Topic: Cb750f2 engine rebuild

[CB750R]

Hi folks, I've been a member for a long time, I build my CB750 engine from rods up after what I bought blew up 2 days after purchase many years ago. So I recently  took the engine out to attempt to repair the damaged case (previously repaired by another owner) to resolve a persistant oil leak and in the process I ended up tearing down the engine, and inspecting the pistons.

I have scuffing on several of the intake sides of the pistons and now considering boring up and rebuilding this motor completely.

I have a 77F2 Chassis and carbs, I have a K8 head I believe, as its stamped 392, as well as 5X and K8. the cases have CB750E-2428786 which if I'm reading things correctly makes it a K6? My Cyliders I do believe have the dowels for oil sealing... I purchased the head and cylinders off ebay or here a looong time ago.
I've always had a bit of a challenge ( can't shift it without a  grinding gears feeling if I'm accelerating hard in 1st, so I'm always short shifting 1-2 so I can give it the beans in 2nd)
I'm comfortable with most thing but I'm in the dark as to how to properly diagnose and fix that.


Questions:

1. There's a 77 CB750 F2 in really rough shape for sale locally, I could use the cases if they look good to build up a fresh motor, any fitment issues with my above list of parts? Or with the exception of maybe getting undamaged cases is this a time waster?

2. with the above listed head/cylinders should I be using a high domed piston vs flat top? I've read through the 18 page Cruizing Image bargain 836cc thread, and it seems that there shouldn't be a problem with a k8 head and these pistons.

3. would I be further ahead doing a cam and a 1mm overbore than doing an 836cc overbore? I want fun reliable power, I have a garage of more modern and newer bikes this is my "classic forever project bike" but I'm a gear head and want to tinker within reason!

I built this bike on a bit of a shoe string budget close to 20 years ago, but I'm in a much better position these days and I want to do this up right, and would appreciate the feedback on what the best path forward looks like.
(I've considered getting a cycle-x shortblock as well, but it feels like the easy way out)

Cheers
GH

[newday777]

Welcome back aboard.

Yes the numbers show early K6 motor.

1. Go for the 77 motor rebuild. The F2 head has bigger valves(better breathing). The head is milled deeper for domed pistons. You will probably want to know if it has stock valve guides and if so replace them. New timing chain guide and rubber wheels.
836 is reliable street motor. A cam is a good thing too.
Might consider porting the head too for a bit more umph.

[CB750R]

Surprised with that reply, I thought the k8 head was considered the better option as the large valved f2/f3 motors had   Low compression/ leaking issues etc. kinda assumed the f2 heads were the read headed stepchild of the 750 line!

[Stev-o]

kinda assumed the f2 heads were the read headed stepchild of the 750 line!

Not really, it's just the valve guides that wore out at around 20K miles [Honda switched the material that was used]

I'd use another bottom end rather than trying to fix your shifting issue, a whole lot easier.

I once blew a hole in the case on my K4 [big bang bike] and bought a bottom end from a buddy for $100. Didnt even think for a second to try to repair that!

[CB750R]

Picked up an F2 parts bike tonight. Seller bought a lot of bikes from an auction was a motorcycle dealer that closed shop. Cases look good motor turns over, hopefully looks ok on the inside. Funny thing is I swear the sellers house was REALLY close to Franks place. I can’t recall his  address but it was really cool picking up a CB750 from so close to his place. (he sure would’ve been Beaten to the punch if he saw the add!)

[RAFster122s]

You do know Frank is gone, don't you?

[CB750R]

Yeah I do, which is why it was so dam odd turning into his neighbourhood to go buy a CB. Honestly on the way down I was wondering if I was buying one of his old parts bikes as he had quite the stash.

You do know Frank is gone, don't you?

[CB750R]

I know that Mike Reick is the man to go to for head work in the US, with the exchange and shipping costs, I'd like to know if there's any Canadians with a solid understanding of the CB head and what would need to be done to make it flow?

