Topic: 69, 69 and another 69 - CB750 projects

[Salman_z8]

Could be 71 at the latest, i have a whole pile, so i picked out the ones that were the easiest to polish. But not 72 for sure.


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[Salman_z8]

I see these ones have a step right below where the gaitors go, pardon my ignorance.


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[Terry in Australia]

Yep, Frank's right mate, they're K1 or K2 lowers, the K0 ones are more squared off under the gaiter mount, here are mine.

Terry's Redneck K0 rebuild 8 Oct 2020 5 by Terry Prendergast, on Flickr

Do you have some of these? There's usually a set or two on Feebay.

[Salman_z8]

Yep, Frank's right mate, they're K1 or K2 lowers, the K0 ones are more squared off under the gaiter mount, here are mine.

Terry's Redneck K0 rebuild 8 Oct 2020 5 by Terry Prendergast, on Flickr

Do you have some of these? There's usually a set or two on Feebay.

Thanks terry.
I will see what i have in the pile, i am assuming the guts are the same just the lowers are different, correct ?
May be some day i will update. Gota get all of these together and looking like bikes first


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[Terry in Australia]

Yeah mate, don't worry what anyone else thinks, it's your bike, so enjoy, but if you have a set of the K0 lowers, your K1 guts will all work within.

[Salman_z8]

Yeah mate, don't worry what anyone else thinks, it's your bike, so enjoy, but if you have a set of the K0 lowers, your K1 guts will all work within.

Yes sir


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[Prospect]

Heres some more, HM300 pipes as well.


I will probably blast the shields, and paint them to do the satin finish. But thats down the road.
I had these handy, so they went on.

Looking great so far! What do you use to paint the shields?

[Salman_z8]

Heres some more, HM300 pipes as well.


I will probably blast the shields, and paint them to do the satin finish. But thats down the road.
I had these handy, so they went on.

Looking great so far! What do you use to paint the shields?

In the past i have kept them chrome, and painted the black in. Then wiped the excess clean.
I use duplicolor satin black lacquer, as it is very easy to clean up. Has lasted well.

This time i am thinking to blast the shields at low pressure, and my understanding is that on k0 both the silver and black are painted, and satin.

I will give it a try soon


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[Salman_z8]

Since the candy gold is up and running, except for hooking up the front brake hose, i decided to clear out the floor a little.
Sold a cb550, that was in the way, also a 73 gt750 and a 73 gt380 that i had just rebuilt the motor for, all those sacrificed so i could get to the 69 cb750 hiding in the back.

Its a 13,xxx number diecast, with everything yamaha neck forward, XS parts it looks like.
Other than that, there is the wrinkle tank, red foam seat, rolled rear rim, correct early swing arm etc
O yea and a kawasaki tail light.

Engine was siezed, and i had it soaking for almost a year now. Well, no go.
I took the motor apart carefully, as i didnt want to replace parts unnecessarily.
Got it all apart, crank is in good shape, main bearings are real good, rods shoe no reason to be further disassembled, tranny looks ok.
Might replace the primary chains, just because, and may be the cam chain tensioner.
Bad news, case was damaged at some point and repaired, probably back in 1971. ( judging by the scribing )
But all in all, not bad, have had worse.

[Salman_z8]

Same things as the previous bikes will be done.
I have enough parts sitting to do a complete front end, with k1 fork lowers and k1 fender, but early front hub, rim, early brake rotor etc
K0 gauges that i restored a couple years back and some more important bits.
All oem stuff basically.

The lower cases i will probably have tig’d, and some material built up on the mating surface.

My question is about the barrels.
I was hoping to just re ring, but if there is an option i will entertain the idea of oversize pistons too.

What pistons do you folks recommend ?
Z1 enterprises, scrambler cycle, cruizin image also which rings, same as what comes in the kit ?

Secondly, anyone on this forum, in canada, offering a boring service ?
I am not comfortable paying what the shops in my area are charging per hole.

[Salman_z8]

Here are some photos, unfortunately i had already started taking off some pices, the home made 4-1 pipe with harley muffler, and the xs gauges and all.
Didnt look too bad imo

[MauiK3]

What part of the case is damaged?

[Stev-o]

What pistons do you folks recommend ?
Z1 enterprises, scrambler cycle, cruizin image also which rings, same as what comes in the kit ?

I used Wiseco pistons on my last rebuild.  Boring was $50/hole.

https://www.ebay.com/b/Wiseco-Motorcycle-Big-Bore-Top-End-Kits-for-Honda-CB750/180159/bn_21807526

[Salman_z8]

What part of the case is damaged?

Chain damage, top and bottom cases, and seems like it was repaired back in oct 71.
But it drginitely needs to be revisited. Will post some photos, didnt have any on my phone

[Salman_z8]

Upper case i cleaned, ground of some of the welding boogers, cleaned the mating surface, etc
I wont be doing any welding to the upper case.
The lower case, i dropped off to the welder today, as i suspect the welding was adequate structurally but had the possibility of weeping oil. Also asked him to build up the mating surface a little so i can file it flat and go from there.

