Topic: 77,78 750 K @ F

[Bodain]

I've just come accross two CB 750's  for $150.00 total... Normally this is a no brainer.... You grab them quick before someone else does. I've only seen some very poor pic of them... Based on these pics I believe them to be 77 or 78 K units. They have the 78 style gas tank and single disk rotar up front, the engines are not black .I'm still trying to get the owner to look on the neck of the frame to identify the year. He's about 80 miles from me.

The last thing I want is yet another 78 F engine....   Still I'm considering these primarily as a possible solution to be 78 F  engine problems.

Ok. We all know we can use a 69 -76 K cylinder and head on a F unit and get rings and pistons easy enough...
So how does the cylinders, pistons, head  on 77/78 K  units compare to 69 -76 equivents... As far as interchangability...

And how are they compared to 78 F interchangability?

[scondon]

 There are a number of changes to the engine throughout the SOHC 750 years. I think your asking whether you'll run into any of the difficulties with the '78 K motor that you have found trying to swap parts for you '78 F motor. The answer, as far as I know, is that you shouldn't have trouble with compression change or incompatibility of head design when swapping top end parts from earlier years of the K bikes with those of the K8. The pistons and rings are a different part #, but that may just have to do with different ring dimensions and not piston shape.

[eurban]

Don't know the exact nitty grittys of installing pre 77K model blocks and cylinders on the F crankcase but it should be no more difficult using 77/78K pieces.  You may want to consider using the entire 77/78K motors, particularly if they are in decent enough condition.  These year engines should be a good basis for your project whether or not you use just the top end or the entire rig.  77/78K engines have updated clutches, new head castings with better flow and bumped compression ratios. The 78Ks used different valves with F model style keepers that may or may not cause issues and they (the valves) don't seem to be available any longer.  If you need new valves, you could always replace with earlier style valves and keepers though.  Pistons are definitely of different design than the earlier Ks, contributing to the increased compression. Hope this helps

[scondon]

Don't know the exact nitty grittys of installing pre 77K model blocks and cylinders on the F crankcase but it should be no more difficult using 77/78K pieces.  You may want to consider using the entire 77/78K motors, particularly if they are in decent enough condition.  These year engines should be a good basis for your project whether or not you use just the top end or the entire rig.  77/78K engines have updated clutches, new head castings with better flow and bumped compression ratios. The 78Ks used different valves with F model style keepers that may or may cause issues and they (the valves) don't seem to be available any longer.  If you need new valves, you could always replace with earlier style valves and keepers though.  Pistons are definitely of different design than the earlier Ks, contributing to the increased compression. Hope this helps

   You'll want the earlier retainers as well if swapping valves and keepers in the '78 K head.

    I only have the '78 K and F pistons to compare. The '78 K pistons have the same flat tops and circumference that I've seen in the earlier K models so assumed that they would have same compression aspects. Are they taller by a smidge? Just curious

[eurban]

Its been a while since I have seen the stock pistons in my 78K as I have been running Wiseco 836 pieces for a couple of years and the stock ones are long sold. The 77/78K and the 75/76F engines definitely have compression ratios of 9.2 to 1 which is a bump up from the earlier Ks 9:1 and the F2/F3 9:1.  I am not 100% certain as to what (pistons, combustion chamber or both) contributes to the increased CR but my understanding is that the pistons have a higher dome. Are you certain about your flat topped pistons being 78K ones? . . . Rxman posted this info in an earlier thread where I was wondering about similar things. Both the 75F and the 78K would have the higher CRs so his pics don't compare with the earlier K pistons . . . . . . .

Quote from RxmanGriff:

"Both pistons are -392. The one on the left is from my 75 750F. It has 392 and 1 by the pin hole on the skirt while the other is a K8 and it has 392 and 6 on it. The total heights appear to be the same.   
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01494.jpg
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01496.jpg   "
 

[Bodain]

I just spent some time looking on some sites  looking for the availability of piston rings for the various models.
As we all know you can get 69 -76 rings almost everywhere. We also know stock rings are simpley not available for a 78F...   It looks me like 77 / 78 K rings follow the 78 F pattern and not easily available. I think I better hold out for the 69 -76 engine or at least the top end of one.

[Jerry Rxman Griffin aka MuthaF'er]

77/78 K and 75/76 F are almost identical and would work on an earlier K. Main difference in the 77/78 K and the 75/76 F I know of (as mentioned) is the 78 (and not 77) K head uses the different retainers/springs which I believe are 77/78 F type. The heads on all these will work just fine on a 77/78F with better flow than earlier K heads as long as you use the matching cylinders also.

