Topic: CB750 Motor rebuild... for dummies

[seebee750]

I am ready to go with the rebuild of my 1976 CB750 K6 engine..
I have the tools, the parts, the ability,the books,micro fiche cards and the time.
 All I need is an audience to cheer me on.,
 Over the next 3 months I will replace the crankshaft bearings, rod bearings, primary chains and primary tensioner.
Also the cam chain,sprocket, idler,tensioner and slider. All 8 rocker arms,valve giudes,points plate assembly.Piston Kit includes. 4 ART pistons, rings,wrist pins, clips.
The majority of the gaskets and parts are genuine Honda. $75 for the head gasket, $22 for the valve cover and $24 for the base gasket. Far superior quality gaskets to any I have seen.
 

[paulages]

use a magnet tool to grab the retainers while you have the spring compressed.

[eurban]

Truth be told, I have on occasion disassembled the valves, springs, cotters etc by using a socket and a hammer.  Pick a socket with an od a bit smaller that the spring retainer (Paulages I assume that what you mean by "retainers" is what Honda calls the "cotters" that fit into the groove in the valve stem?) and with an id and depth to allow the valve stem and cotters to fit easily inside.  Set the socket on the retainer and give it a quick smack.  Most of the time the hammer blow pushes the retainer down (compressing the valve spring) quickly enough that the valve stays put, the tension hold on the cotters is temporarily removed, and the cotters just fall out.  Probably wouldn't do this on nice performance parts  but I have yet to damage any parts using this method.  And no, the spring doesn't fly up and smack me in the face.  Wearing glasses would be wise tho.  Of course you still need the compressor to put things back together. . . . 

[SandwichEnthusiast]

Yeah I had my dad (who's got that graceful mechanics touch) have at a valve spring with a hammer, made me wince but he knows what he's doing more than I.

I'm probably performing the most delicate motorcycle teardown/rebuild known to man.

Does anybody have any wisdom about using a camshaft holder with non-matching caps? I might get screwed by this ebayer and end up with two different sets.

[City Boy]

Here's what I would do if you end up with no match cam bearing cap.Install cam in holders bolting only the no match cap.Hold down other cap locations without caps.Rotate cam.Turn nice?If yes,install other caps too.Still turn nice?If yes,plastigauge,refer to the book for spec,if in compliance I think you are good to go.

[Terry in Australia]

Yeah, good advice mate, if the top cap isn't binding, it shouldn't present a problem, as it's not "load bearing" like the bottom half. Cheers, Terry.

I am ready to go with the rebuild of my 1976 CB750 K6 engine..

All I need is an audience to cheer me on.,
 

Go Seebee! Woohoo! 

[seebee750]

Here are some books that are useful for an engine rebuild.
-Honda Tool catalogue,
-Honda Electrical systems,
-Honda CB750 Shop Manual,
-Honda Common Service Manual
-Honda Service bulletins for the CB750, (including the one for crankshaft bearing selection)


I bought a few tools from Honda that are listed in the tool book.

[seebee750]

Some of the useful measuring tools.
Digital vernier,
Plastigauge,
Metric feeler gauges.
0-25 mm micrometer  .02 mm graduations
25-50 mm micrometer  .001 mm graduations, for the crankshaft journal measurements.
Thread gauge
Torque wrench 0-250 inch pounds (0-20.8 foot pounds)
Micro fiche cards for part numbers.

[City Boy]

Those scuffed pistons remind me of the sheer toughness of these old 750's.I had occasion to "rebuild" a good friends '72 4 years ago.He complained to me of erratic shifting,lo and behold the shift drum was split in two so apart she came.Owner had only money for gaskets and seals,but also had 3 junk motors.I mixed and matched,measured everything,emoried the scuffed pistons,honed the cyl.with a cleanup hone,even picked best plain bearings regardless of colour.Reassembled and added Dyna green coils and electronic ign.Turned key and crossed fingers.Well,Engine starts after barely a revolution,burns no oil[15-40 Rotella]holds 60 lbs.at anything over idle,never misses a beat and has been a daily driver,[Apr.-Nov.]for these past 4 years.As someone who would normally change all the usual parts on a teardown,I was and am amazed at how this unit turned out.Tough old beasts and then some!                     

