[jk316650]

I have a 1985 Nighthawk 450 with a spun rod bearing on the right side. Of course, finding a replacement engine is not easy and I'm not very confident that I'd end up with something much better. I decided to tear down the engine to see if it might be worth repairing. I enjoy doing this stuff plus I like the idea of keeping as many of these old bikes in service as possible.

The crank journal with the spun bearing definitely needs machine work to clean up the scoring and out-of-round. I'd need to take it to a shop to be sure, but in my estimate it would need to be ground down to the very edge of allowable service limits at best (no undersized bearings available, to my knowledge). The rod to crank side clearance is also getting close to service limits. I think I'd be better off replacing the crankshaft and maybe the rods too. Of course, I should try to understand what caused the bearing to fail in the first place. I'm pretty sure the damage was done before I bought the bike, I suppose the best I can do is make sure all other clearances are within spec and all of the oil passages are clean with a healthy oil pump.

My question is if I buy a used crankshaft and rods from eBay, do I need to be concerned with careful balance between the crank/rods/pistons, or is this even done in production? Since this is engine design is fundamentally imbalanced, with the need for the external chain-driven balance weights to keep vibrations at the frame minimized, it would seem that small production variances in weight between the crank/rods/pistons wouldn't really matter much for the mechanical integrity of the engine, worst case I might feel slightly increased vibration getting transferred to the frame.

So can I just slap some randomly acquired rods and crankshaft in and expect it to balance well, or should I have a machine shop sort this out? I'm not looking for better than factory performance.

An interesting observation- my crankshaft has holes drilled in the counterweights, while the majority of crankshaft photos I see on eBay and elsewhere show no holes. I'm also aware that the 400 crank part number is the same as the 450. I'd assume that 450 pistons would be slightly heavier and would need adjustments to the crankshaft counterweights if balance is much of a concern. I've seen photos of both (presumably) 400 and 450 cranks with and without drill holes in the counterweights, mostly without.

Any advice or comments are greatly appreciated. Thanks for taking the time to read this.

 

[doode]

There's a couple folks on here who know this engine well. I'm not one of them.
The 400 and 450 balancers are made of different materials.
The pistons rise and fall together.
I have a 450 spare crank, bearings and cases, if you need.

 

[longdistancerider]

You can mix and match the rods and cranks pretty much, balance isn't an issue per se since the balancer system is used. 400 rods are not interchangeable with the 450's, different wrist pin size.
The crankshafts can't really be machined since there are no undersized bearings available. It could be welded up and turned to spec but that's an expensive repair.
I've seen the same thing on the cranks for drill reliefs, not sure why that is.
You didn't state which rod, left or right, failed but I'm betting it's the left rod. Failure caused by loss of oil pressure due to never being checked. Look at the oil flow schematic in the FSM.

 

[jk316650]

You can mix and match the rods and cranks pretty much, balance isn't an issue per se since the balancer system is used. 400 rods are not interchangeable with the 450's, different wrist pin size.
The crankshafts can't really be machined since there are no undersized bearings available. It could be welded up and turned to spec but that's an expensive repair.
I've seen the same thing on the cranks for drill reliefs, not sure why that is.
You didn't state which rod, left or right, failed but I'm betting it's the left rod. Failure caused by loss of oil pressure due to never being checked. Look at the oil flow schematic in the FSM.

Thanks for the info. I did find a crankshaft with 450 rods attached, so I grabbed it. It was actually the right side bearing that spun, but the left side was starting to eat into the copper. I suspect the PO either ran low on oil or ran with contaminated oil. There's evidence it ate a plastic dipstick in the past as I found the tip of it in the pickup screen. Perhaps some chunks of dipstick found their way into the oil system, or more likely the PO didn't keep proper oil levels because of a broken dipstick. Everything else appears to be fine and reasonably clean. Main bearings look good and plastigauge says they are well within limits. Pistons/bores look good and I'll have to make do with what I have since oversized pistons are not available. It's just the rod journals that have considerable scoring. I'm a little concerned about what this engine may have been through, but my sense is there's enough life left in it that I won't be completely wasting my time putting it back together. Arguably it would be more economical to part out, but there's more fun in getting it running again.

Other than the occasional sound of rod knock, it ran really well before I tore it apart and didn't burn enough oil to worry me.

 

[jk316650]

There's a couple folks on here who know this engine well. I'm not one of them.
The 400 and 450 balancers are made of different materials.
The pistons rise and fall together.
I have a 450 spare crank, bearings and cases, if you need.

Thanks for mentioning that you have some parts. I found a set of rods with a crank on eBay, so I went ahead and bought it. If the crank turns out to be a dud, I might be interested in what you have. I figure what I bought was worth the rods alone since 450 rods are hard to find.