Topic: Never Ending Maelstrom of Engine Woes

[SD750F]

Will the insanity ever end? So I bought a CB750 F3 Super Sport knowing the cautionary tales about the valve guides. I knew that the motor would require head work which is being done. But I didn't know, and I can't find anything about the problem with the piston rings until myself and one other member talked about them recently. I and my machinist thought I might be able to get rings from a major manufacture with a slight size difference (thickness that would be milled down) or bore sized slightly larger so I can hone down the ends to make the gap correct once inserted. Now the guy who has milled down rings will not do the job. And contrary to my current machinist, I can not modify the ring ends to gap them properly if I bought 0.25mm oversize rings that I can find at the moment...

The machine shop hasn't even checked my bore dimensions so no one knows if I need to up size anyway. So re-groove the pistons for the standard thickness K1-K6 rings... But that is going to cost me another $125! So I am back wondering if I should put in the Cycle X 850cc Big Bore for $250?

And I found some glass beads in my crank case. The machine shop says "You MUST open and clean out your case" what sounds right to me, but my friend who has a couple 750's says no way, don't crack the case. You can clean it out good enough and a couple glass beads would be ground up and filtered in no time...!!!

Will this Maelstrom ever end???

[chrislib]

You need a new friend...Glass inside an engine, I don`t care how small is a recipe for doing the job over again. It might be cost effective to just go with an 836 kit?

[SD750F]

I agree about the friend. Needed my help with the assembly of his bike. Valves, timing chain, and especially the electricals. He is riding, mine is still in parts. And he is hardly available to help me unless it is for useless suggestions like don't worry about the glass beads. And being an engineer I know better...

Scott

[eurban]

The combustion chamber is larger on the f2/f3 heads. Pistons that will yield 10.25:1 compression ratios in the standard K engines will yield lower compression ratios in the f2/f3 engines.  I don't know off hand how much lower but I think that it is significant enough that you wont want to go that route.  Ring options for the f2/f3 have been discussed a good bit recently.  Maybe it was that "total seal" piston ring discussion.  Good luck . . . .

[markjenn]

Where did the glass beads come from?

- Mark

[SD750F]

I had the engine cleaned knowing that I was going to break it down. When I got it back I found a couple of the plugs I inserted  no longer present. And I have read that a Big Bore kit in a F2 or F3 will have a 18% drop in compression. So a 10.25:1 in a K series will be around 8.41:1. This is okay and not in the danger zone of over heating the engine, but it will run warm even with a matched spark plug.

Scott

[TwoTired]

Not that I know anything about this stuff.  But, isn't there an 836 kit that has 12:1 pistons.  How would those work with the bigger chambers on the F3?

Just thinking out loud.

Cheers,

[SD750F]

TwoTired,

Both the Cycle X 850cc and the Wiseco 838cc Big Bore kits were designed for the K series. I called both manufacturers and they calculated a 18% increase in head volume because of the deeper F2 and F3 heads... So there's the quandary. I could go for a Big Bore and at the risk of lower performance and higher engine temps. Matter of fact the individuals told me that they recommend a K series engine to be used in a F frame... They have had the best results with that configuration.

Scott

[Patrick]

Dynoman.com sells a big bore kit for the F2 and F3. It is a bit pricey, though. Twice what the CycleX kit sells for.

Patrick

[eurban]

I really don't think you will be making any forward progress by installing pistons that will net you a CR of a little over 8:1.  Not an option IMHO.  TT's idea (12:1 Wiesco's) is interesting but I would be concerned about piston to valve clearance with the larger f2/f3 valves.  I am sure APE, Dynoman, or Z1 can tell you if this is an option. Decking the head a bit might help but again I worry about piston to valve clearance.  Dynoman sells the specialized big bore kit for a reason; if there was an easy solution there probably wouldn't be the need . . . .

[SD750F]

Dynoman does make a Big Bore kit specifically for the F2 and F3 engine that does take in account the 18% increase in head volume and the larger valves. So I guess I have to decide if my project is worth the additional $500 plus the unknown $ for machining out the bore...

Scott