Topic: 73' CB750 complete rebuild, DONE! just in time to be moving to KODIAK ALASKA!!!

[USCG_C130]

Hey guys first of all id like to say hi and I love this forum I've read all sorts of great stuff, I recently bought a 1973 CB750 and am planning to completely rebuild it so far I have the frame stripped and the motor out and taken apart all the way to the case not sure if im going to split it though and I have some questions about my piston and piston sleeves and also I have never done valves but am thinking I will have to replace those too. Well a little background on the bike I was looking for a classic/cafe bike to build up  (since the wife doesn't think I need a toy car so i sold he on the motorcycle) and I found the 73 on CL it was all there but hadn't ran in a few years it was clean had a bone dry tank and a decent amount of oil and kicked freely but would not start he wanted 1200 for it and I ended up taking it home with title for 750$ so here are my build pics so far and would love to hear all your guys wisdom and things you may see that I may be missing. Thanks ahead of time to everyone.

[USCG_C130]

ok guys heres the progress pics after my first few days i thought id show a little before i get to my technical questions but there's on picture in here that  u may wonder about, its the oil filter bolt that the previous owner apparently stripped off which putting ever inch of strength into so we ended up trying everything we could think up and had to just weld a new bolt to the old one and with that and the heat it came off fairly easy but thats the explanation of that bolt, (which killed me because i had to wait a few days for my buddy to come back to weld  and so that one bolt was the only reason i couldn't get the motor out of the frame) and the other ones are pretty basic. so far the engine is pretty clean although the previous owner loved RTV

[USCG_C130]

okay well now that i have the opening stuff done heres where my questions are this is my first rebuild and i don't know a ton about motors but i have lots of manuals and am following this thread  http://forums.sohc4.net/index.php?topic=29364.100  which is very help full but when we got to the piston and gaskets the piston sleeves looked okay except for one which looked like there was a burned/ charred spot at the top and it has some ripples that you can see and feel and are pretty high it almost feels and looks like little welds. and i was hoping someone might know the cause of this and what i should do about it if it can be honed out? (which im not counting on) or if i have to replace the cylinder sleeve?  and also is it normal for for the gaskets to be out of round as you can see in the pictures? i already have my new gasket kit here from cycleX so im set on those and will be replacing it all anyways but is it a sign of trouble or just normal wear? heres the pics let me know what all you guys think.

[bollingball]

Wellcome USCG You will be able to get plenty of help here. I'm in NC now but was born in north Tampa. Looking at your sig. are you at the Air Force Base?
Still going the cafe route?

                                               Ken

[USCG_C130]

hey ken im actually a coastguard c130 mechanic and aircrew guy so im at the st. pete airport we have our own little unit attached to the runway there with our 130's and jayhawks, where about in NC are you i want to A school or tech school in elizabeth city out that way, and im not sure  i think im going to go with a mix between cafe/classic look a new seat and keep the stock forks and boots and tank but yea it will have a bit of that naked cafe look in it, and with my job i do a bit of metal fabbing and have pretty much anything and any tool at my disposal. 

[USCG_C130]

does anyone have any input on the cylinder wall that has the  burnt or dissimilar spots on it i am not sure what the cause could be or what to do about it? does it need to be replaced?

[Dave K]

If you had a compression check before taking her down, it May of helped to determine if there was ring/cyl. problems or valve problems. Only an uneducated guess, but what marks I see will probably or possibly disappear in a few miles. Naturally the valves we don't know about. Since you are this far and I don't want to spend your money, I would possibly try to just clean the bores by hand and put it all back together. But............... Looks like you did good though on the purchase and with the all important title!

[USCG_C130]

thanks dave yea it did have compression but i didn't do a real compression check on the motor, and the valves seem to be in good enough shape, im going to lap them and get them all as perfect and of course do new rings i took the jugs to a motorcycle shop today but they tried telling me to get them honed by them for 60$ and then they would let me know if i needed new sleeves and that if that one wall was that messed up that i might as well sell the motor and buy a running one or spend 2k to get that one going, which is all BS i took the oil pan off and that motor show no wear and it looks really clean inside i am hoping new rings and lapping the valves will be good enough, also i lightly and carefully took a greenpad and wire wheel and most of that stuff came off its still a little rough there tho no like glass  as in the other sleeves..  heres what i found under the oil pan.

[HondaMan]

does anyone have any input on the cylinder wall that has the  burnt or dissimilar spots on it i am not sure what the cause could be or what to do about it? does it need to be replaced?

