Topic: CB750 Motor rebuild... for dummies

[Honda?]

Terry in Australia: Thank you very much! 

SandwichEnthusiast: Thats a terrible shame what happened to your engine. Lucky for me I wont have to venture into the bowels of my lower case. I'm pretty sure it is going to be not so much a footrace but rather a standoff between the rest of you guys to let the guinea pig go into the lower case first.

The reason my oil jets clogged and made my camshaft grind into the cam tower like yours did was because a previous owner used liberal amounts of gooey gasket sealer on the HG. Some of the excess gasket sealer jammed up one of my oil jets which oil starved the camshaft.
Pull and check these two oil jets for dead mice, tubes of gasket sealer, bodies, and other junk that might clog them. Then make sure it NEVER happens again. 



Sorry I cant help you in any other way.

-Tyson

[Terry in Australia]

Now is a good time to reinforce what most of the "old timers" here already know, don't use any silicon sealants in CB750's, or the excess sealant on the inside of your engine will drop into your oil, turn into little blobs and block your oil jets, killing your engine dead.

It happened to me 27 years ago, and cost me mega-bucks! Threebond 1194 (which replaced the old 1104) is the only thing I'll use now. "Be alert, but not alarmed"!

[andy750]

Tyson (Honda?) since you are going the 836cc route you should also think about getting some heavy duty studs  - this should prevent any chance of leaky head gasket once its back up and running. They are about $80-90 from APE or Dynoman - cheap investment in my opinion. This is the minimum I would do...if you can afford a little more, think about getting a mild port job since you have the head off.

Dont forget to replace the rubber pucks under the camshaft towers!!!


Good luck,
Andy



 

[mustangcar]

great job guys,love the pics thanks!,

[Honda?]

Andy750: Haha the whole reason I started to take my engine apart was because one of those rubber pucks started leaking oil all over my engine. There is no way in hell I'm going to forget those little trouble starters!

Also if I were to stick with the stock studs how common do you think a HG leak is on an 836 kit? I'm running short on money and I know I wont be able to get a port job done. So if the leak isn't that common I think I will take my chances with the stock studs.

[legendary]

Hey Honda? ?

Read this: "I've decided to go for the 836cc kit for my bike since it costs about the same(in my area) for an 836 as it does for a first overbore."

I was thinking the same today... If you are going to overbore and replace the pistons seems like you might as well call CYCLE X and go 836.

Anyone with more experience see any flaws in this logic?

[.RJ]

Also if I were to stick with the stock studs how common do you think a HG leak is on an 836 kit? I'm running short on money and I know I wont be able to get a port job done. So if the leak isn't that common I think I will take my chances with the stock studs.

New studs are probably cheaper than another gasket kit if it leaks...

[andy750]

Honda?   - from my leaky experience.....likely!!! Follow .RJs logic......

cheap insurance,

cheers
Andy

[Honda?]

*in a shakey voice* Well damn money is getting tight but here I go. *insert nervous(really nervous almost crazed) laugh here*

[Terry in Australia]

Yeah, as painful as it is mate, get the H/D studs, that'll save you having to pull the engine again when you discover how much the standard studs suck! Cheers, Terry.

[andy750]

Yeah, as painful as it is mate, get the H/D studs, that'll save you having to pull the engine again when you discover how much the standard studs suck! Cheers, Terry.

.....as I found out....now pulling my engine.....

[SandwichEnthusiast]

Thanks Honda?,

I was looking at the left jet (where the camshaft snapped under the holder) and its still holding oil, as in not draining out the oil like the right one did so hopefully thats what did it.

Also does anyone have any insight about using camshaft holders from a different year model? The rocker arm shaft off the old one (1975) uses 3 bolts per shaft, and the one I'll be switching out to only has 1.

also has anyone ordered bolt sets from this dude:
http://www.metrocast.net/~spillane/screwsite_005.htm

Thanks everybody

[andy750]

also has anyone ordered bolt sets from this dude:
http://www.metrocast.net/~spillane/screwsite_005.htm

Thanks everybody

Yeah hes  solid guy - lives in NH and has been a list member for a long time (SOHC mailing list days - pre-99) - owns a CB750K3. I and several others on the Forum have bought off him.

Cheers
Andy

[legendary]

This may get a little long winded. But I learned some valuable lessons in the last few days.

I need to start making decisions about my cylinders. If my experience is anything like finding a guy to rebuild the motor I am glad I am doing it myself.

Here is the dilemma. If my cylinders are in good shape, I need a hone and a set of rings (around $70.00). If they aren't, I need to overbore, replace the pistons, pins, rings etc (Ballpark $600.00) Once you cross that threshold, I think I would be better off going 836. If I go 836 I can  mail my cylinder "core" to a place like Cycle Exchange have a bored head ready to bolt on in a week or two.

This is where the question gets tricky. I need to pick a shop out of the phone book and by gut instinct decide who sounds like the kind of guy who will be honest.

I called a machine shop referred to me by a co-worker.  "This time of year we are too backed up to even look at it."
Called the machine shop my local car mechanic uses. " I don't bore anything smaller than 2.9"
He referred me to a bike shop about 15 miles drive south of me.

I am about to get to a lesson here and the lesson is this. The guy who makes decisions and runs the machine shop, is too valuable a guy to be answering phones!