[eurban]

I just spent some time looking on some sites  looking for the availability of piston rings for the various models.
As we all know you can get 69 -76 rings almost everywhere. We also know stock rings are simpley not available for a 78F...   It looks me like 77 / 78 K rings follow the 78 F pattern and not easily available. I think I better hold out for the 69 -76 engine or at least the top end of one.

Sounds like you are trying to talk yourself out of the deal that you found!  David Silver Spares has rings in standard and oversizes for the 77 and 78K  pistons. In fact, they list standard and oversizes for the F2s as well. . . . Do what you like but as has been already mentioned the late K engines have some perks that will make them strong performers.

[Bodain]

Actually I'm trying to avoid another bastard engine like the 78 F... Indeed. You must understand the history. I had an excellent 1975 750 that I cafe'd.... Excellent bike I loved it, but I always admired the 78 F, the cosmetic differences, comtar wheels, dual disks up front. I finally decided to sell the 75 cause I wanted to work a 78 F. I sold it and found a 78 F... Little did I know what I was in for concerning the engine and part availability.

I was actually on David Spares web site way back when I first started doing the F  battle and I thought I saw standard rings were available for 78 F which I thought was an F2. I placed an order for the rings and other things. Fortunately they request frame and engine number just to verify your ordering the correct parts. Turns out what they refer to as an F2 is not the same as what we refer to as an F2.... (Numerous conversations on this item else)

I've not found stock rings for a 78 F available anywhere. It seems that most sites that deal in the vintage upper engine part have stock readily available 69 - 76   Then they just jump  to DOHC in 79.... The infamous black hole of 77 - 78. I was kinda under the impression this problem only existed with the 78F...Black engine.

However you could be right. I did not check David Spares on the availability of 77 parts.

Actually my F runs great, but smokes like a pig...and I've just invested $327.00 into the head to replace the exhaust guides and a valve job and it still smokes. I've been battling this engine too long. It's wearing me down. <grin> Only thing left is a piston / rings.

So now I'm just considering my options, since I'm pretty much out of money pump into this thing.

[scondon]

  The 77-78 K motor is a good, direct replacement for the 77-78 F motor. The only real difference in the bottom end is the F3(78 750F) has stronger rod bolts giving it a higher redline. If you can get a good 77-78 K motor cheap that has good compression you can have the best of both worlds( a solid motor in a F3 bike ). Research the availability of K7-K8  rings/pistons if you want to think about the long haul, valves as well, but two bikes for $150 sounds like a pretty good gamble to me. The only thing you'd want to note is that the F2-3 intake spigots are larger than the other SOHC's so you'd want to either change the carb boots to K7-8 type or buy thin hose clamps to create a better seal using F2-3 boots.

Its been a while since I have seen the stock pistons in my 78K as I have been running Wiseco 836 pieces for a couple of years and the stock ones are long sold. The 77/78K and the 75/76F engines definitely have compression ratios of 9.2 to 1 which is a bump up from the earlier Ks 9:1 and the F2/F3 9:1.  I am not 100% certain as to what (pistons, combustion chamber or both) contributes to the increased CR but my understanding is that the pistons have a higher dome. Are you certain about your flat topped pistons being 78K ones? . . . RXman posted this info in an earlier thread where I was wondering about similar things. Both the 75F and the 78K would have the higher CRs so his pics don't compare with the earlier K pistons . . . . . . .

"Both pistons are -392. The one on the left is from my 75 750F. It has 392 and 1 by the pin hole on the skirt while the other is a K8 and it has 392 and 6 on it. The total heights appear to be the same.   
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01494.jpg
http://i233.photobucket.com/albums/ee84/RxmanGriff/DSC01496.jpg   "
 

   It's been a while since I pulled the K8 engine apart to build a 836 too   Pistons are still in the garage so I can have a look at them tomorrow, but I can't be 100% certain that they are original to the motor since one external engine bolt was missing that could only be removed with the valve cover off. Serial #, head and bottom end definitely K8. Judging from your pics it looks like you and rxman are right, I've only viewed from the top before and the dome on the early F pistons are easier to spot than on the K8 pistons. I'll set my old K8 pistons next to some NOS K1 pistons and see if I can't set myself straight on this

[eurban]

Scondon,

If you could please post a couple of pics of the early piston next to the K8's I would appreciate and I think it would be good info to have on this site.