How much power can stock con rods handle?

So I'm in a dilemma with project creep. I'm working on this big bore radical head package for a KZ750 twin. I expect about a 60-65% increase in power. From about 47-50 at the wheel to 75-80 at the wheel. I'm worried that the factory connecting rods are not up to the task. The later style 750t rods look to be much stronger than the early style but I cannot find any NOS late style rods. As far as I see it, I have three options. 1. use NOS early style rods. 2. find someone to recondition late style rods(recommendations?). 3. get custom rods. I have been looking at the kz1000 stuff as a measuring stick for what I'm doing. What kind of power increase can the stock kz1000 rods handle? 70% increase? Do you think the stock early style kz750 rods can handle 80HP? Custom rods are quite expensive ($1000+) and I cannot decide if its worth it. But i also don't want to scrap thousands of dollars when one of these stock rods decides to create additional crank case venting. thoughts?

TLDR:

Can stock kz750 twin rods handle 80HP? should I get custom rods?

I think RPM may be a bigger factor than hp. As long as you aren't going to spin the engine super fast I think those stock rods should be okay. Maybe go with the late style and have them polished and shot peened? And can you fit them with high grade bolts?

In terms of custom I suppose that would be ideal, but in terms of ideal some larger wrist pins may be good as well. Too late for all that though.

KZ1000 rods will never work on a KZ750 be it 2 or 4 cylinder.
Hint; look at the big end. $1000 for 2 custom rods?
Stock rods can be improved.
What I have done on automotive V8s is polish the beams and lighten the balance pads on the big and small ends.
Then had the machine shop install ARP bolts shot peen and balance the rotating assembly.
Check out ARPs site, they have the measurements listed with the bolts.
Find a shop to at least shot peen them if not resize the big end.
You can balance the rods yourself.


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In answer to part of your question regarding 1000 rods.
They can take over 250 hp , so 3 times stock easily.
This is with a polished , shotpeened , resized , and balanced assembly.
Never blew a rod while racing . Melted pistons and melted heads , but never blew a lower end .
ED and Nebr Rex is on a sound path . Dont raise the redline too much and I would use rebuilt stock rods
Dave

You should contact Steve Darlington (goes by darlington on here) he is into the vintage road racing up here in Ontario, Canada.

He has a friend who races by the name of Fernando who campaigns a KZ750 twin punched out to 880cc, not sure if he runs stock rods but he has done a ton of self development on that motor. I think he is/was putting out 82HP and 68ftlbs to the rear wheel. I believe with that setup it was unreliable but it had all the go fast stuff done to it.

He could give you insight into the bottom end, oiling issues, balancers, etc.

Just PM Steve and ask.

Just my two cents on stock Kawasaki rods - They are tough forged units in stock form and can take a huge amount of abuse, the weak link would be the bearings and oiling system. If your running a highly oversized bore piston speed at high RPM becomes the critical issue.

Yes I was quoted $1000 for 4 con rods from CP Carillo, the minimum setup fee is a killer.

If the KZ1000 rods are good for a 300% power increase I think its safe to assume that the stock 750 rods can handle 150-180%. I will have them shot peened and resized - I was told that shot peening can release stress which can make the rod go oval or twist so they 100% need to be checked after. Does anyone know any reputable shops to recondition rods? APE told me to piss up a rope, and i haven't found anyone locally that is willing to do it. It seems aftermarket con rods have gotten so cheap very few shops are reconditioning OEM.

I was planning on having the whole rotating assembly balanced by a local guy who specializes in that type of work. But before i can get to that I need to have the con rods sorted out. I found some connecting rod bolts form a Mitsubishi Evo that are pretty much the same dimensions but the bolt head is a different shape so the head twists when torqued down, thoughts on using these bolts?

Injected wrote: You should contact Steve Darlington (goes by darlington on here) he is into the vintage road racing up here in Ontario, Canada.

He has a friend who races by the name of Fernando who campaigns a KZ750 twin punched out to 880cc, not sure if he runs stock rods but he has done a ton of self development on that motor. I think he is/was putting out 82HP and 68ftlbs to the rear wheel. I believe with that setup it was unreliable but it had all the go fast stuff done to it.

He could give you insight into the bottom end, oiling issues, balancers, etc.

Just PM Steve and ask.

Just my two cents on stock Kawasaki rods - They are tough forged units in stock form and can take a huge amount of abuse, the weak link would be the bearings and oiling system. If your running a highly oversized bore piston speed at high RPM becomes the critical issue.

thank you I have reached out to him.

Also another question on the con rod bearing side: the crank journals are exactly 1.500". I'm assuming this is a fairly standard size. Is there a resource to cross reference bearings to find something for a late model car or bike? Would getting a modern bearing be worth the effort?