Topic: Build of CB750 K2 1975 - Europe had K2 until 1975 - Engine started feb 17 2019

[PeWe]

Flat front tire! After only 3385 km.
Bike felt really swampy and difficult to handle when leaving a bike meet last Saturday. It was enough to ride bike to a boat shop almost 10km away where a helpful guy helped me with a Ryobi handheld compressor looking like a screwdriver/ drilling machine. Home quickly while air still in the tube, 35km. Change to the blue K6 and continued the bike festival around the lake here.

My idea to always have parts in stock is right way.
A new tube is waiting for the wheel and my probably angry handling of the tire bars.

 I have extra tubes if making a mistake with not enough air in hose when wrenching on the tire.
Headlight reflector off since the light pattern shaping metal bowl inside has broken off and rattled around inside. Now taken out and lamp soon complete.

[PeWe]

Did the tube today. Rather painful with damaged ribs. No cracks just got a hard hit a few days ago. Painkiller gel helps.
Wheel on tomorrow after a check if pressure is OK
Still 3.0 BARs? Non alcoholic bar this time!
If OK adjust to 2.1 and wheel on. If not continue to ride my blue bike

[PeWe]

Wheel on this morning, bike is ready for a ride again.
Gave myself a breakfast for that.  Porridge and coffee. Not a full English as Terry's.

[CBJoe]

I hate changing tubes even without bruised ribs...can’t imagine trying it with injuries


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[PeWe]

The tire is fresh, not difficult to lift one side getting the old tube out. Lifting the wheel hurted more than bending the tire on again which helped me to decide not taking it to a tire shop.
I made sure to lube the tire on both sides with tire assembly lube so it could slide on.

[PeWe]

2 refurbished cylinders back to the garage.
+0.50 pistons to F,F1, K7 to get a little bit more compression.
Cruzinimage cheap 392 pistons.

This might enter the K2 this winter. The cylinder has right look for the 1975 model.

836. I found a cylinder in decent shape bored for Yoshi 812. Not that much to bore further to fit my old but not much used RC cast pistons.

My old 836 cylinder got 849 kit earlier this year together with a refurbished ported head that got 34mm inlet valves so I have spares    Horder warning....!!

[Terry in Australia]

I had a set of those RC pistons years ago Per, I thought I still had them, but can't find them? I think they are Arias brand? If you use them, make sure whoever bores the cylinders measures them beforehand, because they're slightly smaller than the standard 65mm 836 pistons, I ran mine in a set of 836cc sleeves and they were too loose, so pulled them out and installed a set of those cheapie CI 836cc pistons, and they were a much nicer fit, without the piston slap I experienced with the RC pistons.

The RC pistons came in two compression ratio's, 10.5:1 and 12:1, from memory, mine were 12:1. My old K1 went like a rocket with those pistons, JMR head, Megacycle 125/75 cam, 29mm CR carbs, and Dyna ignition, even with the piston slap.

[PeWe]

I have not measured the bores yet. They look rather tight. We talked about a clearance around 0.03mm (0.00118") when I left it in the shop.

Same guy bored my K6 to 836 and ported it late 1983.
He has refurbished the head after that. Installed 5mm stem valves 2016.

The RC pistons had rather good compression when I measured with the ported head with opened chambers which was the way to do it with the old RC pistons with wider crowns.
190 PSI with Action Fours SS-1 cam, chambers 24.4cc.

[PeWe]

These RC 836 pistons ran fine in my old K6 cylinder giving 83 whp on dyno 2015. Clearence closer to 0.2mm without smoking!

They had old style oil rings, 1-piece. I replaced them direct with 3-piece rings from my older but newer RC 836 forged pistons purchased 1983-84 that I blew after 8000 km.

[Terry in Australia]

Thats right, I forgot about the single piece rings, my mate SteveK0 told me that it was puffing smoke whenever I backed off the throttle, so yeah, 3 piece rings would be a bonus. Have you got CycleX 850 pistons in one of your engines? I've got a set here I haven't used yet, I might put them in my K1.

[PeWe]

Yes, my old 836 cylinder got 849 pistons fitted earlier this year.
I found a new set cheap on eBay, $250 US total with shipping and customs.

Not used yet. They look really cheap with grey frying pan surface. Look even older than the old RC836 cast pistons from the 70's, probably early 70's.

I had to fix all parts  for future use since the mechanic guy at my local shop 2km away will retire next summer. Other shops are located in other towns 1 hour away or more and charge much more for a bore. Not many with long experience of CB750 either.

I'll let him replace guides on my smoky K1 head too. I need to check the valves to see if I need to order new. I have been warned about cheap China valves with wrong stem thickness, ex valves.

[Terry in Australia]

Fair enough. I'm lucky, I do my own rebores, so I can only blame myself if it fcuks up. The only problem is it takes a long time to remove 4mm of cylinder bore, so I like to find an old set that's been bored to 810 or 820 and just take it out another mm to 836, or get an old 836 block and bore it to 849 for the CycleX "last chance" big bore kit.

