Topic: Engine rebuilds.. vs buying an engine

[hardus77]

Hello everyone,

I've been contemplating buying an old honda CB750 and then rebuilding it. If I successfully rebuild it, I was considering then I could convert it into cafe.

My question for you all is, considering how simple (or complex?) the engine is, do you think someone who has experience wrenching on a car/bike doing basic maintenance but no experience doing engine rebuilding work could do this and have it be a decent VALUE? Another thing is, I know it would certainly be a great


I have a feeling with enough money for parts, persistence and due diligence, I probably could eventually do it myself.. but would it be a good value then?

I look on ebay, there seems to be motors that are used but working condition for ~400-800 dollars. Would appreciate it if anyone that has insight on this kind of stuff could chime in!

[ekpent]

  I have bought a lot of 750's over the years that did not require an engine rebuild,some have never even ever been cracked open yet for the first time. Hopefully you can find a nice project bike that you can easily make a runner without going through all of that.

[Bankerdanny]

Yes you could do it, no it wouldn't be "good value" especially if you think you will how the cafe route where the bike rarely ends up worth more than iut cost to build. If you want the experience I encourage you to did it, but like the man who owns half the CB750's in Central Michigan told you in the answer above mine, it is rarely necessary.

[Bodi]

Just repeating that a customized CB750 is almost always worth less than a stock one. There are exceptions but, basically, yours won't be one of those.

Engine work is kinda fun for me. However, if you have little experience, limited mechanical aptitude, and don't already have the required tools: unless you want the experience - bucket list maybe? - or plan on doing more of them... forget it. It's a money and time black hole and there are many ways to feck it up severely.
Although far from easy it is possible to find professional shops that will take on an ancient engine and do a competent job of restoring it. It ain't cheap, and doing it yourself won't be a lot cheaper if you value your time at all.

Used engines are a crapshoot. In fact, buying ANY 40+ year old motorcycle (or used part for) is a crapshoot. You'll find honest sellers with encyclopedic knowledge of everything they sell (not often), guys who don't know a screwdriver from a tractor and call any motorcycle part they list "SANDCAST!", and everywhere in between. Caveat Emptor.

[MauiK3]

These engines are pretty straightforward. When you get into one you may find that a lot of what is needed is a fresh set of seals and o rings. If done well, the result is gratifying. The fun is in the doing.
Value is in the eye (pocket) of the owner. I would not recommend going into these bikes for profit unless you find a very low number bike that is mostly all there. A CB750 with an engine number 7414 or lower is almost priceless.

[DickL]

I think you could buy a good running 74 CB750 engine from me for $$500. Dont think of buying a 1970 basketcase for much less than 300. Cheapest route is a,find a running bike for about 1500 to 2000 and run it.

[setdog]

Is rebuilding a ~50 year old motorcycle engine a good value?

NO. Not in my opinion anyway.

Absolutely not a good value in dollar terms.  It's a terrible value.  You lose money. I for one have never heard of
Anyone making a substantial income "rebuilding" vintage motorcycles. (Apart from the super skilled guys doing the extremely high quality  restorations) Parts resellers I consider a different business than "rebuilding". My cousin resells pre war imports from Russia and makes a few bucks, but he does not rebuild.

Is the effort "worth" it?  To me it is worth it.  It's a great hobby.  The only time Ive gotten a decent return or value is selling some parts I won't be using. 
In summary, I dont think there is much to be gained in monetary value.  For the me the value has been non monetary because I both admire these bikes, i enjoy the hobby and Ive learned a ton and gained some great experience.

Perhaps other people have a different experience.

[Redline it]

if it's cheap, buy it for parts and buy another one that works. i got good experience rebuilding motors, and 6 months ago i took the top end off of a 400f, and that's when a bunch of other things in life started bombarding my time, so the 400f (my primary means of transportation, my second means is a 33cc scooter that i found on the side of the road on trash day,) doesn't get put back together still. getting the base gasket off so far was the most time consuming. it's ready to put back together, so now i'm thinking it might be reasonable to take the pistons off to get "all" the carbon off of the rings, and grooves etc. I've never reassembled a motor using new piston circlips, but i'm doubting myself now, it's not like i can go to Lowe's or home depot to get some. where could i even think of going to get circlips that doesn't use modern day china metal trash. it's quite a challenge to make things work these days. everything is different. i'd watch someone, pay them to rebuild a cb motor, before i'd attempt it in modern times, when they were a dime a dozen, you had some room for errors.

[przjohn]

It depends on what you want to do and how much swag you got. No offense, but you're not the first single post guy to come on here wanting to do "A Cafe Build" Many of these bikes turn out to be Sawzall Fodder and not much more. My advice would be the same as others, find a good runner and don't worry about the motor. The other thing I would add is to ride the bike and get use to it's idiosyncrasies and work on it a bit, and oh, have some fun riding it as is.
 

[PeWe]

You need good torque wrenches to tighten the M6 screws with. Very easy to strip the alu threads. An old bike has bad threads you need to fix. Time-sert thread insert kit is a must.

Honda recommend too high torque on some bolts. All engine covers (M6) max 10Nm if thread lenght is min 10mm. I have started to tighten less, 7.5 Nm and tighten again a day after.
Cam holder threads in head very sensitive.
I have 2 wrenches, a small 1/4" socket 2.5-25 Nm for all M5-M8, 1/2" socket 10-60Nm for head/case nuts and clutch. Be aware of mixing up the M6 screws for engine, check thread lenght so you do not use too long screw that will hit bottom and crack it getting a alu bit inside that might crack a gear or worse. All gaskets work fine to apply dry without additional sealer. Only rubber pucks under cam holders need sealer under them.

