Author Topic: CB750K6 Barn find Chopper - Engine Rebuild and Chassis repairs (Read 4904 times)

Trikeopath · « **on:** June 05, 2014, 04:06:29 AM »

I've just picked up this 1970's CB750K6 SOHC Hardtail about 2 weeks ago.

It's been with the previous owner since 1984 but other than what he has told me, the origins are unknown.

The motor seized a few weeks ago so I pulled the Head and Barrells off the other night and found No4 Rod has welded it's self to the Crank.

Also does anyone recognise the Frame? It looks straight and very well made. So far I think it's either an Uncle Bunt or Desperate Dan Challenger??

Trikeopath · « **Reply #1 on:** June 05, 2014, 04:09:23 AM »

2nd inspection of the engine has shown up a shattered clutch spring plate and some prizes in the sunp and oil pump.

Trikeopath · « **Reply #2 on:** June 05, 2014, 04:12:51 AM »

Cam and Top end are perfect. The 2 middle pistons are a bit scored but I can re-bore that myself so no drama.
When opened the rockerbox there was plenty of clean oil up there.

The Crank problem appears to have been human error. I found one of the Mains hammered in backwards so there is no reason why the same fitter hadn't done the same with the Big End, resulting in it spinning....

The Main had one half in the right way so that couldn't go anywhere as easily.

Rods have arrived now so I can get on with matching them to the Crank and start ordering the stuff to do the bottom end.

The shopping list for the Motor looks something like this:-
Crank & Oil Pump
Gasket set
Con Rods
Main Brg Shells
Big End Shells
Primary Chains
Cam Chain
Primary Tensioner
Guiness
Seals
O/S Piston kits
Vapour Blasting

Trikeopath · « **Reply #3 on:** June 05, 2014, 04:14:54 AM »

Before I go any further with the Engine I found a few questionable repairs and modifications to the frame.

The Footrests that were on the rear motor mounts had been well bodged about and the Battery box was hilarious.

Old Footrests

L/H Engine Mount (whats going on with the giant lump of weld??)

R/H Engine mount with bodged C90 pedal mechanism.

70CB750 · « **Reply #4 on:** June 05, 2014, 04:29:33 AM »

Welcome!

Nice project you got going here.

Trikeopath · « **Reply #5 on:** June 05, 2014, 04:35:30 AM »

Thanks man, its one of many. I normally have 3 or 4 builds on the go at any one time.

Just ordered a bunch of stuff from Holland to work out the bottom end.
In the meantime I've got some machining to get on with for the Footrests.

70CB750 · « **Reply #6 on:** June 05, 2014, 04:37:58 AM »

Well, glad you joined

Trikeopath · « **Reply #7 on:** June 09, 2014, 05:21:17 AM »

Rear Engine mounts now fixed.

Old half arsed Side Stand looked pretty #$%* so I adapted one off an Enfield.

I made this battery box but it doesn't fit (Oil tank makes it difficult to remove).

Trikeopath · « **Reply #8 on:** June 09, 2014, 05:44:53 AM »

Here is the Battery box finished.
As I said, it's no good for this bike but I have managed to donate it someone with a similar frame on another forum.

Trikeopath · « **Reply #9 on:** June 11, 2014, 09:13:44 AM »

Machined up a couple of footpegs out of stainless Hex bar. They will also be the nuts for the rear engine mount.

roadgoeson20 · « **Reply #10 on:** June 11, 2014, 12:38:40 PM »

Your machining skills outclass mine. Nice work.

Sent from my SAMSUNG-SGH-I337 using Tapatalk

Trikeopath · « **Reply #11 on:** June 12, 2014, 02:34:49 AM »

No skills here man.
I just bought a cheap Lathe because I was sick of waiting for other people to make silly parts for me and I'm having a go.

Trikeopath · « **Reply #12 on:** June 16, 2014, 11:13:32 AM »

Well, I'm on Battery box MK3 here, which has led to Oil tank MK2 and Mudguard Mk2. $:-\$

<a href="http://s278.photobucket.com/user/Trikeopath/media/CB750/DSCN09531_zpsf26f69fc.jpg.html" target="_blank"><img src="http://i278.photobucket.com/albums/kk107/Trikeopath/CB750/DSCN09531_zpsf26f69fc.jpg" border="0" alt=" photo DSCN09531_zpsf26f69fc.jpg"/>[/url]

Trikeopath · « **Reply #13 on:** June 17, 2014, 04:21:22 AM »

I take it there's not too many Chopper guys here then...

I started mounting the Rear brake master cylinder last night, only after welding up the first spacer boss, I realised I'd left the secend one in the lathe at work. Never mind, to be continued....

Trikeopath · « **Reply #14 on:** July 02, 2014, 09:03:47 AM »

Been messing round with a new rear guard.

Started off with this one that was left over from a Skirty I built the other week.

Chopped off the fat and tightened up the radius.

Added some 5mm round bar as a bead and to add strength.

Notched it out to clear the Battery and re-fitted the piece of frame I cut out when I moved the Battery.

blue collar · « **Reply #15 on:** July 02, 2014, 11:23:16 AM »

Love it. The foot pegs are boss.

Trikeopath · « **Reply #16 on:** August 20, 2014, 09:06:42 AM »

The original owner of this CB wanted his new bike modifying so, as the Front end off the CB had a bit of sentimental value, we did a deal last week for it to be fitted to his Dragstar.
The Forks are 160 longer than the Stock Yam set so I made a new Headstock and re-raked the frame with bit more stretch.

The Dragstar front end is now mine and I've put 6" over stanchions in to make it fit the CB frame.