Not replaced: bored to an oversized piston (all 4 at once). You can get good pistons from Z1 Enterprises in the stock 0.5mm and 1.00mm oversizes, or from Honda if you have the $$. If you go the big-bore route, you'll have to start adding other expensive stuff, like heavy-duty cylinder studs.

The color of the bad bore looks like rust, from sitting with a valve open for a long time. This is real hard on the bores, and usually requires a rebore to some larger size is needed. If you stick with the stock overbore piston sizes, the head gasket and other parts you already have will work fine.

[mrtanner101]

With as much rust as you have in those cylinders honeing isn't going to get rid of all the pitting that the rust as made in the walls. If you hone it your rings prob. Won't seal causing blow by. I would replace all 4 sleaves. As for valves I would do a 3 angle grind on them and mate the seats and call them good.

[USCG_C130]

@ hondaman and Mrtanner  thanks for the input the pictures i think look a little worse than it really is i took it into a local shop to a guy who works on a fair bit of vintage motorcycles and he thinks he can clean it all up and hone it to within specs but i did take him the pistons too so he can measure and give me his input if they wont work ill probably just got with a slight overbore kit (1.00 mm over size from Z1). so well see in a day or two what he thinks about them and if im going to have to buy new pistons… what do you guys think about splitting the case it all looks good inside and very clean i will replace the cam chain guide and tensioner and anything else i can get to  and ill try lapping the valves and if needed ill get them ground and/or seats recut. again this is my first experience with opening a motor and  really sinking myself into a project so i appreciate the input and knowledge

[chewbacca5000]

I would take things one thing at a time, and not worry about spitting the cases now.  Judging by the look of the oil pump and screen everything looks really good.  The only reason IMO to split the cases is a known transmission issue ie. slipping out of gear.  If you can shift through the gears statically I'd call it good for now.  Can't really tell what kind of condition those bores are in from the pics. 

You might want to try a rigid 3 stone 220 grit hone with 3 in 1 oil as the lube and try to hone it out with a cordless drill.  Make sure you use a crosswise pattern back and forth so it looks like cross hatch pattern on the bores.  Avoid staying in one spot for any amount of time as this will cause rings / groves that can cause a ring to catch.  A hone can be got for $30 bucks at any auto parts store.  If it does not work your not out alot of money and can always have it bored to the next oversize.

[USCG_C130]

ALRIGHT!!! well i had my sleeves honed and pistons measured just to be sure and to make sure it was done right, a really great guy who had no problem with it.

@ chewbacca i think you right everything look pretty good in the case from what i see ill have to make sure the shifting it good too but ill probably leave the lower end alone  i don't see any signs of an oil leak any where.

so having the pistons and jug back all i have to do it order some new rings and the cam chain roller assembly because the upper roller is pretty chewed up is there anyway just to change the top roller? i haven't seen anything and assume you have to bye the whole assembly but the lower roller looks good and the guide seems almost new. them ill be ready to start re-assembly. any input for things to check  or do while im at this stage? and heres some pics of the hone job



the #1 sleeve has some stains from the  stuff that was in it but is as smooth as glass and no pitting at all so im happy and the re decked the top for me and clean my pistons up to almost new and as you'll see im well with in tolerance on all the holes

[VTCBike750]

Nice work. Keep it up.
Ill be doing the same thing for the first time in a few weeks. Cant Wait!

[Xnavylfr]

What was the cylinder bore size??? It looks to me the pistons have a little bit of a dome to it. This could possibly be bored out to 836cc. if so you can only bore 1 more time to 850cc without changing sleeves( very little meat left on sleeve walls after the bore!!!!)
Also HOW much meat did they mill off the head ?? You can't take very much off because these pistons/valves are very close tolerance!!
The stock bore is 61.1mm(2.406)
the standard warp allowed is max of .05mm(.002in) anything greater than .25mm(.009in) you must replace head or have it resurfaced.
Chedck out this site for more TECH info http://hondachopper.com/

Good luck with it We just did a topend job that had been bored to 836cc.


Xnavylfr(CHUCK)

[USCG_C130]

as far as i know it is 736 cc if you look on top of the pistons the guy who honed them wrote the measurements  and they appear to be all with in  the specs of being stock and  not bored at all and as far as the head i don't think he took much off just a few runs of a block to sharpen the edges up a little  but again i don't know 100% he has worked on  many older bike so i hope he knew what he was doing!! lol and the pistons are pretty flat on the top they have a little bit of convex curve but not much at all. also the pistons are marked as honda, and have 300 and 8 marked on the sides i donna if anyone can tell me if that has any bearing on what they are.