I called the local bike shop and told them what I needed. "Aw a bike that old you can't even get parts for."
I laughed at that.
Then he said, " We close for lunch noon to 1pm. You can bring it down if ya want."
I drove down all the while thinking I was wasting my time and setting myself up for (being mistreated).
I walked in and met the counterman. "Bike that old you might just as well bore it."
Then the machinist came out, " Hey, a CB750!"
He ran a finger over the ridge at the top of the cylinder, looked the bores over for a minute, and said, "These are in nice shape. How many miles?"
I answered 25K.
He said, "If it were mine I would hone it and replace the rings."

Bottom line $40 (in writing) for having it honed by someone who has the tools and knows how. I'll let you know how that turns out. 

[Honda?]

Oh you lucky fool! I hate you so much!  Good thing I have 500 dollars just laying around and 400 on it's way in at the end of the month. I just hope my BMW doesn't break down. Haha it's like my car can sense when I have extra money so it can break down and force me to spend that money on it.

[Terry in Australia]

Oh you lucky fool! I hate you so much!  Good thing I have 500 dollars just laying around and 400 on it's way in at the end of the month. I just hope my BMW doesn't break down. Haha it's like my car can sense when I have extra money so it can break down and force me to spend that money on it.

Well if it's a BMW bike, don't worry about it, it won't break down, but if it's a BMW car, you're in trouble! Do what I did, buy a BMW bike and a Toyota car, that way you can waste all your spare money on your Honda resto!

Z1 enterprises have Tsubaki H/D cam chains for under 30 bucks, this is an absolute bargain, make sure you get one  for your rebuild!
 

Also does anyone have any insight about using camshaft holders from a different year model? The rocker arm shaft off the old one (1975) uses 3 bolts per shaft, and the one I'll be switching out to only has 1.

Thanks everybody

You can use any of the camshaft holders from any model CB750 mate, and according to Hondaman (who really knows his stuff) the earlier single bolt holders will actually give you an extra BHP or two via less drag on your valve train. Make sure you get all the goop out of your oilways before you reassemble it! Cheers, Terry.

[SandwichEnthusiast]

I was popping out all the gaskets and oil guides and such out of the cylinder head. I'm looking at this tiny oil guide that looks just like his brother, except I can't see through him and I'm saying: your the reason I'm tearing this bike to pieces and dealing with the nuttiest ebayin' dude in the world for camshaft and holders that YOU broke.

[Honda?]

TerryIt's a BMW car(96; 328i-5speed). But I love it to death. In my eyes a BMW car is as close as you can get to a motorcycle funwise w/o actually getting a motorcycle. I cant wait till I can get a Beemer! 2005 R1100S is my wish.

[Terry in Australia]

No worries mate, but old BMW cars will break your heart, and eat your wallet.

[riverman]

add another newbie ripping apart a cb750 to the list.

 I picked up a 78 cb750k tonight which I wheel up my stairs tomorrow (not looking forward to that). I've never owned a streetbike or ripped apart an engine before so I will be sure to fully document all the mistakes I am bound to make

[SandwichEnthusiast]

Thanks for the info terry, andy and everybody else so far.

I pulled the cylinder up off the pistons, (required more force than I would have thought to get it loose) also I think it's kind of vague in the manuals I have when to take the cam chain tensioner and cam chain guide off. It's after the cylinder comes off, the tensioner comes off from the top of the crankcase and the chain guide comes off from the bottom of the cylinder.

Also I removed the Valves from the cylinder head, using a different style tool than in the guides:
http://autorepair.about.com/library/images/bl254a-lib.htm
see K-D Tools P/N KD 3271 in the middle there. It was a bit of a hassle, putting the spring down and then getting those half moon clips off the guides (any worldly wisdom on that?)

Two of the valves required a little force to get out of their guides (again the ones closest to my oil starvation problem) but they seem pretty true, straight and tight in the guides.

so here's some pictures of them (the roughest one is on the right):


Here's and average valve seat:


Here's the worst one in the head:


and here's some of the cylinders and a piston (they are all looking about the same)



In the next installment: pistons come off, and into the crankcase!

Any suggestions about the condition of those valves and pistons/bores would be appreciated, also any information about cleaning up the inside of a valve guide (I would like to avoid getting into replacing valves and seals and such if I can avoid it)

[Patrick]

For future reference, Autozone has a C-clamp type valve spring compressor in their loaner tool inventory that works perfectly on CB750 valves. I like it cuz it's free.

Patrick

[bert96]

Removing the cylinder studs?  Penetrating oil and a few heat cycles with a carefully applied torch to start.  Now get a helper.  Double nut the threaded portion of the stud.  This means that you thread one nut on, a second nut on, and then hold the lower nut with a wrench while you tighten the top nut down onto the lower nut. This "locks" the nuts to the stud. Clamp a GOOD PAIR  of vise grips to the same stud about an inch off of the case.  While one person turns the stud at the top with a socket on the nut, second person turns the stud at the bottom with the vise grips.  Avoid bending the stud while you turn.  This method has worked well for me on 2-3 engines.  IMHO if you want a leak proof engine, replacing the stock studs with heavy duty ones is a good move.  Enjoy.

Do i torqued them when the installation or just tight them with a ratchet?
maybe a little bit of lock tight too?

[eurban]

I've never used loctite; once under torque they shouldn't want to back out.  Make sure the thread holes are completely clean, particularly if the cases were glass beaded etc.  Lube up the stud threads with engine oil and twist it in.  Finish the tightening with ratchet, no need to go crazy.  Apply about as much torque as you will to the nuts when you torque the head down . . .

[bert96]

Thanks.