Restore close to stock look will sell better. Rebuild as long as you replace parts is always reversible when having the original.

Check build threads and see how these engines are done.

[seanbarney41]

If you want to rebuild your first engine, get a small block chevy.  Cheap, easy, and abundant.  These are not really like that at all.

[dave500]

If you want to rebuild your first engine, get a small block chevy.  Cheap, easy, and abundant.  These are not really like that at all.

yeah and youll need to expand your tool set first if ya wanna tinker with old jap bikes!never torque any 6mm bolts forget all that torque wrench #$%*!

[bryanj]

You need a torque wrench for con rods, head bolts, big crankcase bolts etc just not the 6mm threaded ones.

[Dolomite]

I'll just throw in my 2 cents having just gone through this. The basics is bought a barn bike for $300, engine turned, rewired and cleaned everything, finally got it to run, was working on synch and timing when clank, clank , BOOM. Number 4 connecting rod bolts dropped out (in an engine that had NEVER been split open, so no one has any idea what really caused this on a bike with a 'bullet proof' lower, but that's just the special kind of luck I have), seized the crank (to the point where even after the engine was out it couldn't be removed without a very large mallet), shattered the sleeve and bent the con rod. Opted to rebuild than buy a runner, so $50 engine (K6, local shop had shelves of them in as is condition), $1000 in parts (cx1 cam, 836 kit because the hone was no good, cyclex cam tensioner system, super gasket kit, offset sprockets, etc), and $1600 in shop labor (thats WITH my mechanic charging $55/hr and discounting me, he had over 40 working hours into the thing). For those of you keeping score at home. Thats $2950 so far, not including everything else I purchased and did (full re-wiring, dyna s system, 5ohm coils, every piece of wiring and electrical component is new except for the negative ground and the positive to the starter).

TLDR: Buy a bike that already runs.

If you have the time, space, tools and ability to rebuild it, then MAYBE. But we ran into problems at every step; ie can't reuse pistons, new bore is no good, can't use this chain it's too short, etc etc etc.

I know I will NEVER get back out what I put into this bike so far so my solution is to keep it until one of us dies.

[PeWe]

WOW. really no torque wrench for M6?? You miss something very important. Not all can have the correct feeling tighten the M6 correct without stripping them.
Cam holders very important to tighten correct. Covers can always be tighten afterwards if leaking.

[DickL]

Buy a bike that runs

[DaveBarbier]

Having done two complete engine tear downs and rebuilds (one stock one not) I say buy a project and have at it. Few things more satisfying and frustrating than bringing an old bike back to life. You’ll learn a lot and loose money.

[Mark1976]

"I have a feeling with enough money for parts, persistence and due diligence, I probably could eventually do it myself.. but would it be a good value then?"

Lets answer a question with a question, define "good value."

In terms of learning and improving your tool and knowledge inventory, without a doubt!
But as a physical return on investment, yeeeahhhh, not so much...
Yet, if your inclined to do so, you ll have an experience with a result that you'll never forget. Whats the worst that could happen, you end up with a bike that cost 5 times your original investment that you'll never wanna sell....

[Scott S]

 While the original post and poster ended up having some spam links inserted,  there is some decent information in this thread.

 He's been removed, as well as the links, but the thread can stay.

[PeWe]

Might be better to find a restored bike or a good survivor very well taken cared of as dhall57's K4
http://forums.sohc4.net/index.php?action=dlattach;topic=45183.0;attach=404960;image

Restore an engine can cost $2000US. 
Only fix the head with probably blown ex guides and valves, cut all seats. That cost some.
Change guides, cut seats and bore cylinder is a job for a professional shop that has worked with bikes before. Best if they had done CB750 before too.
Rockers might have flaking chrome and cam junk. OK on eBay for $150

All chains and tensioners inside, maybe new crank bearings. The primary hubs cush rubber stuff inside  is hard. There are kits and shops that can do the job. Maybe the needle bearings inside too which are cheap and shops can fix.

New pistons can be cheap if order the cruzinimage OS pistons and just bore to next  over size as +0.5mm cruzinimage has.
Complete gasket set + all o-rings $100US (The complete kit has not all o-rings) Maybe CycleX has. Their can cost more.

New clutch fibers. Order from Cruzinimage when order pistons, their clutch is OK.
https://www.cruzinimage.net/category/item/itemgenre/motorcycle/honda/

Paint for cases, sealer for cases like Loctite 518.
+ Sealer under the rubber pucks under the cam holders. Fix bad threads, time-serts best but cost some....
Maybe need tools too? Torque wrenches that work correctly. I have 2.5-25Nm (1/4" socket), 10-60Nm ( 1/2" socket)

[Dunk]

It's not rocket surgery. Simple to do with a Manual and Hondaman's book is great too. Better to completely go through a bike, engine and everything else, and have a brand enw bike than be frustrated with a 50 year old neglected barn find that always has some minor issue. If you're going to cafe rather than restore, please get a '73-'78, or preferably a '77-'78. Please don't hack up a rare and desirable '69-'72. Leave them to those of us who do a proper restoration.

[BenelliSEI]

So MUCH  excellent advice.......... I wonder if the guy even read it??

[PeWe]

Good info for others with same thoughts
CB750 has many bolt off - bolt on parts without raping the bike and save for future back to stock.