How to tell the year of my engine?

[Jorgy]

Morning members, I have a 1971 CB 750.
The engine that came with the bike, has the ID numbers missing by the starter cover.
My question is, is there any other way to tell? It is a stocker internally but was hoping someone might know?
Cheers

 

[Qaxe]

Sorcery!
I've never seen an engine number on any location other than the one. There would no need for Honda to write it twice. All other parts are not unique to that specific engine case, so there's no way to find the number that way. Does your engine still have that rectangle where the numbers are meant to be? Could the numbers have just been covered with Bondo or painted over? If so, maybe you could get in there with a probe and find them. If they were ground off, then you're out of luck.

I don't know about other countries, but in Washington State our engine number is not recorded on our title either. The engine is "just another part" in terms of documentation.

<rant>
In fact, my odometer reading isn't even listen on my title or registration, because my bike is old and therefore exempt from "trivial things" like odometer readings. Yet somehow my paint color, which could change at the drop of a hat, was important enough to document. Ha!
</rant>

 

[brettp]

Sorcery!
I've never seen an engine number on any location other than the one. There would no need for Honda to write it twice. All other parts are not unique to that specific engine case, so there's no way to find the number that way. Does your engine still have that rectangle where the numbers are meant to be? Could the numbers have just been covered with Bondo or painted over? If so, maybe you could get in there with a probe and find them. If they were ground off, then you're out of luck.

I don't know about other countries, but in Washington State our engine number is not recorded on our title either. The engine is "just another part" in terms of documentation.

<rant>
In fact, my odometer reading isn't even listen on my title or registration, because my bike is old and therefore exempt from "trivial things" like odometer readings. Yet somehow my paint color, which could change at the drop of a hat, was important enough to document. Ha!
</rant>

Yup, that's Washington

 

[Qaxe]

Jorgy, I got to thinking, and there are indeed some ways you can tell the difference between some, if not all, K models. However, such detective work may require the engine to be open, and a lot of tedium comparing parts numbers online. In case you want to go that route, K1 parts are listed on this page: https://www.cmsnl.com/honda-cb750k1-four-1970-1971-usa_model14354/partslist/ I sometimes go down that road myself.

Strictly as an example of something external that I can see, on my engine, that is different from K7 or K8 is that my K6 front sprocket is held on with 2 bolts and a fastener, whereas K7 only uses 1 bolt to hold it on. Ya can't just slap a K7 sprocket on a K6 and bolt it up.

Our aluminum engine covers, at least from K1 to K8 can be swapped easily, so we can't rely on them to tell us the actual year of the internal parts.

I have read that there were changes made to the crankcase in K1 and again in K2. I'm sorry that I don't know what they are, but somebody here will.

 

[dirtdigger]

More than likely you have a replacement case. Dealers were supposed to transfer the number but didnt. Have also seen guys building race engines from bare new cases which wouldnt have a number. http://www.honda750expert.com/ this is a good source of information if you take the time to scroll through.

 

[Jorgy]

Jorgy, I got to thinking, and there are indeed some ways you can tell the difference between some, if not all, K models. However, such detective work may require the engine to be open, and a lot of tedium comparing parts numbers online. In case you want to go that route, K1 parts are listed on this page: https://www.cmsnl.com/honda-cb750k1-four-1970-1971-usa_model14354/partslist/ I sometimes go down that road myself.

Strictly as an example of something external that I can see, on my engine, that is different from K7 or K8 is that my K6 front sprocket is held on with 2 bolts and a fastener, whereas K7 only uses 1 bolt to hold it on. Ya can't just slap a K7 sprocket on a K6 and bolt it up.

Our aluminum engine covers, at least from K1 to K8 can be swapped easily, so we can't rely on them to tell us the actual year of the internal parts.

I have read that there were changes made to the crankcase in K1 and again in K2. I'm sorry that I don't know what they are, but somebody here will.

I have been told the advance mechanism is different between 1971-1978 one is larger and one is smaller in size.
But not knowing which one it should be.

 

[Qaxe]

Just as a thought, there are 2 books that might help. Books aren't cheap, and they don't provide answers fast.

First is a book that I personally had, by John Wyatt. "Original Honda Cb750: The Restorer's Guide to K & F Series 750 Sohc Models, 1968-1978" Long title. It has a lot of high quality photos, and there is discussion of changes between models. $50. I would look up info for you, but I no longer own the book.

The second book is one that I've only heard of, but that people rave about. "My CB750 book; On Putting Them Back on the Road " by Mark Paris. That book is $150.

This topic came up on this forum before, and there was talk of replacement crankcases without numbers. https://www.cb750.com/threads/no-vin-on-my-engine.4301/

 

[Qaxe]

Somehow I missed Dirtdigger's reply up there. The old thread I was referring to was one where he made the same comment about the engine.

...buying a new crankcase and moving everything over... That's a whole different tier of gearhead.

 

[dirtdigger]

I have been told the advance mechanism is different between 1971-1978 one is larger and one is smaller in size.
But not knowing which one it should be.

The advancers did change as the years went on. They changed total timing and the rate of advance. Its not a good way of telling the year as they are easily swapped out as any of the sohc motors can use any of the advancers.