Topic: Alan's CB750 K8

[Alan F.]

I had spotted this on Craigslist for $500 for a few months now, yesterday morning I noticed that it had dropped to $300 and I knew I would probably pick it up.
It's got a title so worst case it's a titled frame for 3 bills.

After getting some things done around the house I backed up and unloaded.











Wheels and exhaust are toast, carbs are completely disassembled, forks may be ok, wiring needs a closer look.
I dug out the shifter and was able to find all five gears and 2 randomly occurring false neutrals between third and fifth gear. I was able to shift up and down one time without false neutrals though so that's encouraging.

The base gasket has blue sealant, what kind of PO does that?

And while I was looking for the shifter I found 4 stock pistons, they look to be in ok shape and probably show wear consistent with the 24k miles on the odometer.


I'm not sure

[SOHC4 Cafe Racer Fan]

Woof!

[Alan F.]

I guess I was NOT SURE about something...

Idiocracy, if you haven't seen it, see it. It becomes more true every year.


So I've solved a few mysteries.
The seller stated that he can't turn the engine with the kicker.
Neither can I so off came the points cover hoping to be able to turn the engine. Nope, the stub bolt had already been broken off, I think I spotted a shiny new one in one of the boxes.
So off came the stator and shift covers and I put a 19mm box end on the rotor bolt. Now I can turn the engine, but only about 50 degrees before something contacts something else... well at least the rings aren't frozen.


So off comes the cam cover, everything looks to be here and assembled correctly. I fully backed off all of the valve adjustments and gave it a shot, now I can turn the engine 360 degrees! Whoooo! Since plugs are out I put a finger over each each hole and gave the wrench a turn, compression is very strong.
Unfortunately looking down the cam chain tunnel I see more blue silicone, the PO used it on the head gasket too.

I don't suppose i can leave it like this can I?
I admit I'm very curious to know what pistons are in there.
There were some markings on the cam, can anyone tell me if this is a stocker?



Is this a new camchain?

Looks like cases were never split, I wonder where the advancer is?

And I can see red assembly lube in a few low lying areas, this engine was never run I think.

[Alan F.]

So I'm guessing I need to get this engine apart to get that blue sealer out of there. I'd like to hear from some members with successful engine building experience first though.

It looks like the cylinder studs are stock, I'll probably want to swap them out for APE heavy duty studs like I did on my K3.

I've got a few questions though, since this engine has never been run, can I reuse the existing head gasket after cleaning the blue sealer off? Base gasket? What products will I want to use? Is there a preferred gasket set that won't break the bank?

[MauiK3]

Get a new gasket set, you can’t be sure what quality gaskets are in there. Head gaskets should not be re crushed.

[Alan F.]

I'll get it torn down tomorrow and figure out what pistons are in there and if the bore is stock or not.
I suppose if I find an 836 kit in there that I'll be needing a special head gasket?

[Davez134]

So I'm guessing I need to get this engine apart to get that blue sealer out of there. I'd like to hear from some members with successful engine building experience first though.

It looks like the cylinder studs are stock, I'll probably want to swap them out for APE heavy duty studs like I did on my K3.

I've got a few questions though, since this engine has never been run, can I reuse the existing head gasket after cleaning the blue sealer off? Base gasket? What products will I want to use? Is there a preferred gasket set that won't break the bank?

I'll try and answer some of these, hopefully most agree. Cam looks like a Factory Honda cam just based on those markings alone. Weather it is original to that motor?? no idea.
I would not reuse the head gasket after things have been torqued down, unless it is a copper one that can be annealed. (based on the blue sealant, I highly doubt it!)
 I have had very good experience with gaskets from CycleX. Others may chime in too with other good options. If the pistons are in fact larger than stock, you will need a corresponding size head gasket.
I have kept factory cylinder studs with stock displacement/compression builds with no issues. I have used APE or Kibblewhite studs otherwise.
Now, as far as the blue sealant...in my opinion, that is potentially one of your biggest issues. If it were my motor and I was spending my time doing anything to it, the cases would be split and all passages cleaned out. Who knows where that stuff has found its way into. It can kill a motor quick! Hopefully this is of some help. I've been away from here a while and happy to provide some assistance again to someone if possible!