I've got 2 sets of their pistons as a couple of years ago they sold me a kit that one of the staff literally threw into a box, and by the time it arrived it was damaged, so they sent me out 4 much better packed pistons when I b1tched about it here. I must pull the K1 engine apart soon, it'll be interesting to see what damage the broken cam chain made.    

[PeWe]

Chain must have broken by a reason like stuck cam due to no oil or somthing hard blocked the chain.

Thats what I have seen on the forum.

[Terry in Australia]

Yeah mate, I remember them dropping cam chains quite regularly "back in the day", it'll be interesting to take the cam cover off to see what's left.

[PeWe]

Terry, you can make bets about the reason in your build thread where people can guess why chain broke.
New project?

The winner will just win the pride guessing right.

[Terry in Australia]

Terry, you can make bets about the reason in your build thread where people can guess why chain broke.
New project?

The winner will just win the pride guessing right.

Ha ha, funny you mention a new project Per, I've just been melting my visa card all over Ebay looking for parts for the K1, I think another "patina build" might be the way to go? I'm not that good with shiny bikes......

[PeWe]

It will end up as a shiny bike anyway
CB750 parts are very expensive. Try to find good looking  tacho, speedo setup with upper bar holder to K0-K2 for a small amount of $$$. Impossible!
K3-K6 easier but might end up in NOS anyway.

I had to order a NOS K1 rear fender to my EU K6 while Honda fenders are still available a month ago.  Original fender has a little bit rust and a few small dents.
Now new from Vietnam for not much less and maybe not that good.

[Terry in Australia]

It's funny you say that mate, I just bought a K2 "Rhino Horn" top (and bottom) triple, speedo,tacho, idiot lights and wiring, brake hose splitter, brake hose, front caliper, speedo and tacho cables, all off the one bike in the US, for $350 USD. I tried to "make an offer" of $300, but he rejected it. I've got K1 gauges on my "Bitsa" K2, so I'll swap them over. I found a good K1 swing arm for $20 USD, a rear fender and tail light (rough) for $25 USD, and a few other bits and pieces, but I still need lots of parts for the K1, but no rush, it's got a couple of bikes in front of it.

[PeWe]

Good prices  for those parts.
I have found many nice parts in USA for very good price, its the shipping that make it impossible. Part for  around 50, shipping 50- 120 USD + 30% customs/ Vat on that

[Terry in Australia]

Good prices  for those parts.
I have found many nice parts in USA for very good price, its the shipping that make it impossible. Part for  around 50, shipping 50- 120 USD + 30% customs/ Vat on that

Yeah, that 30% tax is bullsh1t Per, ours is 10% and that hurts, especially on used parts from private sellers, because if I was buying some used parts from a guy who wasn't a professional business here, there's no tax. I've got a MyUS.com account, and their shipping prices are pretty good, and what they'll do is repack several purchases into one consignment, and get me a better deal (normally) on shipping. Pretty good for $7 USD per month.

[PeWe]

Our tax is "only" 25% + customs for these things around 4%. All on total price written on the package.

 plus custom fee that start at 10USD, large package as 4-4 pipes 40USD.
Most often around 12-15 USD.

So it hurts to order a lot of things.
CycleX and Dynoman use expensive shipping this way so that will be a stopper.

[PeWe]

I found pics from last season.
The metallic need sunshine to show the true color

[PeWe]

Prepare for next  step.  Cruzinimage 61.5mm 392 type pistons.
Rings gapped, cylinder painted.


Old 61.0mm pistons K0-K6 style at left vs 392 61.5mm pistons with a crown that gave higher compression

[PeWe]

Cylinder with new pistons got a test ride today.
It runs better and stronger with better power passing 6000 rpm.

My initial plan was to use a Kenny Harmon D cam. I could not get enough valve-piston clearance.
Lift at 0 lash: 0.350/0.340
Open /close  at 0.004 (1mm) lift
IN 27/52
EX 74/25
Comparing in open with ex closing show a slightly advanced timing by 1 degree.

K7-K8 cam went in. Stock sprocket as is.

Head had got Kibblewhite springs for the D cam, so let it rev some extra!

MLS head gasket's 8 oil return holes were smaller than head holes. 9.5 vs 10.5mm. Not much gap around HD cylinder studs for oil to flow.
I grinded the holes from both sides with a conical stone to same diameter.

MLS use really hard steel.
I had to restore the embossed sealing rings around the holes with Loctite 3020 gasket spray.
The heat when grinding had damaged the Viton surface.

[PeWe]

Checked the compression after 310km.
1- 180 PSI
2- 170
3- 170
4- 185
All plugs dry, no oil in bores. Good compression but could be more equal.

Old cylinder with CruzinImage stock 61mm K0-K6 type: ~145-150 PSI

My old cylinder might get a K7 61.5mm CruzinImage piston kit too.
I have an old K1 engine to be restored.