Topic: engine number Q?

[Don R]

  cb750e-2509317  I think it's a 76K but not positive, there is no F even in my F's engine number, so I'm not positive. On the rear of the head near the vent is a Honda logo and an X or inverted Y maybe and a 3.
  How do you tell a 341 series head from a 392 head? 

[david 750f]

Don, sounds like a 1975 F engine, the engine serial number range is CB750E-2500004 through to CB750E-2515085 (from the Honda parts book). It should be stamped 392 but my original 1976 CB750F head is stamped X7, so there may be other markings out there.

[gifv]/gifv]

[jaytee-nz]

I fitted a 392 head to a "bitsa" engine a while ago and it had "392" cast in to it. From memory the marking was on top and in the center of the head.

[newday777]

  cb750e-2509317  I think it's a 76K but not positive, there is no F even in my F's engine number, so I'm not positive. On the rear of the head near the vent is a Honda logo and an X or inverted Y maybe and a 3.
  How do you tell a 341 series head from a 392 head?

The F engine numbers could have an E or an F according to this chart.
I'm not sure of the differences between the K and F heads.

[Don R]

  Thanks that helps a little,  recently Hondaman mentioned that a 76 could be a hodgepodge of parts including pistons and heads which may be unusually fast or fuel efficient. That's the reason I wondered. I'll eventually have the heads off of two or three 76 motors so I'll post any differences I find.
  The motor that actually came from a 75 build but made as a 76 F1 begins with 238 so anything is possible with these old bikes. The  250 prefix motor is more likely correct for the F but they didn't come from the same place. 

[Don R]

 I did find a head with 300 clearly cast into it. Several with the HM logo, X, and a number. 

[Jerry Rxman Griffin aka MuthaF'er]

Mid year 75 F engine. The "year" went from January to ~ 1st week of May.

I would expect a 392 head to have 392 cast into the top intake side fin between 2 and 3. I have seen 392 heads from the 77/78 K without a 392. I "think" my 78 K head has 78 K stamped. That's one way to externally tell that it has the "updated" different valves, springs, and keepers.

If the head was off it is fairly apparent when looking into and/or comparing chamber shapes 

[Don R]

 I believe the story is correct about 76 being a free for all when they were building 750's. They were going to drop the K but a lot of people said no to the F.  Hence the scramble to make anything they could sell to us in the US.
  Much like the switch to K2, Honda used whatever parts were on hand or suppliers could churn out to get bikes on the boat. 

[Don R]

 The 76 A head wasn't identified either. (should be 392 also I think) It really doesn't matter except for a numbers match bike. I can see the waters get muddied on newer bikes. 

[scottly]

I have a K7 head that doesn't have the 392. IIRC, the K8 head was different than the K7 head, with a narrower cam chain tunnel? That was a claim by Lucky, so who knows.

[Don R]

 I put the head back on one and haven't had the head off of the other one. I'll have some fun this winter looking at them.  250 has broken champion plugs in it and 238 has a little cylinder rust. I need a dial bore gauge.

[Jerry Rxman Griffin aka MuthaF'er]

I have a K7 head that doesn't have the 392. IIRC, the K8 head was different than the K7 head, with a narrower cam chain tunnel? That was a claim by Lucky, so who knows.

My understanding is they both used the 392 head but the 78 switched valve train components. I think parts books would identify this.

[Don R]

  I had planned on a low mile K6 case and crank with a back cut f2 trans in it, my ported K head and big cam with whatever big bore I can put together.  All assuming my 1124 isn't fixed by then.