[seebee750]

Also some tools that are nice.
-Valve lapping stick with jar of lapping paste.
-Cylinder Hone,Made by Blue Point ( with extra stones.)
-Alternator Rotor Puller, Honda Part # 07933-3000000 It was $20.20
-Punch,valve guide, Honda Part # 07942-6570100
-Carburetor float level gauge
-Carburetor Vacuum gauge set

I need to get an oil pressure gauge to verify oil pressure on initial start  up after the rebuild.
What do other people use?
It will not be a permanantly installed gauge.

[754]

Tough .. no kidding.. I havent been in a Honda shop for years..

The last 8 months I rode it I bought  nothing at a shop for it, did look for a chain though .. which it needs bad.

I ran the absolute p!ss out of that 836cc that was built 25 years ago.. one burnout contest.. 28 seconds to blowout!!.. 3rd gear!!
countless days at the drags, running on the dirt track without aircleaners.. for a long time I didnt care if I broke it..used to warm it up just to do a few burnouts.. then finally this summer I took it easy on the old bike and it gave me one more memorable ride to Bonnevile.. then it finally gave up .. 4 hrs run time after I got back!!

I ran only pennzoil.. religiouslly.. will see how it looks when I teardown.. but I know that last 10 minutes probably hurt it..

[City Boy]

Get yourself a spare end cap from the oil gallery behind cyl.Drill and tap for pipe fitting to connect to gauge and line.Voila!

[seebee750]

I will do that.
thanks

[City Boy]

Further,all I use is a cheap auto mechanical gauge.I mounted mine to a stand I have that holds oil pressure gauge,vacuum gauges,lawn mower fuel tank,syncro screw driver,and vacuum fittings.

[Terry in Australia]

I run permanently mounted oil pressure guages on all my bikes, they look trick, but more importantly, they give me an indication of the oil pumps function all the time, not just occasionally. Rebuilds are too expensive to trust to a 50 cent light bulb. Cheers, Terry.

[Kevin D]

So...................... My top end is apart, I'm decarbonizing valves and pistons, scraping gaskets, everything is measuring up good. I'm hoping for a successful rebuild and wondering what are the the things to not do? You know, the rookie mistakes. If I can stay away from that kind of stuff, things usually work out pretty good.
 I also found slightly chewed camchain rollers and a worn guide.

[Terry in Australia]

Make sure you replace all of those cam chain components Kevin, and of course, the cam chain. You can buy them all components from Z1 enterprises, including heavy duty Tsubaki cam chains (the best available in the free world) for only 27 bucks, and they sell the OEM Honda tensioner components for a good price too. Have fun! Cheers, Terry.  

[legendary]

I worked the last few days, but the project continued forward. I had my cylinders honed $40.00

I had some custom pieces made to adapt a traditional engine stand to the CB750 (more about this below) $20.00

I also just returned from the hardware store with an oil drain pan, gallon of degreaser, nuts, washers, and threaded rod $21.58 

So my total cash outlay so far is $161.57 (US dollars) give or take a few.



Engine from the left



Engine from the right



Engine stand mounting bolts. Copied from Gordons design 1/4" thick, 3 " long, 1" wide. Welded to #8 bolts with 1/2 holes drilled near the end. I dropped a drawing and the 4 bolts off at a metal shop and 4 days later.... "Come and get 'em...20 bucks!" how they made any money on that I have no idea.


Cylinders were honed and thats all they need. They are protected with Grease now and one of my next moves will be removing the old paint from the cooling fins and refinishing them.

Also need to get the valves and pistons out and post some pics of them. Thanks again for the help!!

[Terry in Australia]

That looks good mate, nice welds, which is good, as there's a lot riding on them! Make sure that you clean all the grease out of your cylinders before you fit the pistons, so as not to glaze your bores.

Just a drop of oil on the piston skirts, or even better, a squirt of WD40 down the spark plug holes and no other lube at all. Cheers, Terry.  

