engine make very sure that mixtures are safely in the rich areas before starting to test with any amount of boost up.

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