1977 KZ1000 Restomod

Mikaw wrote: This seemed to be a good place to ask this as we are dissecting pros and cons of upsized pistons. I found a set of 1015 wiseco but they are 12.5 to 1. Any thoughts by the experts on the compression for a total street bike.

Based on reading, no direct experience, 12.5 is high for pump gas. Modern bikes run very high compression, but they have much more sophisticated cylinder head design. The kz1000 has a simple hemisphere, which isn't as efficient as modern designs, and is more prone to detonation. Again, that's what I've read.

But here is something to consider. The effective, or dynamic, compression ratio is dependent on the camshaft duration. If you have a long duration camshaft, it lowers the effective compression ratio. There are several calculators I found, and the key detail is at what angle (how late) the intake camshaft closes.

Are you planning to use the RC370 cams you mentioned in the other thread? Those have a very long duration. It's possible that 12.5:1 would work well with those cams.

Using this page: uempistons.com/p-28-effective-compression-ratio-calculator.html

bore : 2.756"
stroke : 2.598"
rod length: 4.647" (from a post by Larry C, so...)

Here are some examples:
Intake Close ABDC
Web 118 : 47.5 ("Performance street cam for stock engines")
Web 110: 54 ( "hot street" )
RC388 : 74
RC370 : 83

Here is the calculated dynamic compression ratio:
Web 247 : 11.076
Web 110: 10.663
RC388 : 9.091
RC370 : 7.267

If the numbers and calculator are right, and the dynamic compression ratio is really directly related to the risk of detonation, I'd say if you're running those RC370 cams you're going to need the 12.5 pistons.

Oh, and the RC370 specs came from the timing card you posted a photo of. The web cams come from webcamshafts.com timing cards. And the RC388 I used this site to calculate Close ABDC.

mgispeedware.com/camshaft-calculator/

And here is another dynamic compression calculator that has adjustments for boost and altitude.

www.wallaceracing.com/dynamic-cr.php

slayer61 wrote: Keep it up calum & post more pics as you progress. My Kawasaki was a jet black '77 KZ1000 just like yours. Very cool.

Will do, thanks.
Thinking this one will stay black but with the original A1 stripes in yellow to match the Öhlins shocks. Won't be painting the engine but maybe matt black for the forks, swingarm and frame with a black Kerker. Still messing around with a few ideas though.

Getting back to the engine...

I got in touch with the builder and with the guy I'm getting the bike from (he has been working almost exclusively with KZ's for a couple of decades).
To his credit, the builder said that he'd go for 1075cc in the original liners (more bang for your buck). The guy I'm getting the bike from said that 1075 would leave the walls quite thin, especially if it were to see any track time.
As far as I can tell the OD of the stock liners is 75mm - is this right? That leaves the stock wall thickness at 2.5mm and with the 1075 kit that goes down to 1.5mm. This does seem a bit thin but the consensus here seems to be that 1075 is they way to go and fine in the stock liners. Are my numbers wrong or is 1.5mm OK?

I’ll let the experts answer but 1075 is ok. Trying to get 1105 out of the stock sleeves is a potential problem..

The guy who is saying that 1075 (72mm ) is too thin is talking absolutely enormous bollocks.
No need to say anymore really.
The 250 hp turbo Orient express funny bike i built that runs 30 psi and low 8 second quarters has been doing just fine on 1075 in a stock block for the last five years with no issues.
When you go to 1105cc (73MM) is where the liner get a little thin and are prone to oil burning due to microscopic distortion of the liner surface under high heat and load and it's also not uncommon for them to crack at the spigot .

Thanks, everything I've read seems to support this. I'd still quite like the numbers, though.

zed1015 wrote: The guy who is saying that 1075 (72mm ) is too thin is talking absolutely enormous bollocks.

Thanks
Is the liner OD 75mm? I saw an old post from Jay of APE where he said 1.9mm was their lower limit on wall thickness. Just made me wonder about the 1.5mm (assuming I have the correct OD).

The liner OD is on average 75.70 mm where it sits in the block.
Believe me, i've been building ,running and racing these bikes for over 40 years and i currently own five of them in various states of tune from Turbo, Supercharged, N/A race tuned and my Z1R that i've ran daily for over 20 years.
They are all stock block 1075 cc.

Sweet, thanks. The builder said it was fine as well, but 40 years of experience sets me at ease

Is there really much of a power difference from a 1045CC engine to a 1075CC motor? I went with 1045CC that should allow for one more rebuild to 1075CC if needed at some point.
Rick H.

Not sure about the jump from 1045 to 1075, but my engine guy said it would probably be about 7-10ps difference between 1075 and 1135 (assuming comparable systems re. porting, valves, cams etc.).

OK, I've been doing some research and taken the advice given here. The current plan is as follows:
Engine:
- Wiseco 1075cc piston kit (w/ welded crank)
- bigger valves and ported head (letting the engine builder do his thing here - 38mm intake valves)
- WEB CAM Stage 2 cams ("hot street")
- RS34's
- K & N pod filters

Chassis:
- Shorten the back end. Keeping the original tail but moving it forward so I end up with a single seater. Chopping off the pillion footrest brackets.
- Frame bracing
- ZRX1100 forks and brakes with a new front wheel hub to fit (getting a triple clamp made for the conversion)
- 110 front wheel, 160 rear wheel (spokes)
- GSX R750 swingarm
- Öhlins S36PR1C1L rear shocks
- minimized front end (smaller speedo and tacho, headlight, clip-ons).

Before anyone goes too mental about me chopping up the frame - I got a second frame for cheap as the pillion footrest brackets are rusted and someone's been cutting things off the back end. Will use this frame and keep the original one in storage.

A couple of things I'm not sure on:
- I'd like to do away with the battery and just use the kick-starter. I've read that this works with the Tarozzi rear-sets but I'm not sure how practical it will be. Are these things a pig to kickstart?
- Earls oil coolers are crazy expensive here. Does anyone based in Europe have any recommendations? The engine guys stock Setrab coolers and one of them will cost about half as much as an Earls cooler.
- Does anyone have any gearing recommendations or should I stick to the stock 15/33?

When I tally up everything that I'm replacing, I'll be left with a frame, triple clamp, forks, swingarm, front wheel, rear rim and spokes (no hub), front and rear brakes incl. master cylinders, switch blocks, levers, side covers, carbs, footrests and a few other things. Basically, I'll almost have a second bike minus the motor. I'm considering just getting a new hub for the rear wheel and keeping the old one as a spare (and keeping my eye out for a motor). I could use the 'spares' bike to help finance the project. Any recommendations for another model that I could pull a spoked rear wheel and swingarm off? I thought about a Zephyr but the 1100 swingarm is supposedly heavy and I don't like the idea of the excentre for setting chain tension.