Author Topic: 750 K2 Strip & Rebuild - help please  (Read 5694 times)

Offline royhall

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Re: 750 K2 Strip & Rebuild - help please
« Reply #120 on: June 04, 2022, 09:38:54 AM »
An interesting last Friday with Lindsay, who is rebuilding my K2 engine.

The first job was new cushs into the Primary drive sprocket component, which had to be disassembled due riveted. Within there are 12 cylindrical rubber cushs; at 50yo they were rock hard and if deformed slightly  (using a pliers) simply split and stayed that way. We stuck ‘em in a bag, and when I put it down on the bench there was a solid clunk like it was a bag of marbles.

Initially 2x new needle roller bearings were pressed into the shaft. Next the 12 new cushs went in to the lower sprocket, with upper sprocket then pushed in place. (needed a press to coax this - very tight). Then finally 6 new Allen screws, into the holes pre tapped and countersunk, using thread locker of course. All now good as new.
Interesting write up on the cush rubbers. I wonder what makes the rubbers rock hard on one rebuild but still soft and pliable on another, after all they are all a similar age. I was going to do mine, but after a good prod through the holes they feel as good as new. Maybe the type of oil used or mileage? Anyway I wasn't complaining as it's a pig to do, just wondering.
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Offline Sesman

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Re: 750 K2 Strip & Rebuild - help please
« Reply #121 on: June 04, 2022, 11:04:29 AM »
Yes, shock rubbers…..🤔. When I stripped my 77 550F2, I found the primary chain to be approximately 60-70% worn (that’s a dimensionless estimate based on comparison with new), but remarkably the primary rubbers were comparable with new in condition. I replaced them anyway. The motor showed absolutely no signs off being dismantled in the past. So primary rubbers seem to be something of a mystery?

Offline Sesman

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Re: 750 K2 Strip & Rebuild - help please
« Reply #122 on: June 04, 2022, 11:07:53 AM »
……though I believe others on this forum find that the ‘norm’ is to find them as ‘hard as marbles’.

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #123 on: June 04, 2022, 01:06:47 PM »
My guess is (my) old rubber cushs had little useful deformation ability left given what I saw; but given their loadings so uniformly applied I’d agree they might have gone on forever - with little real indication to any rider. (tho’ time will tell on this one).

As the first cush came out in my hand, there was little obviously wrong with it…. and I stated so. Lindsay got a pair of pliers and squeezed it… it split and deformed - as any old rubber would. When a new one was in my mitt the difference was then very apparent; rubbery rubber versus hardened.

There was a choice for me…. at extra cost of course, but I figured we were in so deep it was better to go the extra mile. The bike is 50yo, the cushs have experienced numerous heat, compression / decompression cycles, and like the rest of us, must be getting a bit tired and stiff!

In my case, no regrets having seen the old pieces….

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #124 on: June 18, 2022, 01:14:12 PM »
It seems Roy & I pretty much neck in neck… though it’s certainly no race at the pace I work.

Engine now complete, and very pretty. Head completely reworked, with valve guides re-lined, new seats, valves worked. Bores taken to 0.5 o/s, with new Cruzinimage pistons.
New chains, seals, bearings, primary cushs, cam tensioner / followers, and obviously painted & baked. Old clutch refitted as in gc.
Starter motor now with new brushes, positive wire…and painted.
Frame and all black components powder-coated.

Various bits now Cobalt-Nickel plated, which is a pain as needs to be done at 50-60C for an hour, and best preceded with a copper ‘flash’ plate (15 mins / 50C). My garage looks like something out of breaking bad. The toolkit freshened up…

Carbs and wheels next.
« Last Edit: June 18, 2022, 01:16:04 PM by Lobo »

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #125 on: June 18, 2022, 01:22:15 PM »
Comments please…

Currently the wheels with SS spokes, which have been fine over the past 10 years, but notably 3 or so failures on the rear wheel in those spokes with the acute bend. Luckily these are the ones that are easily replaced by deflating the tyre enough to unlace the bad one and thread in a new…

Anyways, am planning to polish both hubs, and so will be dismantling the wheels.
Thoughts please on switching to Chrome spokes versus SS. Notably the bike will be ridden regularly, and that I live on the coast. Low maintenance and retaining good looks are key…

Ta.