VTCBikes this has been a blast and its moving pretty quickly i was planning on a bit of a longer project for fun but now all i want to do is ride it!!

[USCG_C130]

UUUHGGGGG i hate waiting on parts guys i can start the prep/paint for the  case and head

[USCG_C130]

well got most of the frame done ground off almost everything i don't need waiting on a seat to see how ill be mounting it before i grind anymore off

[USCG_C130]

and i had a question there are some welds that you can see on the picture with out the tank they do not look stock and were kind of crappy weld i cleaned them up a little but why are those there? and are they stock? its on the top rails  about half way down the tank

[grcamna2]

Nice Quality work USCG_C130  !

[lucky]

There was no such thing as a CL750.  Only a CB750.
Good job so far and it is a good thing you got your gaskets from Cycle X. They are the good ones.

Do not put those 33 year old rubber cam chain rollers back into the engine.
If it was me I would put in a new cam chain too.
See Cycle X about that.
The frame and engine is the foundation to a good motorcycle.
If you do all that work and spend your money on things would you want some kind of engine problem a couple months after you start riding it?

BTW I also used to be a C-130 jet engine mechanic.
I basically tore down four engines per day( every day)with my 4 man crew for 12 months . Then I did inspections and repairs on engines for 6 months.

[lucky]

and i had a question there are some welds that you can see on the picture with out the tank they do not look stock and were kind of crappy weld i cleaned them up a little but why are those there? and are they stock? its on the top rails  about half way dow,...

Everyone asks about those factory welds.
They did not have TIG welding at that time. But they were sound welds.
It is just that now all the young guys have gotten used to perfect TIG welds even on a muffler.
They are plug welds in overlapping parts of sheet metal stampings.
They are factory welds. They have lasted 33 years.
Do NOT grind them down!! You can SAND spatter off,
but do not do anything to the weld or the height of the weld. Only sand NOT grind.
The frame is only .062 thickness. A grinding disk can take off half of that material in a split second!
The crown of the weld is a structural part of a weld
I know what I am talking about.
I am a certified welder and a certified welding inspector through the American Welding Society.

It is good to strip the paint off of the frame and inspect for cracks.
If you find a defect(crack), have a competent welder make a TIG weld repair.

[lucky]

ok guys heres the progress pics after my first few days i thought id show a little before i get to my technical questions but there's on picture in here that  u may wonder about, its the oil filter bolt that the previous owner apparently stripped off which putting ever inch of strength into so we ended up trying everything we could think up and had to just weld a new bolt to the old one and with that and the heat it came off fairly easy but thats the explanation of that bolt, (which killed me because i had to wait a few days for my buddy to come back to weld  and so that one bolt was the only reason i couldn't get the motor out of the frame) and the other ones are pretty basic. so far the engine is pretty clean although the previous owner loved RTV

About all these engine questions....

You have to measure everything properly to know what is going on.
You cannot just look or have people online look at internet photos and tell what is going on.

The reason the gasket may not be round is sometimes the gasket has to allow for the edge of the valve to clear the cylinder edge. Depends on the valves, which cylinder head and so forth. It would be unlikely you would have to replace a cylinder sleeve unless it was paper thin, and had a defect.

That cylinder head and piston may not be a 1973. You need to find out what cylinder head you have. Very important.

[USCG_C130]

hey lucky thanks for all that input sorry i haven't gotten back on here in awhile i been busy with coastguard school and progress has sort of stalled but i have been doing alot of sanding and wet sanding and polishing of all my aluminum which is looking good but takes forever, and as for the CL you read i meant craigslist that where i ended up finding it. i really appreciate the input. where did you work on 130's? military or civilian?

[lucky]

hey lucky thanks for all that input sorry i haven't gotten back on here in awhile i been busy with coastguard school and progress has sort of stalled but i have been doing alot of sanding and wet sanding and polishing of all my aluminum which is looking good but takes forever, and as for the CL you read i meant craigslist that where i ended up finding it. i really appreciate the input. where did you work on 130's? military or civilian?

I worked on C-130 engines while serving in the US Air Force.
I was also a mechanic in the civil service working on many types of plane engines, including Jets and recips too.

I also worked for the Civil Service for a year on all types of plane engines.

That was funny...CL =craigslist ooops!