Crankshaft Steel

[Ian B]

Long term project. I've wanted to convert a 4 cylinder kawasaki Z650 engine into a 6 cylinder for some time now. Something similar has been done with the Z900 / 1000 motors, Google Allan Millyard for a brilliant series of videos. What he does with a hacksaw and a BBQ are astounding.

The 900 / 1000 parts are expensive, and it results in a big heavy lump. Z650 cases & cylinders etc are cheaper, and end up more compact, still giving 100BHP. "If it can be done with a 900, it can be done with a 650" was my logic.

Then I found the snag; the 900's had pressed together built up cranks and roller bearings, just like the 2 strokes that preceded them (H1, H2). Making a 6 cylinder crank just means getting 2 normal cranks, pulling them apart and repressing the bits together, properly timed at 120 degrees.

Wnen the Z650 came along, they went for split big ends and a solid crankshaft running in shell bearings, so forget pressing anything apart. My thoughts are to make 12 crank webs from barstock, 6 crank pins and some big end pins, press them together and then MIG or TIG weld the outer ends of each pressed joint. I'll reuse some bits from an original crank, the centre main bearing pin that has an integral HyVo drive chain and cam drive chain wheels on it; cut the webs off, machine the ends and press them into new webs. Maybe also do the same with the taper that goes inside the flywheel.

All much harder than with the 900, but - I think - still do-able. All of the rest is a la Millyard.

Any recommendations of a steel to pick for the webs and the pins? It needs to be weldable. I'd like to avoid any post-welding heat treatment.

Ian

[Doozer]

I just gotta think that the factory KZ inline 6 is pretty well engineered
as compared to building one up from two 4 cylinder KZ engines.
But who am I?
The KZ1000 (4 stroke) had a roller press together crank.
Maybe another model was a plain bearing motor, I dunno.

-D

[Ian B]

To my knowledge, the only inline 6 from Kawasaki was the watercooled K1300. Pretty much all the rest were 4 in lines. There seem to have been noise problems with the pressed together roller bearing 900's & 1000's, and I think this design was abandoned for everything but 2 strokes after that model. Certainly, today's 1000's are forged solid cranks, all plain bearings.

The eventual project involves hack (or band) sawing the hell out of 2 complete aircooled KZ650 engines and welding the bits together to make a single 6 cylinder lump. It sounds totally implausable, until you see someone who's done it. All the "that'll never work"s disappear. Camshafts? Ah, chop them up with an angle grinder, bore the ends of the bits out, press together with old piston pins, weld 'em up. Doing this with air cooled engines is easier than watercooled, but it seems even they're possible to work with.

Ian

[Ian B]

It seems that Kawasaki were the only ones amongst the Japanese to build a big 4 cylinder crank from pressd together pieces; the Honda 750, which preceded it was also a solid unit.

[JRouche]

"I've wanted to convert a 4 cylinder kawasaki Z650 engine into a 6 cylinder for some time now. Something similar has been done with the Z900 / 1000 motors, Google Allan Millyard for a brilliant series of videos. What he does with a hacksaw and a BBQ are astounding."

The 900 / 1000 . That was the bump that Kawi needed. The 900zx? Maybe the best consumer bike, ever.

Hahaa. Yes! (I am a Honda guy so F Kawi ). Honda was making 460cc two stroke engines and folks said lets see more.

So they came out with a 900cc that was right out of their race team.

Then the 1200cc two stroke, two cylinders came out (not production). Mad Max type chit. Thast was a monster. JR


Yeah, I still feel the pain. My bud got a new honda, CBR650 I think, Im on my old Suzuki air cooled 750. We never went canyon riding. Go thing. JR

[nickel-city-fab]

I think it'll work, the heat from welding might cause your press-together fits to lose some of their tension. I think you could use generic 4130 structural for the cheeks, and 0-1 for the journals. Chemically they're very similar anyway. And I know that TIG filler for 4140 is available, the race car guys use it.

I've wondered for a while about building up cranks by using agricultural spline shafting, that stuff is usually 4140 or similar (1045) 6-splines and 1-3/8 dia and they sell it by the foot. Turn the spline part down to round for your crank pin, keeping the ends splined and broach the crank cheeks to match. Then assemble with green loctite.

[Ian B]

I also wondered about the effect of welding on a press fit. I'm hoping the heat won't penetrate all the way along the joint (I was only going to weld the outer end), which I hope would leave the rest tight. 4140 does sound like a good choice. I believe the mains are 38mm, big ends 36mm diameter. Not sure if I can find ground rod of these sizes and bore the webs to fit, it would certainly give me consistent pin diameters.

It seems common in the 2 stroke (ie. built up crank) world to "heli-arc" (ie. TIG) weld the joints, that's been done for decades. No ill effects that i've heard of. The trick seems to be strip tha crankshaft down, install new bearings & seals, press it together, straighten it with a lead hammer and then weld it. My version would probably go the same, without the bearings & seals.

