Topic: When do aftermarket rods become necessary?

[brandEn]

1978 CB750 F3
836 Wiseco Kit
Webcam 41
Dyna 2000
Cycle X valvetrain/mild porting
stock crank/rods
Approximately 70 HP


I recently was on a roadtrip with friends and riding my bike pretty hard. I was riding twisty's with the RPM's staying in the 5000-8000 range for extended periods. The outside temps were around 85-90 F degrees . Long story short I spun a rod bearing. I just split the case and found the #4 rod bearing is toast and so is the crank. So it got me wondering if this failure could have been avoided if I had better rods? I am considering new APE Super rods and a balanced crankshaft and was hoping to get get some input on other's experience in this department.

[Don R]

It sounds like you might be at the tipping point. does the rest of the motor look OK? As in, can you find anything that would cause a rod bearing to fail? Compression ratio and fuel? Sometimes detonation will affect a rod bearing. pistons look good on the top and sides? Spark plugs OK? How about the other Rod bearings, check the top half for flattening of the shell. Dragster guys measure the thickness at the center and compare to the thickness at the side to see if the rod bearings are getting pounded.

[lucky]

Just old age ,heat etc.,.

[brandEn]

It sounds like you might be at the tipping point. does the rest of the motor look OK? As in, can you find anything that would cause a rod bearing to fail? Compression ratio and fuel? Sometimes detonation will affect a rod bearing. pistons look good on the top and sides? Spark plugs OK? How about the other Rod bearings, check the top half for flattening of the shell. Dragster guys measure the thickness at the center and compare to the thickness at the side to see if the rod bearings are getting pounded.

Well my bike was just tuned on the dyno so I believe it was pretty well sorted as far as that goes. I never had any signs of detonation. The plugs, valves, and pistons all appear ok. I suppose I should post some pics just to give everyone an idea of the condition of things. I just rebuilt everything and the top end has only about 3000 miles on it.

[matt mattison]

It was really the bearing that failed, not the rod. So a better rod would probably not improve your odds. The way you were treating that motor also didn't seem too extreme either. These bottom ends are almost bullet proof in stock form,unless you are really abusing/racing it or have a pre existing condition. You could have had an oiling failure at that journal, I would look there first and the supporting oiling passages. Also, I recently had 200 mile old oil fail on me recently. I was using Valvoline motorcycle oil. Good luck!

[MCRider]

It was really the bearing that failed, not the rod. So a better rod would probably not improve your odds. The way you were treating that motor also didn't seem too extreme either. These bottom ends are almost bullet proof in stock form,unless you are really abusing/racing it or have a pre existing condition. You could have had an oiling failure at that journal, I would look there first and the supporting oiling passages. Also, I recently had 200 mile old oil fail on me recently. I was using Valvoline motorcycle oil. Good luck!

Yeah I'd agee.

I had a real hyper CB750 (actual cc) roadrace motor. Ported head, webers, radical cam, 12-1 Yosh pistons, BolDor pipe. We bent a stock rod in a 4 hour endurance race, grossly and continuously over-revved (for a stock rod).

But I think yours should have been OK. IF the rod isn't bent, I'd agree it wa a bearing fail. Did you or someone set up the crank. Picked a too tight bearing?

Curious, very rare.

[Jerry Rxman Griffin aka MuthaF'er]

Damn Branden! That sux. Sorry to hear about it. I'd say that with your engine it would depend more on how much/how you flog the #$%* outta it. In your case I'd say rods would be a good insurance policy. Dyna 2000 with the rev limiter is also a good insurance policy against missed power shifts. IMO 900 or above if built right/strong should use rods. You using HD case studs too to keep the cases clamped together tightly?

[754]

 Was it a new rod bearing, or used?

[brandEn]

Yea, I am using HD studs on the head and jugs. No HD studs on the case.

All the lower bearings were OG and had about 30,000 miles on them. When I opened the case originally I replaced the chains and tensioner. All the bearings were reused.

So with all this in mind I have a good crank on hand. I plan on sending it to APE for a race balance. Should I use my stock F2 rods with new bearings? Were is the best place to get rod and crank bearings?

[brandEn]

I will take and post pictures in a bit....

[MCRider]

Yea, I am using HD studs on the head and jugs. No HD studs on the case.

All the lower bearings were OG and had about 30,000 miles on them. When I opened the case originally I replaced the chains and tensioner. All the bearings were reused.

So with all this in mind I have a good crank on hand. I plan on sending it to APE for a race balance. Should I use my stock F2 rods with new bearings? Were is the best place to get rod and crank bearings?

All the OEM suppliers have bearings. I got mine from either motogrid or bike bandit. Paid too much. Probably have a few pairs to sell. I suggested we should start a bearing registry as I'm sure that between a couple hundred forum members there are a few dozen extra pairings for sale.

Under the circumstances I might consider letting APE install the rods. They can source the rod bearings. Transfer the liability. They installed mine. Extra $$ but WTH.

[brandEn]

Pictures! I think I found the culprit. Blocked oil gallery on the crank. Should have cleaned under the balls?


This is what I scraped out of the crank



#4 rod journal

[MCRider]

great! I didn't really think it was the rod. For the rod to cause spinning of bearing it would have to be too tight or bent.

I notice APE has a service to drill out the balls and thread in a screw. Assuming a full clean at that point.

I'd read a thread years ago about someone here who cleaned out their crank, and it was dirty. I never have, makes me wonder.

I think short of that, just soaking it in solvent will loosen and flow stuff out.

[Xnavylfr]

Don't go on the pretense that a good soaking will get all the crud OUT!!! I would use some kind of pressure hose to aid getting all the crap out.
You will probably have a machine shop turn the crank and they will solvent clean before and after turning to get rid of any grit.


Xnavylfr(CHUCK)

[Old Scrambler]

Sorry for your problem..........but very good of you to share the pics.........This should be a very good lesson for EVERY builder

[Jerry Rxman Griffin aka MuthaF'er]

Didn't your mama tell you to always clean under yer balls?!   Sorry BrandEn, can't resist sometimes 

[andy750]

Very informative photos and sorry about the bearing failure. I had the bottom half of my engine rebuilt in 2006 (after  a final gear bearing failure) before getting all my top end work done/810cc kit and it was cleaned before reassembly. Now with a few thousand miles on the 810 kit (dyno; ~71 hp, used the stock rods/bolts) riding it in the 5000-8000 rpm range at high speed for long periods as well as up/down on the gears in the twisties the same as you with no problems. So I think using the stock rods is not an issue.

Good luck in getting it rebuilt.
cheers
Andy

[brandEn]

Didn't your mama tell you to always clean under yer balls?!   Sorry BrandEn, can't resist sometimes 

haha!