Offline K2-K6

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Re: 750 K2 Strip & Rebuild - help please
« Reply #126 on: June 18, 2022, 06:59:38 PM »
Engine looks good. For spokes I'd favour stainless with brass nipples and using molybdenum grease on threads to build and tension them.

Consider judging the load on the tight turn spokes in comparison to the less angled (are the breaking spokes the outside on hub) that may improve longevity for you as that doesn't seem that bad for current spokes in lifing terms.

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #127 on: June 19, 2022, 11:44:09 AM »
Thanks Nige; some good advice and I’ll rebuild using SS and arrange a bit more pong than ping on the B Spokes.

Today’s question on the (K2) carbs.

(1) any tips to getting the top rubber seal out so that I can properly service / polish the  carb top? (Pic #1)

(2) any tips on getting the polished strip concealing the choke mechanism out? Better still, does it actually need to come out to replace the mechanism? (Pic #2)

Ta…

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #128 on: June 25, 2022, 12:40:16 PM »
Needing help form the Alchemists…. Ash, Nige etc.

Plating today, specifically Cobalt-Nickel onto carb fittings. All gone very well…. barring the ‘anchor’ bit that screws into the slider and holds the riser bar in situ. The red flag came up as I gave them a short acid pickle prior to the copper Ultra Strike plating… rather like dropping an Alker seltzer into a glass of water… ie a lot of fizzing.

So, the flash copper coat went on, but not great. I continued with the Cobalt Nickel, one piece came out fine, the other three a mess, with patchy black streaks. The plate obviously didn’t take.

I’m not sure of the metallic make up of these bits, tho’ they are magnetic. Not sure how to proceed tomorrow… any thoughts whether Zinc plating will take? If not, what next?

Thanks

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #129 on: June 25, 2022, 12:57:19 PM »
One the plus side to the above post 90% of plating today was a great result. The pics say it all - even if the first two are upside down 🤦‍♂️.

The cable pulley wheel was a real sod, in that it’s two plates riveted together, and sandwiching 50 years of oil. This, of course, felt the need to leech out in the plating, which set me back a day with streakings. An overnight soak in petrol, followed by multiple heated ultra-sonic bathes finally got me where it needs to be.

Do I enjoy it? Not a bloody bit - it’s an awful lot of prep, and in the case of the copper strike and Cobalt-Nickel they’ve to be done at 55C or so which just makes it a bigger logistical pita. This all said, I am appreciating the freedom the plating kits (ie inc Zinc) are giving me in that ANY bit of metal on the bike is now doable on my own terms; and if done methodically can be minimised to a point. The nice bit about the Cobalt-Nickel plate is that it’s the next best thing to a chrome look, and a whole lot cheaper & safer to do. Did the brake torque arm last week - @ 50yo looks just fine now.
« Last Edit: June 25, 2022, 01:03:51 PM by Lobo »

Offline Trigger

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Re: 750 K2 Strip & Rebuild - help please
« Reply #130 on: June 25, 2022, 02:03:03 PM »
You have to keep a eye on the pully wheel ( Butterfly ) as, it has alloy rivets and any type of Pickling weakens the rivets.

As per factory they were BZP with a yellow passivate and riveted together after plating  ;) 

Offline Oddjob

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Re: 750 K2 Strip & Rebuild - help please
« Reply #131 on: June 25, 2022, 03:19:09 PM »
Not that hard to make a new one if it does break though, made one for the 500 earlier this year. Tweaked how to do it now to improve how it comes out.
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Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #132 on: June 27, 2022, 12:33:59 PM »
Cheers Graham. The passed couple of months I’ve been using the Sandcast as my go-to reference for the K2 rebuild (when in doubt), but alas that doesn’t have the pulley wheel. And tbh, I didn’t give it a thought…

So dang when you pointed out the gold passivate finish! (& thanks). Fixed now…

And yep, the acid wash… basically 30 secs in a weak solution followed by a distilled water rinse - ie not concerned wrt hydrogen embrittlement (which is fully covered in the plating manual)

Offline Oddjob

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Re: 750 K2 Strip & Rebuild - help please
« Reply #133 on: June 27, 2022, 01:14:31 PM »
They came out nice, your getting good at this lark  :) :)
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Offline McCabe-Thiele (Ted)

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Re: 750 K2 Strip & Rebuild - help please
« Reply #134 on: June 27, 2022, 01:55:05 PM »
That's an impressive bit of plating you have done - looks like new.
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