The European equivalent of 4140 seems to be 42CrMoS4, I'd need something like 110mm diameter round bar, it should be available. Start with 300mm lengths, profile them with the shaper, cut slices off and machine from there. Once all assembled & welded, take a final skim off the OD of the webs, fixed steady in the middle.

Out of interest, any reason to use a different material for the journals? Would 4140 be too soft? I'll need to drill the journals for oil passages, so the O-1 would have to be at least partly annealed first.

Ian

[nickel-city-fab]

It sounds like a good plan, all of the 4140 and O-1 that I have bought was shipped in the annealed state, the O-1 machines beautifully like that. But after hardening a file won't touch it. So I would use the O-1, or 4140, or 42CrMo for the journals because it can be hardened and tempered to high strength and wear resistance. And simply use cheaper material on the crank cheeks

I also wondered about the effect of welding on a press fit. I'm hoping the heat won't penetrate all the way along the joint (I was only going to weld the outer end), which I hope would leave the rest tight. 4140 does sound like a good choice. I believe the mains are 38mm, big ends 36mm diameter. Not sure if I can find ground rod of these sizes and bore the webs to fit, it would certainly give me consistent pin diameters.

It seems common in the 2 stroke (ie. built up crank) world to "heli-arc" (ie. TIG) weld the joints, that's been done for decades. No ill effects that i've heard of. The trick seems to be strip tha crankshaft down, install new bearings & seals, press it together, straighten it with a lead hammer and then weld it. My version would probably go the same, without the bearings & seals.

The European equivalent of 4140 seems to be 42CrMoS4, I'd need something like 110mm diameter round bar, it should be available. Start with 300mm lengths, profile them with the shaper, cut slices off and machine from there. Once all assembled & welded, take a final skim off the OD of the webs, fixed steady in the middle.

Out of interest, any reason to use a different material for the journals? Would 4140 be too soft? I'll need to drill the journals for oil passages, so the O-1 would have to be at least partly annealed first.

Ian

[Ian B]

Good point on the cheaper material for the cheeks - there'll be a few kilos of it.

I must admit, i wasn't planning on any kind of post-assembly heat treatment; would that be much of a problem? Cast iron was a common crank material for a long time, it can't have been that hard. I guess I was going to rely on the oil film to keep metal for touching metal.

On the welding; does the filler rod need to be a similar alloy to what's being welded (which would most likely mean TIG), or would plain old mig with normal wire be good enough? TIG is definitely an option.

[old mart]

Some research needs to be done on high strength steels and welding. I know that the Reynolds 531 chrome moly was selected because the molydenum reduced the brittle fractures that welding could leave, but that was tubing, not solid. Villiers two stroke engines assembled their built up crankshafts with a light press fit, and then pressed hardened steel expander plugs into each end of the hollow crankpin.

[strokersix]

Villiers two stroke engines assembled their built up crankshafts with a light press fit, and then pressed hardened steel expander plugs into each end of the hollow crankpin.

I like this. I wonder if it may allow disassembly as well?

[mattthemuppet]

I would love to see your progress on this project, sounds like an absolute hoot and should keep you out of trouble for a good while. Nothing to offer on the material selection, but I'm curious. Will it be a 4+2 or a 3+3? Will it have a 6 cylinder firing order or...? Sounds like a lot of fun!

[nickel-city-fab]

Good point on the cheaper material for the cheeks - there'll be a few kilos of it.

I must admit, i wasn't planning on any kind of post-assembly heat treatment; would that be much of a problem? Cast iron was a common crank material for a long time, it can't have been that hard. I guess I was going to rely on the oil film to keep metal for touching metal.

On the welding; does the filler rod need to be a similar alloy to what's being welded (which would most likely mean TIG), or would plain old mig with normal wire be good enough? TIG is definitely an option.

Best practice is to use filler wire that is identical to the base metal, in critical applications. I would use 4140 TIG filler. No heat treatment. Just the heat from the welding will loosen up your press fits a bit.

FWIW, Harley Davidson uses tapered pins with threaded ends, drawing the taper into matching bores in the flywheels with the nut. Seems to hold up OK for them, and those are not small engines.

[Ian B]

I'd seen the taper and big nut approach on some of the older British bikes - 1950 Nortons, that kind of thing.

I can see the sense of using a similar filler metal. Is any kind of preheating of value for welding? I know this will be needed when TIG welding the alloy cases, would a 4140 pressed crank benefit from the same? 200 degC for instance?

[nickel-city-fab]

I'd seen the taper and big nut approach on some of the older British bikes - 1950 Nortons, that kind of thing.

I can see the sense of using a similar filler metal. Is any kind of preheating of value for welding? I know this will be needed when TIG welding the alloy cases, would a 4140 pressed crank benefit from the same? 200 degC for instance?

I could go either way on preheat for 4140. Post heat after welding may be better than pre heat before welding. You could do it in an ordinary kitchen oven, then shut the oven off and let it cool down slowly before taking the piece out. Just for a precaution against cracking since the metal will be fairly thick. I think the press fit is going to do most of the work of fastening the parts together, but the press fit may be ruined by hardening and tempering after.