1973 Yamaha RD350 Crankshaft Replacement

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New RD350 crankshaft installed in case with original crankshaft at left.

It’s funny – and not always in a “ha ha” way – how often simple projects go off course, taking unexpected and typically expensive detours. In our How-To in the March/April 2015 issue, we walked through the process of pressure testing a 2-stroke engine, the object being to suss out engine leaks that can cause a 2-stroke to suck air, resulting in a lean mixture and poor performance. Boy, did we find leaks in our subject 1973 Yamaha RD350. Aided by Yamaha RD350 and 2-stroke expert Brad Obidowski of HVCcycle in Lincoln, Nebraska, we found intake and head gasket leaks, the former likely causing an extremely lean condition in the right cylinder, resulting in the piston starting to burn away.

Where it all began: The right piston on our 1973 Yamaha RD350 was burning up thanks to an intake leak.

That discovery meant we were in for a top-end overhaul, but all things considered, on a 2-stroke like the RD350 that’s a fairly simple and not hugely expensive proposition. New pistons with rings are $180 from HVC, with gaskets and other miscellaneous bits adding another $100. Throw in cylinder reconditioning and it’s about a $350-$400 project.

Since we were going that far, we decided to replace the crankshaft and transmission seals. That means pulling the engine and splitting the cases – which sounds much harder and scarier than it actually is – and it’s a good thing we did, because what we found wasn’t pretty.

The cases split, ready to remove the original crankshaft. No damage is visible, but one of the center crankshaft bearings was about to fail.

Apparently, our RD (actually, it’s not ours; it belongs to good friend and Yamaha 2-stroke fan Tom Howe) had been building up to a major failure, narrowly averted, it turned out, thanks to our decision to reseal the engine. With the cases split and the top half removed, lifting the crankshaft out of the bottom case revealed odd scoring marks in the left crankshaft cylinder cavity, as if something had been sucked into the engine and ground into the case by the revolving crankshaft. At first glance, the crankshaft seemed fine – until we spun the right inner bearing. Notchy and rough, at least one if not more of its ball bearings had apparently broken up. If you rotated the bearing to the “correct” position, you could move the outer shell back and forth axially, and in a big way. That might have explained the metal scoring we saw, yet the scoring was in the left cylinder cavity and the failed bearing was between the cylinders, and right of the center labyrinth seal. It didn’t quite make sense.

So now what? Crank rebuilds are time consuming and expensive; were we getting in too deep? Fortunately, as we discovered, HVCcycle offers brand new crankshaft assemblies for RD350s, and they come complete with new rods, wrist pin bearings, crankshaft bearings and the center labyrinth seal for what’s basically a plug-and-play replacement. Made in Taiwan, they differ slightly from the original. For one, the flywheels are a few pounds lighter (likely unnoticeable in use). That may matter to purists, yet the HVC cranks also feature modifications that should enhance durability. The connecting rods are beefier, and the big or crankshaft ends are machined to encourage more oil flow, while the little or piston ends have larger bearings and are also drilled for more oil flow. Frankly, at $400 we think it’s a bargain, and the quality of the unit we got seems excellent. Time will tell, of course, but we were pretty thrilled to have a complete crankshaft in hand only days after discovering our original had problems.

New crankshaft from HVCcycle installed in lower case half. Note the oil relief cut in the connecting rod big end and the oil hole in the small end.

Installing the new crankshaft was a pretty straightforward affair. After cleaning the insides of the crankshaft cylinder cavities we removed all traces of old sealant from the case mating surfaces. Then, using a small artist’s brush we painted on a thin layer of ThreeBond gray sealant. With the new crankshaft and drive-side transmission seals installed, it was a simple matter to place the top half of the crankcase in place, holding the connecting rods clear while gently tapping the case around its perimeter with a rubber mallet until it was mostly seated; the seals keep it from seating completely until the case bolts are installed and tightened.

A close up of the stock crankshaft and connecting rod. Note the lack of an oil relief as on the replacement crankshaft.

Painting on ThreeBond case sealant on the mating surface of the upper case half. Doing it this way makes it easy to get a consistent application of sealant without applying too much.

The upper case, sealant applied and ready to install on the lower case.

With the crankshaft and transmission positioned in the lower case, the upper case is gently positioned and lowered into place.

Once the upper case was in place, it was gently tapped into place using a rubber mallet.

Once the case halves are joined it’s a simple matter of gently and carefully flipping the engine over to install the nuts and washers to fix the case halves together.

With that done, we’re finally ready to turn our attention to what started this project the first place, rebuilding the top end. Look for a full report soon. — Richard Backus

Motorcycle Classics Magazine
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