[Alan F.]

I've got the top end off, the PO had used black and blue silicone on the base and head gaskets, it was amazing. But I was able to get quite a bit of it off without dropping any into the cases using just my fingernails and an old paint brush to sweep the debris away. I took good notes, found an o-ring was missing from the 3+4 cam tower. Lots of things were loose too. I bagged and marked everything but I'd never be able to tell if these parts were in their correct places as it was.

I found light porting on the intake side, none on the exhaust side. The pistons i found measure 2.401(61mm) so they're stock, but they have different tops than stock, maybe someone will be able to tell me what I've got.
They're marked E 392, while the other 4 I found in the parts bin are marked H 392 (these look more like the pistons in my K3)
The cam tensioner rollers are hard as rocks, I guess I'll have to replace them too, is the guide something I can reuse?



Timing must have been wrong, it looks like the "something contacting something" I mentioned earlier was certainly valves contacting pistons, good thing I was turning gently.





Ugly gaskets with pretty blue goop!




Intake porting





Cam chain tension wheel

[seanbarney41]

The "porting" in your intakes is from the factory...just a quick swipe to clean up the worst of casting irregularities.  Ironically enough, these swipes are just one of the things that needs to be cleaned up if you were really porting the head.  Dave is correct about the cam's r2 marking indicating it is oem and it does not tell you anything about what year 750 it is from.  It is possible that it is a re-grind though.  Look for stamped markings, not cast in.  Regrind stamps are usually found on the ends of the cam.  392 is part number code for 77/78k cb750's, iirc. 

[Alan F.]

Thanks SeanB!
I had hoped to find some hotrod parts in there but stock are good too.

I just pulled the trigger on some stuff from Randakks, a gasket set, A.P.E. Studs, fork seals, dust seals and a cam chain roller. $230 with free shipping, I could do a lot worse.

[Stev-o]

Agree with Dave and Sean above, subscribed.

[Jerry Rxman Griffin aka MuthaF'er]

Looks all stock to me. Correct style of 392 pistons for a 78 K from what I have seen. My 75 F also has 392 pistons but with a "full" dome.

Was the cam timed with the crank properly? From the looks of the valve strikes on the pistons I'd say NO.

Break it down and see what else you find. Probably more surprises. Clean up all the blue goop and only use proper non blue type goop sealer

[Alan F.]

Non blue type sealer?

I'm not wise in the ways so please clue me in.
I've done some searching but I find a lot of useless info that included no follow up.

I'd much rather follow in the successful footsteps of well known members.

It's all apart now, I'll remove the stock studs today, get the weldless frame kit (754) installed and start cleaning up the frame today. Studs/gaskets/cam chain roller should be here soon hopefully.

[MauiK3]

I used Hondabond
No leaks
It's a bit fast to flash over so when you assemble the cases do a dry run to make sure all is in order. I laid out my fasteners in holes in cardboard drawn with the shape so I could just grab them and put them in.

[MauiK3]

Also look at the primary tensioner, it's likely just as hard as the cam tensioners.
The cam tensioners should be put in as a set including the slipper.

[Alan F.]

Did you smear the whole gasket surface or just run a thin bead around the center of the perimeter and around each hole in the gasket?

[Alan F.]

Also look at the primary tensioner, it's likely just as hard as the cam tensioners.
The cam tensioners should be put in as a set including the slipper.

More shopping!

[Alan F.]

Got the 754 weldless frame kit that I bought a few years ago for my K3 installed on the K8 today.

[Alan F.]

Wire brushed the loose rust off then scuffed the whole frame with 400 grit sandpaper, tomorrow I'll scuff up the swingarm and engine mounts, then I'll haul it all over to storage where I can spray some paint outdoors sheltered from the wind without my neighbor's Dodge Challenger catching any overspray if the wind changes.

