750 Katana Resurrection

[KiwiAlfa156]

Hi All,

I'm in the middle of bringing a 1982 Suzuki GS750S Katana back from the dead. I have been posting/blogging on the Katana Australia forum (Hi PeteGSX) but thought I might share the journey here too. It may be of interest to some of you. I'll gladly take any advice or comments, and am happy to answer questions. I've done a few restorations for road use, not show bikes. "Ride it, don't hide it", they say.

With every project, I've tried to do more and more of the work myself to a level I can be proud of. Last build (my daily rider GS650G Katana) it was all the paint and panel work.



This project is the most ambitious to date. The bike was given to an acquaintance of mine, apparently as part-payment on a debt, and had languished in his garage for a year or two. It was a non-runner and he's not a bike guy. He just wanted it gone and the deal was struck. Here is day it came back on the trailer.

[KiwiAlfa156]

Carb clean, new mains, pilots, and needle valve assembles



Fresh oil and filter and...

Going after at least 10 years sitting idle.

[KiwiAlfa156]

Inspection of the frame showed that the Kat was bent...



So I thought why not straighten it?

[KiwiAlfa156]

So after much sitting, staring and imagining I came up with a plan. The plan was essentially two parts. First is to "reverse the damage" and second to adjust rake and alignment.

The swingarm, frame cradle under the engine, and the frame behind the rear engine mounts and airbox are straight. The bending out of alignment is in the backbone and front downtubes. The triangulated and gussetted area behind and below the steering head appears to have rotated 'as a unit' from the frontal impact. Everything in this section of the frame measures up and shows no sign of deformation or cracking. So "reversing the damage" looked to be pushing the front downtubes back out straight, while applying a pulling force at the bend on the backbone and a rotational force via a lever in the steering head. The deflection and deformation at the points of damage weren't overly much, and I'll probably weld in bracing into backbone at those bend points Pushing the down tubes forward meant removing the engine.

The idea was to build a jig to replace the engine and hold all the engine mounts in alignment as well as provide jacking/pulling points. The plan was to exert enough pressure to push out the front down tubes direct at the point they are bent in, while simultaneously holding/pulling on the backbone tubes until both sections are straight measured against a straight edge. The deflection of the tubes is only 2 or 3°. I have a 10-ton 'porta-power' hydraulic ram to push the front tubes out and have fabbed up two large diameter square thread pullers from some scrap large g-clamps (which also formed the main members of the jig) to hold/pull down the backbone.



The second stage will be to return the engine to the frame and construct a chassis trueing jig to hold the frame with the swing arm at horizontal (set using bubble levels) and provide pulling points to twist the steering head so the pivot is plum on the centre line and perpendicular to the swing arm pivot. At this stage I'll also measure the rake angle to see if it is within coo-eee and I have a some lengths of square section steel tubing to construct this.

So far the total cost of materials and tools has been NZD $250. Most of this was for the porta-power which was cheaper to buy than hire for a day. I have access to scrap steel at very low cost.

I did some research on the steel alloy the frame is constructed from in order to determine to what extent the straightening will effect structural integrity, and what, if any, heat treatment either during or after the straightening process may be required. I found a reference on line on a Japanese website that described the chassis as being built from STKM13A thin walled, large diameter tubing. STKM13A is a JIS steel standard which specifies steel with a maximum of the following:

Carbon 0.25%
Manganese 0.30% (0.9% Max)
Phosphorus 0.04%
Silicon 0.35%
Sulfur 0.04%

Yield strength for STMK134A tube is 215 N/mm², ultimate tensile strength is 370 N/mm², and elongation is 30%. So confirming that is classed as low tensile mild (low carbon) steel and is quite ductile (non-brittle/high plasticity). Good news, as this means that the risk of introducing stress and fatigue through bending and straightening are very low. Being low carbon means that heat treatment isn't required to normalise stress or return material properties (annealing/tempering), basically because heating and cooling doesn't change its crystalline structure. Suzuki using STMK13A makes sense for ease of manufacture. Cut it, bend it, stamp it, weld it, paint it. Done. It also makes it easy to repair. 'Slight' bends can be pushed back. As long as there is no problematic 'plastic deformation' in the tube (kinks, necking, etc.) all good. Other fancy steel alloys like chrome moly require a lot more care, especially with welding.

[KiwiAlfa156]

This was the jig just before applying the pressure

Frame straightening stage 1 - reverse the damage. After lots of looking and thinking about the direction and distribution of the forces I was going to apply I decided that as the push force was down the centreline and that lateral and twisting would be neglible, I wouldn't do much more jig fabbing. So apart from a welding up abottom mount cross brace, I'd just get to it.

The jig it is extremely stiff, so I wasn't to worried about flex letting mounts going out of alignment.

