Crank regrind gone bad.

Crankshafts break -- car, truck, diesel or motorcycle. Usually rare, but given enough time and stress, it's possible for any crank to fail. What we have to be careful about is not to speed up the process by improper crankshaft regrinding.

Most farmers and mechanics have to trust specialty machine shops to do many machining jobs, mainly because we lack the tools or training. The results we get aren't always good, as proven by this broken sprayer engine crankshaft.

This crankshaft, from a VW air cooled Spray Coupe engine, was reground in an Ontario machine shop and it only ran a few hours before breaking. Typical of many farm repairs, this overhaul was done in the fall but not put to work until the next June. By the time it failed there was no hope of getting the machine shop to accept responsibility.

The problem, as I see it, is the way the crank journals were ground with a very sharp fillet radius. The rod journals were ground to .040" undersize because of damage to the rear rod bearing. The journal beside the break shows no sign of discoloration, so there was no earlier trauma in that area. Some black and blue crankshafts should never be rebuilt, but that was not the case here.

I didn't have an original crankshaft to compare with the corner radius with the reground, broken crank, but it was obvious that whoever ground this shaft used a grinding stone with a very sharp corner. This fracture started along a line so sharp and perfect you think it was drawn with a glass cutter. And, cracks always start in sharp corners.

Let me explain that. Breaks in steel are very similar to cracks in a car windshield, where a stone spot makes a few short cracks, often so short you can't see them. But they are stress risers -- sharp points where a long crack can start.

This crankshaft failed the same way. The rod journal corner (fillet radius) was too sham. As soon as the engine was put to hard work, the crack took off and spread through the body of the crank until it finally broke in half.

How to avoid this

Obviously, dealing with a good machine shop would be your first step to avoiding such a problem. However, if the guy working on your crankshaft has a bad day, then you're headed for trouble. Warranty doesn't mean much when your truck or sprayer dies in a rush season.

I suggest you express your concerns to the machinist, when you take the job in for regrinding. If he has a set of radius gauges, he could check your original fillet radius while you watch. Now you have something to compare with after the crank is ground. And, if he knows you're going to be checking, your job is more likely to be done correctly.

You can check the fillet radius yourself first, using a set of drill bits, before taking the crank to the machinist. Try one size at a time, holding the drill bit in the corner of the crank journal. When you find the bit that fits in the original shape, record that drill size. The original fillet radius will be one-half the diameter of that drill bit.

When the crank comes back from the machinist, use the same drill bit to check the new fillet radius. If it doesn't look right, it probably isn't. If so, take it back and discuss the problem with the machinist. There are some non-original shapes that are perfectly acceptable, but you need to know if yours is good or bad.

The first machinist job I ever had was grinding crankshafts, and my foreman was only interested in how many cranks we ground each day, not the shape of a fillet radius. It takes time to put a proper radius on the stone, after the stone is trued up with a diamond, and there are different radius specifications for different engines.

"Read" all metal breaks

It's important to know why something breaks. With practice you can learn to "read" most types on metal breakage. Look for the point of impact or stress where a crack first started, often long before it broke. There's always a reason. And, there's no sense in just repeating the error and waiting for the new part to fail. Learn to spot stress riser problems.