[Alan F.]

Frame is done, I'm sure touch up will be required but for now I'll leave it to dry. I used the Search function and found that I could spray Rustoleum Appliance Epoxy over a scuffed frame without a primer coat. So I put down some cardboard, set my frame on it and hosed it liberally with a big can of CRC brake cleaner and gave it a half hour to dry in the sun.
Then i hosed on the Appliance Epoxy, from what I read this paint is pretty thin and runs easily but somehow I didn't spot any runs. I used the whole can but will probably buy another sometime for touch up.

7pm Edit: I just realized that in an hour I'll have owned this bike for four days, I think I'm making pretty good time so far.
Tomorrow I guess I'll tear down the forks and paint the triples, maybe I'll polish the fork lowers too.

[Alan F.]

Search function tells me that my K8 forks take 5.5 oz. Or 165 cc of fork oil after assembling with clean, dry parts.

Looking around for triple tree painting ideas and info I found my own post from 2009, I was planning to do exactly this and had not forgotten the glare I used to get off the top triple on my 750 Nighthawk that I'd painted with gloss black. It was bad then but now after years of flash burns from poorly aimed LED headlights... ouch.

Personally I painted the top triple on one of my bikes in gloss black, never again, it's just too shiny while riding.

Flat black for the upper at least, maybe Appliance Epoxy for the lower, I need to pick up another can anyway.

[Alan F.]

Yesterday I did some research and online shopping but didn't order anything new. I'm thinking I should split the cases and get a new primary tensioner and new circlips for the transmission installed as long as I'm in there. I still need to research the rest of the seals I should replace while im in there.

Today I disassembled the forks and got everything cleaned up reasonably well. The springs are even within spec. One handy bit was the circlips over the seals had already been replaced with the modern style clips, the seals didn't put up much of a fight to come out either. Fork oil that I found in there seemed to be waste engine oil, nice.

Tracking my shipments from Randakks, 4into1 and Partzilla show that I should have a gasket set. For seals & dust seals, heavy duty cylinder studs and a cam chain tension wheel delivered tonight, the rest maybe next week.

[Alan F.]

Today I picked up a spare set of K8 forks from a hoarder on Craigslist for $30. He parted the bike out in 1994 and these were in heated storage since then, so they only had 16 years on the original bike, I'll use the best parts from both sets.

I dug up the swingarm dust seals, cleaned them up, and bolted up the swingarm. I added the shocks from my K3 which I believe are early DOHC CB750 units. Then I found the chain adjust stoppers in a bag of rusty hardware. I have as much rusty hardware as I could find soaking in regular 5% white vinegar (2 big flat plastic dishes with lids from the dollar store) I'll check on them tomorrow but I expect to leave them soaking for 3 days to remove the rust. Anything still rusty after that will get the wire wheel before soaking some more. Then everything gets an acetone rinse and goes right into the plating bath for a quick run or 2 of DIY zinc plating before going into the vibratory polisher with some fresh corn cob pet bedding for a few hours.

My gasket kit arrived from Randakk's, does anyone have any experience with D&K brand? I also received A.P.E. cylinder studs, fork and dust seals and a cam chain tension roller. I had no idea how soft these rollers should be, wow is this one soft. I've got orders from 4into1 and Partzilla on the way too. I finally found a primary chain tensioner at Bikebandit, I'll put in an order over there once I figure out some other needs so if anyone has spotted something I need please speak right up.

[HondaMan]

Chance are that if the PO did not install new valve guides, it will leak and burn oil. They only last about 15k-20k in the stock engine. Installing bronze ones will make them forever.
PM me your address: you will need 2 of the thicker O-rings (2.6x10.77mm) for the oil ports in the head-cylinder joint. If you don't use these, it WILL leak oil there, showing up in front-middle of the engine.

[Alan F.]

PM sent.