Crank bearings wont be changed, as they hardly show any wear.

The head will need the seats touched up, or new seats cut, and will replace the valves with replacement japanese valves, so hopefully i will be able to get away without doing the guides. They didnt show much slop, atleast the one i checked.

I have decided to go ahead and order one set of std piston rings from vintage cb750, just to see how the ring gap is in the bores. I did run the hone more than i had wanted to already to clean up the top ring wear and the ridge, its still not fully cleaned up, but i wana check where the gap is at before i do more.
Not building a bike which will see regular use, so 170psi of compression isnt a requirement.
But no shade tree stuff either.

[Salman_z8]

Cylinder 1

[Salman_z8]

#2

[Salman_z8]

#3

[Salman_z8]

#4

[Salman_z8]

These are after about 2 mins of honing each, the second ring wear is almost gone, there is still some first ring wear.
I think i want to try a new std ring to see where the ring gap is, to get an idea if i should hone some more or not.
Under ideal condition all the marks should be removed, but with a 3 stone glaze breaker, i dont wana introduce more wear and out of round tbh.

I want to reuse pistons if re ringing is the course.
I wont post my measurements, as i am going off of a vernier rather than a micrometer.

I generally use rings as somewhat of a measuring instrument

[Terry in Australia]

Those bores all look pretty good to me mate, but as you've already said, measuring the ring gaps will give you a good idea, try measuring them at the top and down an inch, then down another inch on the thrust surfaces (front and back of the cylinders, the sides won't show as much wear) and of course slide a feeler gauge betwixt piston and cylinder in each bore.

I was building my K0 around the same time you were kicking off this thread and had similar issues, busted cases (chain damage) that I repaired using a torch to heat the bent pieces which I carefully tapped back into place and used JB Weld to fill. I used second hand CB77 pistons with stock size 61mm rings that I bought from a member here several years ago in bores similar to yours. Two years and over 3000 trouble free miles later, I still ride it most weekends in warm weather and it's probably my favourite bike. I'm looking forward to following your efforts!

Before:

K0 by Terry Prendergast, on Flickr

Two weeks ago:

K0 Thursday 20 October 2022 5 by Terry Prendergast, on Flickr   

[MauiK3]

Great transformation!

[Salman_z8]

Those bores all look pretty good to me mate, but as you've already said, measuring the ring gaps will give you a good idea, try measuring them at the top and down an inch, then down another inch on the thrust surfaces (front and back of the cylinders, the sides won't show as much wear) and of course slide a feeler gauge betwixt piston and cylinder in each bore.

I was building my K0 around the same time you were kicking off this thread and had similar issues, busted cases (chain damage) that I repaired using a torch to heat the bent pieces which I carefully tapped back into place and used JB Weld to fill. I used second hand CB77 pistons with stock size 61mm rings that I bought from a member here several years ago in bores similar to yours. Two years and over 3000 trouble free miles later, I still ride it most weekends in warm weather and it's probably my favourite bike. I'm looking forward to following your efforts!

Before:

K0 by Terry Prendergast, on Flickr

Two weeks ago:

K0 Thursday 20 October 2022 5 by Terry Prendergast, on Flickr

Hey terry, thanks man.
I did read through your build, you do good work.

Are the cb77 pistons straight drop in ?
Any clearance issues with the valves, with stock cam ?
How much of a compression bump do you get ?
I had 12 thou skimmed off of the last head i did to make it flat, and i coukd feel a difference kicking the bike over.
I was intrigued to see the cb77 pistons, they have quite a dome i think, right ?

[Terry in Australia]

Thanks mate, it was a great "Covid Lockdown" project for me, the only downside of the lockdown was that it was really hard to get stuff shipped in time so I used whatever I had lying around. I reckon I'd had those pistons sitting in a box in my garage for at least 15 years, I bought them from an old American drag racer who I'd bought some RC stuff from, he swore by them and said I'd notice the difference, and he's right. I think stock CB77 pistons are only 60mm so mine must be 4th oversize, but my sleeves were much like yours, pretty good, so I fitted some new stock CB750 rings that once again I had sitting in another box.

I was worried about clearance because the domes are so high, they certainly fill the combustion chambers, but as I said above, after more than  3000 trouble free miles I'm no longer concerned. The head is stock K0 as is the cam, I'm running 120 mains in the stock K0 carbs with a K&N filter in the stock airbox, a Kokusan electronic ignition, Dyna 3 Ohm coils and Bosch plug leads and caps from a car parts store. With the Delkevic 4 into 1, it rips and doesn't sound anything like my K2 with 836cc Wiseco big bore kit, mild street cam etc, but I reckon it's quicker.

As far as the compression ratio, it's only a guess, but I'm gonna say it's gotta be around 12:1 as the domes are higher than the Wiseco's. It really is great fun, I have a lot of other bikes to ride, but the K0 is my favourite "Fair Weather" bike.

K0 engine build 7 Nov 2020 3 by Terry Prendergast, on Flickr

[MauiK3]

When compression is that high is there and engine temperature issue?