[CB750R]

Started tear down on the F2 engine on the weekend. Did a cursory clean of the outside and then started removal of the side covers and valve cover. Noticed the first example of some “modifications” internally. The cam cap studs have been replaced with the machine screws used on the cases for the right 2 cam caps. I was able to get one loose with my JIS impact driver but no luck with the second. I of course snapped my EZ out on my first attempt at extraction as well. Hopefully that’s the extent of the ham fisted “repairs” internally.

[CB750R]

Little more progress on the tear down, the f2 pistons are in much better shape than the ones that were in my running engine. This bike must’ve been stored well as no rings rusted to the cylinders and the carbon on the piston tops was all gooey so I’m guessing it was stored with oil in the holes.

[newday777]

Yes those pistons do look better. It probably had an easy life before you, other than the bungler putting in a screw where the bolt should have been. Good to hear it was put away wet with oil as we were taught to do in the 70s, Honda even mentioned in the owners manuals putting oil in the cylinders and in the gas tank(after emptying) for winter storage.

[newday777]

Be sure to replace the valve guide bushings.

[newday777]

http://forums.sohc4.net/index.php?topic=110858.0

[CB750R]

Cheers for the info, I’m talking with mr Rieck about getting the head done properly. I will post up the measurement and photos of everything to get some opinions on stock or time to bore out soon.

[CB750R]

Got a few bits blasted today on the F2 engine.

Sand blast cabinet either works or it doesn’t. It’s very frustrating. I have a nice new 60gal compressor so air isn’t the problem,  super finicky pickup of the media. But it’s still faster than sanding.

I give the parts a wash in the ultrasonic tank, then blow dry them before I put them in the blaster. I’m using glass bead.

[CB750R]

Pretty excited.

It’s going to be an F2 head built by MRieck, paired with a set of 836cc Dynoman F2 pistons. This should be a pretty peppy combination!

[RAFster122s]

Media needs to be very dry to have consistent draw in most syphon blasters.

[CB750R]

Media needs to be very dry to have consistent draw in most syphon blasters.

Thanks for the tip, new gun and different siphon tube is working much better. It’s also winter here so relative humidity stays pretty low

[newday777]

Pretty excited.

It’s going to be an F2 head built by MRieck, paired with a set of 836cc Dynoman F2 pistons. This should be a pretty peppy combination!

That should be a good ride indeed.

[CB750R]

Got the stripper and scraper and brush out for a party tonight. Top case got the first pass on paint removal.

Question about the cylinders? I see a lot of casting flash closing up holes intended for airflow, does filing these do anything besides make me feel like I’m doing something?

[Don R]

  I'd pretty it up a bit if it were mine but then it's been OK so far.

[CB750R]

Hi folks,

I moved my crank into the house as the garage sits at around 10c this time of year. I am pretty sure that’s a bit cold to get an accurate reading with the Mic, (I think I read that somewhere).

I tried removing this post on the ignition side of the crank using double nut method, I ended slightly deforming the threads, I used propane on the crank to heat it up, (see above I probably didn’t do enough) I looked up the part name and found other queries and see some have had a hard time and needed a stud extractor. I’m worried about transport to the machine shop and it getting bent or broken. 

Is measuring the crank journals irrelevant as it’s going to get polished up when I get it balanced?

[CB750R]

Pretty excited to see this starting to take shape! I had grand plans to build a big bore motor 14yrs ago only to realize I had caviar dreams on a Kraft dinner budget. I’m fortunate enough to be in a spot where I can build my CB out to be a fun unique machine and glad to get a head built as the foundation of making this motor performing!

[SOHC4 Cafe Racer Fan]

Are you doing all the head work yourself?  Porting?

[CB750R]

Are you doing all the head work yourself?  Porting?

I did clean up the casting lines and such in my old head but this time I’m getting things done correctly!

No I am getting this F2 head built by Mike Rieck. Pistons are going to be the 836cc F2 pistons from Dynoman.