Topic: Need some help with 78 CB750K Con Rods.

[fastestz1000]

I have lurked here a long time but never posted.  I am currently rebuilding a 78 CB750K and have run into some issues with my connecting rods.  I found a score in the piston pin end of the #1 rod as well as the piston pin itself. I want to replace the con rods with used ones and I was wondering about sizing and weights.  My current rods are marked as follows

#1 F3 with brown bearing
#2 F2 with green bearing
#3 F2 with green bearing
#4 F2 with green bearing

My crank is marked with "A" s and the case is marked with "B" s

My question would be do I need to find used rods with the same markings or could I get a complete used set with different markings.  If I got a used set with different markings, what would I need to know to get them in the right crank locations.

I hope this makes sense and any comments/info would be useful

Thanks.

Just for other info. The bike was given to me not running with 26xxx miles on the clock. It had sat outside for several years, covered with a tarp, but turned over freely.  It had a Kirker four into one exhaust on it but otherwise appeared very stock.  I have disassembled the entire engine and found the following:

The intake valve stems above the retainers were mushroomed and pitted and a complete loss
The exhaust valves had pitting on top of stems but were not mushroomed.  They will need to be replaced
The cam lobs and journals all looked good with no pitting or scoring
the cylinders all have visible cross hatching but there is a small amount of surface rust on the top area of the #1 cylinder that I am concerned about. I have not measured them yet
The pistons looked good with minor scuffing on the skirts. They cleaned up well and I want to reuse them
The cam chain tensioner was very brittle and broke during disassembly
The crank journals all look good but I have not measured them yet.
The crank bearing are all greens and appear to have been replaced at some time. There is little wear on theme but it appears that there was a lot of dirt trapped in them upon assembly and some have very severe scores in them. they will need to be replaced. 
The transmission and bearings appear to be in good order.

[754]

Any rod should fit on any crank, use the chart in the manual to select correct bearing shell.

[Jerry Rxman Griffin aka MuthaF'er]

Match the weights too according to the charts in the manuals.

[HondaMan]

There will also be a number on each rod: this is their weight, if you want the smoothness back. These match the crank weights at each journal.

The damage you describe to the top end and cam chain tensioner point to use of too-light oils in the engine at 29k miles. When rebuilt, use 20w50 oils. You will find that unless they have been replaced, the valve guides will have between .0050" and .0080" clearance and need to be replaced, too. APE makes excellent guides for them.

Be sure to get a new cam chain while you're at it: it will be worn, too. 

And, check the top plate in the clutch: it has slant-cut corks and they might be loose on the steel plate. And the rivets in the dual-steel plate in the clutch pack, which will be either the 3rd plate in, or the 2nd from the back (if in stock locations) might be loose. There's several 'fixes' for this one.

[fastestz1000]

So my understanding is that I just need to find a set of rods with all the same weight markings and then use the size markings on the new rods to figure out the bearing size with the chart in the manual.  If I understand this right, that is easy enough.  My understanding is that the weight markings are the "F" on my old rods and the size markings are the "3" an "2" markings. Does that sound right?

[Jerry Rxman Griffin aka MuthaF'er]

Tolerance = numbers and weight = letters

[lucky]

I would want to see a photo of that "scratch".

[fastestz1000]

I will post some pictures up when I get a chance to take some

[fastestz1000]

Here are the pictures of the damage to the connecting rod and pin as well as a little gem I found the other day.  I'm sure that ding did not occur at the factory.  Any suggestions?