Z650 / GPz engined Restomod

Back in early January I bought a frame from a 1978 Z650, complete with its all important V5 log book. I didn't buy a complete bike, barn find or otherwise, because I planned to change most of the parts on the bike, except the frame and bodywork. 

Over the previous few months I'd been slowly buying the parts I'd need for the bike, including forks, swing arm, wheels, brakes and most of the bodywork. Still looking for a tank at a good price, while the side panels and tail section are reproduction parts made in ABS, with improved, stronger mounts.  

The goal of the project was to build a bike which still looked like a Z650, but with much better suspension and brakes. While making more power, and being as light as possible, without resorting to the expense carbon wheels and Ti parts.  The simplest route to more power was to use the 650's bigger brother the Z750 engine, the most powerful of which is the GPz motor, by chance I knew of someone with such an engine he didn't need..

Not sure yet if I'll go to a 810cc conversion, as the only pistons I can find are from Wiseco, and they are three times the price of a standard set of pistons. Target is at least 80bhp. 

Its not easy to find out the weight of individual parts of a standard bike, but again the target is to get as close as possible to 400lbs dry weight.

An additional target was to complete the bike this year, as my projects have a habit of dragging on for years. 

The frame arrived on a pallet one cold, wet January morning, and it didn't take long to start work, first by narrowing a ZRX1100 swing arm to fit the frame, and making spacers so the 16mm pivot bolt will work with the 20mm bearing in the swing arm. But first some crash damage to the underside of the swing arm had to be repaired.

I checked the price of a new pivot bolt, since I didn't have one, then decided to make one in stainless steel. Even with having to pay for a bit of welding, it turned out half the price of a new standard pivot bolt. I'll make the engine bolts in stainless too, as its the same process. 

The bike is having wheels, brakes and forks from a 2004 Yamaha R6, which is the year before the model switched to USD forks. I choose these donor parts as I like the 5 spoke wheel design, which are also impressively light, while the 43mm forks and 4 pot blue spot calipers are some of the best fitted to a Japanese production bike. I've also used these wheels before, and knew they'd work with the ZRX swing arm.



However, when the rear wheel was fitted into the swing arm, it was clear something was very wrong.. it didn't fit!

With the wheel hard against the left side of the arm, it was still miles out from being central in the arm. Very odd, took awhile to work out what was going on, as the ZRX1100 is similar in  width as the ZRX1200, which I'd used successfully before.

Eventually found that the rear wheel wasn't in fact from an R6, it was from a later model, an FZ8, though it was sold on Ebay as from an R6. Despite looking identical, they are not, the hub is much wider, even the cush drive is different.

I've since bought a cheap R6 rear wheel, but its silver, not the gold of the original, so now I have a mismatched front and rear wheel. Will probably have them both powder coated. A pity as the front wheel is in perfect condition.

To fit the wider back end, which has a 5.5 x 17 wheel, the pillion mounts on the frame have to be removed, not a problem as they were rotten inside in any case.

FZ8 wheel wont fit the ZRX swing arm..




 

It feels like the project has taken one step forward and two steps back, but these things happen when your mixing and matching parts from different bikes. However some good news as I've recently bought the GPz 750 engine I've had my eye on. The cases, barrel and head will be ceramically coated, and so the engine has to come apart, as well as needing a new cam and primary chain etc. 

The engine last ran 24 years ago, but it turns over ok by hand, and fingers crossed its long sleep hasn't caused too many problems with its internals.

Coming up to date, the engine came apart ok, had a problem with the oil filter bolt, as it was rounded off. Had to chisel it off in the end. While the top fin on the barrel was found to be broken. This was repaired by welding in a shaped piece of alloy, not by me, but sometimes its not what you know, but who you know... 




One problem was that the splines on the gearbox output shaft were badly worn, as if the front sprocket had been run loose. I later found out the the engine came out of a bike which had CMA wheels, and the rear wheel had no cush drive. Since I intend to fit a 5.5 inch rim / 180 rear tyre, it seems that the later ZR7s output shaft is longer so I have bought a good used transmission from a 2002 ZR7s, hopefully it'll fit with no issues.   

What is a V5 log book?

TexasKZ wrote:

What is a V5 log book?
 

This is the UK registration document that shows vehicle details (Vin, engine number etc) and ownership

Here are two great 750 restomod threads that you may find useful...

www.kzrider.com/forum/11-projects/610573-saving-a-81-kz750-e

www.kzrider.com/forum/11-projects/602462...0e-resto-mod?start=0

With the engine taken apart, its cases have been sent off to be coated, while the frame has gone off to be powder coated. But not before the frame was welded, to add a few tabs etc.

Meanwhile the clutch basket has been modified to remove the looseness caused by its cush rubbers getting old. This was done by grinding off the rivets holding on the back plate, then inserting steel sleeves around the cush rubbers which take up up any slack and prevent movement of the rubbers. Finally the back plate was welded back into place. New cush rubbers are now available, but at £108 a set, this option is cheaper.




 

Got the frame back from a local place, I'd not used before, happy with the powder coating, and the price was very good for a change. Next, I fitted the plastic front part of the rear mudguard, along with a stainless splash plate I made to replace the heavy chromed rear part of the mudguard, along with the battery box. 

Finally, made a bracket which goes over the lock stop post beneath the steering head, as it had to be wider to give the correct lockstop with the R6 front end. It was still drying from being painted, so couldn't fit it yet.
  

That frame looks great!

Maybe you know something I don't, but I thought the GPz engines used rubber engine mounts, and the frame uses different mounting tabs than the early bikes?  

They are slightly different, but will make spacers to accommodate those differences, so the engine will no longer be rubber mounted,

For example, this is another GPz750 engine, a turbo, being fitted into a Z650 frame..

  

Got the alloy bracket finished to mount the rear ZX6R tail light, while bought a good tank which is rust free, only slight problem is a small dent just behind the badge on the left hand side. But I'm assured by my local painter that dents are much easier to deal with than rust. Hopefully will get the swingarm and yokes back from powder coating next week, which will allow me to fit the back and front ends for the first time.