[legendary]

All right!

Pistons are out. I have to admit some of these fairly easy tasks are more of a mental hurdle than anything.
So far everything I have touched has been in nice shape... I have given some thought to just doing the top end and re-assembling. But in the end I want to put the motor in the frame and enjoy it.

Also based upon the guys I see on this forum--- I would say the likelihood of this being the last old bike I work on is slim. Might as well learn as much as I can while I am in here.



Heres the block as it looks with the pistons out. Pistons are held in with ring clips which keep the piston pin from sliding horizontally out of the piston. You need only remove one of the clips, then push the piston pin toward the ring you just removed. I slid the pin far enough to clear the connecting rod and they're out. Then re-inserted the pins to keep them organized.

I have to do something to keep on of my sons from spinning the crank and letting the connecting rods drop beneath the upper case. Ideas???



This is the piston from Cylinder #4.  This is the worst looking of the 4. And it doesn't look bad. Measurements will tell the tale.  I previewed the pics and they are pretty poor...



Here is the same piston (#4). Wiped off with a paper towel. The 4 scratches you see on the skirt are truly gouges, not just discoloration. Enough to catch a fingernail in, maybe 1/4" long maximum. I'll measure these and if they are in spec I will re-use them.
Any words of wisdom.... I'd love to hear. Throw these out? Re-usable?

Other questions:
I know that allen style bolts are available for the exterior of the engine.For example the bolts/nuts that hold the cylinder head on.... the nuts that are beneath the black rubber "pucks" beneath the cam holders are rusty.

 Does anyone sell a replacement nut-bolt set for the internals?

Thanks for all the help!

[legendary]

Almost forgot question #2.

What do I do about removing the carbon without damaging the pistons?

[Kevin D]

I used a wire brush with my bench grinder on the tops of the pistons, stayed away from the sides. The manual talks about some sort of scraper. The pistons weren't too difficult to clean.

I got the small wire brush for a buck from Sears. It did a good job on the exhaust ports.

[Kevin D]

Here's my bad news - a rusted knock pin, it's coming out in bit and pieces. I need a little rotary cutter to carefully go in there. How the devil does water get in there? You can see that the corner stud is also rusted. It doesn't bode well for retorquing. 

[legendary]

Guys,
          I need a little timely tech assistance. Too cold today for the garage so I am ordering my parts.

Here is what "Terry from Australia" provided on page 1 of this thread- and what I have done

Full gasket set (CycleX, $59.00)- Ordered
Set of Piston Rings (Rusty Riders $30.00) Couldn't find them on RR ordered from Cycle X
Cylinder Hone (buy the tool to fit your cordless drill, around $10.00)- had it done for $40
Heavy Duty Tsubaki Cam Chain (Z1 Enterprises $28.00)- Ordered
OEM cam chain tensioner, wheel, and guide (Z1 Enterprises $61.00) Ordered
Engine seal set ($20.00 from Rusty Riders) Couldn't find them on RR ordered from Cycle X
New OEM Primary Chains (Terry Quail, $70.00 per set) Ordered from Terry I also got the primary chain tensioner
New NGK Spark Plugs ($10.00 from your nearest auto parts store)Not yet
New points ($10.00 from Rusty Riders) Still looking
Engine Enamel ($10.00 from your nearest auto parts store) Today

I am also considering this cam from Cycle X- # EN-617 about 2/3 of the way down the page here:

http://www.cyclexchange.net/Engine%20Parts%20Page.htm.

Does the benefit of a cam on an otherwise stock engine justify the $200? Or would the money be better spent on porting the head.  Do I need to replace the tappets if I replace the cam? I know on a V8 engine new cam means new lifters???

Anything else I should be getting??

Jury still out on all new cylinder studs?

Thanks All,   Steve

[Sweep]

You'll need paint stripper for the motor.  Do yourself a favor and check out dynoman.net.  I got most of my parts from them and there's a great section on cb750 parts.   Get the adjustable cam sprocket from dynoman and you might want new ignition wires.