All the mounts where torqued up and the ram and screws were preloaded. I started applying hydraulic pressure checking for deflection of the front downtubes while adding tension through the screws on the backbone. As the front tubes deflected forward, tension on the back bone eased, so I added more tension, effective pulling the backbone down. I added more hydraulic pressure until the front tubes deflected slightly past straight to allow for elasticity.



I released all pressure and tension and checked for straightness. Here are the results

[KiwiAlfa156]

[KiwiAlfa156]

Everything is equi-distant on both sides and all mounting bosses are in alignment. No cracking at welds. So far, so good.

Checked the deflection on the front fork stanchions (bent) and I think the next step might be to rebuild these with the new stanchions I got for it (cheaper and easier than straightening and re-chroming due to pitting - which cost the same as new), pop in the wheels and check alignment using string/laser. The bent forks might be the cause of the front wheel being a little cocked to one side. To the naked eye, referencing against the horizontal rods I've it on the frame, the steering head doesn't look twisted. Although a small deflection is all that's required. But if I'm lucky, and the steering head is in the vertical, I'll save myself the trouble of having to do some unnecessary jig building.

[KiwiAlfa156]

Fork disassembled and cleaned


Out of the parts washer


Worn bush next to one from the spares box that dates back to the late 80s...


Comparison with replacement aftermarket part. Some minor differences, but correct where it matters.


Bend apparent on the glass plate.

Next step to polish the lower tubes. Drill the damping tube orifices - I have a set of fork cartridge emulators I was going to fit to the 1100, but I think I'll keeping her as standard as possible. So into the 750 they go.

[KiwiAlfa156]

Damping tubes drilled


Legs prepped for polishing. Hopefully forks will be finished tomorrow.

[KiwiAlfa156]

Clean enough for street use.

[KiwiAlfa156]

Reassembled the forks. Replaced the worn bush. It needed a little filing of the end gap of the bush, as the insufficient gap meant the bush sat proud and the fork tube couldn't be inserted. Set up the cartridge emulators with blue springs and 3 turns reload, which is baseline for the Katana. Fresh seals and 10wt synthetic oil, emulators and springs in and the fork caps won't go in deep enough to bite the thread....

Upon inspection the alloy sealing buttons are binding in the top of the fork tube which has an internal taper up the top starting below the threads. Measurements with internal and external calipers revealed the difference was around 0.1mm so the aftermarket tubes have slightly thicker wall thickness.

The easiest fix was to reduce the diameter of the button by 0.1mm, but I don't have a lathe. So I came up with a solution with what I had to hand: plastic pipe, some wooden dowel, tiny panel pins, hand files and emery paper. The buttons are essentially static, when fitted, so while roundness and clearance is important, precision tolerances aren't required.







Kept taking off fractions until they fitted the tubes and the caps could be screwed on without resistance. Reused the standard orings as they were in good nick and had enough give not to jam or twist. A little rubber grease for assembly. Tested action of preload adjuster, and all good.


Done

[KiwiAlfa156]

Wheels cleaned up better than expected. Tyres were a bitch to get off. Ended cutting the front one off. Hard old rubber and cool temperatures. Next step to get the wheels and frame bolted together and run some strings to check alignment and then decide whether to build the straightening jig.

[KiwiAlfa156]

Straight forks fitted and tweaked steering head confirmed. Next steps degrease the engine, build straightening jig, refit engine and reset the steering head.

[KiwiAlfa156]

Frame jig 2 is coming along nicely. Used a plumb bob and spirit levels to centrepunch some centreline reference marks on the frame and swing arm. The datum was string lines off the sides of the rear rim -checked square against the swing arm piovt- and pleasingly this mapped the exact centre of the swing arm pivot and frame cross tube. Did some diagonal measurements between front engine mounts and the swing arm and they were identical I have confidence that apart from the tweaked steering head, the frame backward from there is straight.



So after some more sitting, staring, and imagining the forces I'll be applying and paths of transmission, I cut some steel and did some arc welding. The chassis with engine bolted in forms the rigid mass for the jig frame.



I'm going use my push ram as I can't get a pull ram under NZs lockdown. There will be an arm for the ram to sit on and push a heavy tube dropped through the steering head. The idea is have the ram operating as close to the horizontal plane of the jig in order to increase leverage on the steering head and minimize leverage and torsion directed through the jig. There will be some triangulation to resist the main jig beam bending.

[KiwiAlfa156]

Jig pretty much finished. I have a crossbar to fit to which the ram will push at 90° to the centre beam.


Final checking will be front axle centreline alignment. The defection is approximately 15mm at the axle.


I've begun pulling down the top end -which was always part of the plan- to make fitting and removing the engine easier for the straightening process.


Already a few issues have become evident


Inlet on number 2, is number twos