Topic: has anyone put 78 f3 pistons in to boost compression?

[Hockers Choppers]

Hey, Here is the deal. I have a 78 cb750f and this winter tore it all down and was going to put an older head and cylinder, big bore and cam in it. I found a 76f engine one ebay for $700, it was supposed to have big bore cam etc. I got the engine, it has the webcam 63a, aluminum retainers, heavy springs, adjustable cam sprocket, but no big bore! I was going to put it back together and was looking at the pistons of the f3 and they are domed up much higher, but he reliefs for the valves aren't as deep. Could a guy cut out the valve reliefs and be OK or will the compression be too high? Has anyone ever tried this?

Also, anyone have any specs on the cams of these two bikes. I plan on putting them both back together but may use the stock f3 cam in one of them or should I just by another performance cam for the second one? I think the f3 was also to have heavier valve springs?

[Jerry Rxman Griffin aka MuthaF'er]

Don't know if you could mod the pistons. My gut says no.

As far as cam specs go, the lift info isn't readily available but the opening/closing/duration is:

75/76 750F
 Int Open 5° BTDC
      Close 35° ABDC
 Exh Open 35° BBDC
      Close  5° ATDC

77/78 750F
  Int Open  5° BTDC
       Close  40° ABDC
  Exh Open 40° BBDC
        Close 5° ATDC
 

[mwohlenhaus]

it has been said many times on motorcycle forums; if you have the money anything is possible.  I would think a good machinist could put the pistons in a mill and do em up for ya.  But how much are you willing to pay.

[Hockers Choppers]

I have a buddy at a machine shop that can cut the valve reliefs but the rings are very expensive. I'll just wait for santa to bring me the wiscos I guess.

Wow bout some cam info? I have an f3 cam and a webcam 63a I've mic'd them both and the web is only .030 more lift? Is this right? Does the f3 cam have more lift than the 76 and older stock cams and could I run it in a 76f engine?

[Big Jay]

With cast pistons, don't get it any thinner than .200" under the valve pocket.

Jay

[Rocking-M]

you could also put the pistons in there and modeling clay and rotate engine around and see what you get.

[MRieck]

You can set the engine to overlap and pry down on the rocker to get clearence too. Put a dial indicator on the retainer if it is close.

[Hockers Choppers]

Thanks for the info guys. I have a buddy at a machine shop that can cut the valve reliefs. But the rings for these pistons are pretty pricey. So I might just spring for the wiseco's. I have a spare engine that I might just try the mock up and see if it will work. Just curious.
Jesse