Goodbye crank bearings

Welp, after my oil pressure sensor took a dump, I guess I didn’t notice until I got to work. Less than 5 minutes later. Filled it with oil and Limped it home. I toss on the new oil pressure sensor and boom. It’s knocking like a Jehovas witness at 8am on a Saturday. Double checked my timing and started it back up. Still knocking, drain the oil and see this…
so now I don’t know what I’m gonna do. I can rebuild it, try to find a used donor and swap them, or just start over. I’m truly unsure of what to do. I’ve put so much into this motor and everything and now this. I could part it out, it’s got some decent stuff on it. Electronic ignition, 4-1 exhaust, stainless steel brake lines, rebuild calipers, good carburetor, clean tank, new tires, new chain and sprockets, new seat cover, rebuilt gauges and so on. Really unsure of what to do now. 

With a rebuild, you will be sure about the engine, though it will be pricey. A used engine may be cheaper, but it will also be a gamble since you would not know about its history. If you have the money, I suspect you would be happier with a rebuild in the long run. You have a great deal of emotional investment in it already. Also, one of the main purposes of a hobby is to empty your bank account. 

Youmay want to back away from it for a day or three before seriously considering your options.

Well, a swap is kind of out of the question. The closest kz750 to me is 200 miles away. Biggest issue is that one is that it has 52k on it. I don’t want to swap a bad motor with one that’s on its way out. I don’t want to downsize to a 650 or smaller. If I sell it, I will lose my rear end, no one will pay over $1000 for a really old motorcycle with a known bad engine and no title. I think I’m looking at a rebuild. But I’ve never rebuilt an engine before… I think I can but I have never to date. 

With the help of the guys here you should be fine. Just take a load of photos when you're stripping it down and read through the service manual a couple of times before putting it back togather (the GPZ 750 manual in the service manual section is better quality than the 750 fours manual).
The 750H engine was the first engine I rebuilt and mine's still running

Thank you for your kind works Calum! Obviously I will need crank and rod bearings, do you think I will need rings too? I want to do it cheap but I’m not gonna skimp on the things that matter. I browsed Z1 and saw no crank bearings, and rod bearings were $16 a piece!

will it be one of those things like you can only tell once it’s apart?

If your cylinders look okay and it was running fine before, I'd skip the rings, just making sure to clean out the old oil. You could take off the rings and check the gaps, but there's some chance of snapping them when removing/installing. 

If you do decide to rering, I've got a ball hone I can lend you for the bores. 

I was definitely stressing last night, I’ve calmed down and decided to Look back at marketplace and used some different search terms and found a KZ750 of the same year! Seller wants $1000 for it, bill of sale only, says it runs and drives. Seller is an hour away and we are scheduled to meet Sunday morning. If it falls through then we can continue the conversation about rebuilding. I know a rebuild could be done for less but I just want to get it back to working order before long. 

Would be a ZR7 engine or crank/rods parts swap an option or possible?
Complete engines are super cheap here in Germany (180 - 275 Euro).

www.ebay-kleinanzeigen.de/s-anzeige/kawa.../2059267900-306-3365

www.ebay-kleinanzeigen.de/s-anzeige/kawa.../1700607502-306-8135

www.ebay-kleinanzeigen.de/s-anzeige/kawa.../1766096577-306-9199

I think a ZR7 would work but I’m not in located in Germany. Thanks anyway though. If I have to then I’ll find a ZR7 here in the states and use it. 

Seems like getting the nearby bike and transplanting that engine will give you breathing space to effect a rebuild on a bench at whatever pace suits you while you keep on riding. Then, when that engine is ready, you can swap 'em out and start rebuilding the donor. Rinse and repeat, ad infinitum.
Plus, that gets you spares for all kinds on non-engine stuff. Use what you need and sell the rest on the forum here.

Yes buzz, I’m thinking this will be my course of action. I will be posting the carbs for sale once they are cleaned up and balanced, I’ve seen them go for a pretty penny so I’d like to recoup some of my initial cost. 

Hi Elfarm, I would suggest you take a breath and evaluate what you have - is a $1k for another 40 year old engine of unknown provenance the best way to go ?
The fact that it will re-start after you have turned it off multiple times is a "good sign"
I would suggest pulling the cams out and have a look at the bearings ( these are the furthest from the oil pump so the first thing to go dry)
It is unlikely that the rings are damaged (check the colour, if they are blue they need to go) as I would expect bore damage at that point and it is unlikely it would re-start
If the bores and cam bearings are ok it's just a bottom end rebuild which will cost a lot less than $1k and you will know you have a good motor (worth replacing the oil pump)
It would also be a good learning curve to have seen and know what is happening on the inside. If you take your time and follow the manual it will give you a huge sense of satisfaction (just my opinion)