Trikeopath · « **Reply #17 on:** August 20, 2014, 09:08:01 AM »

Just got the Crankcases back from CRC Restorations in St Helens.
Nice job and much easier than S.O.C, Petrol and a toothbrush.

Trikeopath · « **Reply #18 on:** August 28, 2014, 03:24:42 AM »

Got the new front end test fitted. The Stem needs a bit of alteration bit it'll work fine.
Also built up the new Crank and started replaceing bearings in the Gearbox.

CBJoe · « **Reply #19 on:** August 28, 2014, 02:57:11 PM »

Those cases look great! You said they were vapor blasted?

Cheers, joe

Tews19 · « **Reply #20 on:** August 28, 2014, 06:35:59 PM »

Interesting build! You should PM 754... He a chopper guy and knows all the old school tricks. Subscribed.

seanbarney41 · « **Reply #21 on:** August 28, 2014, 08:28:45 PM »

You might try putting something alluding to this being a chopper project in the thread title...that should draw the fans in.

andy750 · « **Reply #22 on:** August 29, 2014, 03:45:35 AM »

Quote from: seanbarney41 on August 28, 2014, 08:28:45 PM

You might try putting something alluding to this being a chopper project in the thread title...that should draw the fans in.

+1 plenty of chopper fans here including myself and enjoying your progress so far. Dont forget http://www.hondachopper.com/ either.

cheers
Andy

Trikeopath · « **Reply #23 on:** August 29, 2014, 03:59:10 AM »

Quote from: CBJoe on August 28, 2014, 02:57:11 PM

Those cases look great! You said they were vapor blasted?

Cheers, joe

Yes mate. It only cost £90 and they came out really well.

Quote from: Tews19 on August 28, 2014, 06:35:59 PM

Interesting build! You should PM 754... He a chopper guy and knows all the old school tricks. Subscribed.

Cheers man, I'm just getting back into reassembling the motor so I'll put more detail of that up as it happens.

Quote from: seanbarney41 on August 28, 2014, 08:28:45 PM

You might try putting something alluding to this being a chopper project in the thread title...that should draw the fans in.

Quote from: andy750 on August 29, 2014, 03:45:35 AM

Quote from: seanbarney41 on August 28, 2014, 08:28:45 PM
You might try putting something alluding to this being a chopper project in the thread title...that should draw the fans in.

+1 plenty of chopper fans here including myself and enjoying your progress so far. Dont forget http://www.hondachopper.com/ either.

cheers
Andy

Done guys. Thought this thread had died on its ass and maybe folks were more into restoration than alteration. This build is a bit tame compared my other projects, so might not attract as much debate.

BPellerine · « **Reply #24 on:** August 29, 2014, 05:42:20 PM »

you have great fab skills ,adapting the hd brake was a good move.bill

Trikeopath · « **Reply #25 on:** September 01, 2014, 04:38:13 AM »

Started putting the bottom end back together on Friday but hit a wall when I realised I'd missed 2 of the Gearbox bearings off the order. DOH!

Trikeopath · « **Reply #26 on:** September 03, 2014, 06:18:09 AM »

I've extended the steering stem by 10mm and welded a sleeve to the top bearing landing to make it fit the Honda bearing I/D.

Once I've re-laced the wheel, the front end should be sorted.

754 · « **Reply #27 on:** September 03, 2014, 08:14:23 AM »

I am late to find this thread, can't help on the maker of the frame..
That front end is clean looking...great score..very nice..
Nice fab work you are ding on it..

Trikeopath · « **Reply #28 on:** January 05, 2015, 04:49:09 PM »

Stuck waiting for parts I decided to spend this weekend doing stuff that wouldn't cost anything, so the seat base looked like it might chew up a bunch of time. I don't like doing them in Alloy or flimsy sheet like most are, so this one is all done in 2mm. Its a bit harder to work but it'll be tough as fcuk.

Front pan cut and formed.

Pillion section marked out for forming.

Shaped sides added and blended to fit exactly on the rear guard.

I threw a bit of black rattle can paint on it so you can see it against the guard.

Trikeopath · « **Reply #29 on:** January 05, 2015, 05:06:20 PM »

Been off this one for a while working on another (big) project so thought I'd drag it out for another look.

Over Christmas I got the Head and Barrells Vapour blasted to reveal a few new problems.

First up was the Cylinders for rebore. There was some water damage to 2 of the liners so I set the tool to the biggest tolerance I dared.
It wasn't enough, there was still a shadow on one of the bores. After me and bsadave doing lots of measuring and research on acceptable specs, it would have been on the very limit of a good motor so I bit the bullet and put them back in the machine and went to the next oversize.
Other than lost time, the real bummer was I'd already bought the 4 new 0.25 o/s piston kits :oops:. So if anyone wants a CB750 rebored with new slugs, give me a shout!!

Trikeopath · « **Reply #30 on:** January 05, 2015, 05:08:19 PM »

The Head wasn't such a sad story. Over all the condition was ok but one of the valves was slightly bent, which had in turn snapped a Valve guide. This would explain why it was burning a bit of oil before the Crank locked.

I'm still waiting on a new guide from Honda but the others have all be re-ground and fitted now.

Trikeopath · « **Reply #31 on:** January 05, 2015, 05:11:11 PM »

The #$%* up Fairy has been at it again. When I torqued up the Crank, the Mains nipped up. :evil:
I must have got a measurement wrong somewhere or messed up with the plastigauge.
I've just spread it all out on the living room and I'm going to go through it all again to find out whats up.

Tews19 · « **Reply #32 on:** January 08, 2015, 08:33:41 PM »

Progress, progress, progress!

News:

Author Topic: CB750K6 Barn find Chopper - Engine Rebuild and Chassis repairs (Read 4904 times)