Topic: Just started my 750 engine rebuild - first of many questions

[Tugboat]

Well, that wasn’t so bad. About 6 hours to get down to the pistons on my replacement 750 engine. You can read here (http://forums.sohc4.net/index.php?topic=69470.0) about how I destroyed the top-end of my ’76 F motor.. and now I’m replacing it with this here ’76 K motor which had been sitting under some dude’s carport for who knows how long but otherwise uncovered. Not locked up, shifts through gears. I knew water had gotten down into it and so I figured while I have it off the bike I might as well replace gaskets, bore the cylinders, new pistons/rings etc. Here it is all broken down.



SO, some questions (I’m sure they won’t be the last ones):

1.  This is the only cylinder stud that has any rust damage. I literally have only just taken it all apart and haven’t had time to fully clean & inspect, but there is some obvious pitting on it. Would you replace it or if it cleans up ok just leave it as-is? And if you replace one, should you replace them all?



2.  When I first dragged this thing outside to initially clean it with engine cleaner & a hose I’d noticed this but thought it was just sticky chain lube goop that would come off. But upon closer inspection it looks like someone’s chain snapped at some point and slapped against the cases and possibly put a crack in it. The JB Weld repair looks okay – no signs of it leaking or degrading. What do y’all think about it? I mean, if I’m going to put all this work into rebuilding the top-end it would suck to end up with a problem here. I REALLY don’t want to crack the cases open to weld it proper. I do have the F motor still, and though I haven’t pulled it off the bike and taken it apart to see what damage it sustained I think the bottom-end might be okay. Is it possible to put a K top-end on an F bottom-end from the same year? Just thinking out loud here….





3.  Finally, can someone give me a ballpark figure of how much it is for a new piston/ring set and to have the cylinders bored? And how much is a valve job if I choose not to do it myself?

[ksmith0034]

I can only speak to your cylinder stud question.  Answer:  you should never reuse original studs - they have already stretched.  Buy APE studs - they are heavier duty and don't stretch as much.  Save yourself the hassle of having to pull the top end off again very soon; replace all your studs. 

[haggeo]

I can only speak to your cylinder stud question.  Answer:  you should never reuse original studs - they have already stretched.  Buy APE studs - they are heavier duty and don't stretch as much.  Save yourself the hassle of having to pull the top end off again very soon; replace all your studs. 

and don't be tempted to over-torque the APE studs beyond 22 ft lbs, they will snap. don't ask me how i know.

[Jerry Rxman Griffin aka MuthaF'er]

All depends on your "budget". Those engines are basically the same.

Bottom ends have different tranny ratios with the F being better for acceleration.

One/some of the 76 F rings are narrower and harder to find. That switched over at engine serial number 2551567 which I believe is near the end of the 76 run. Rings prior to this use the standard K rings so that's good. The K had lower compression 9.0 vs 9.2:1 which MAY be partly due to the pistons in addition to the definite head difference. I've seen 392 pistons that are identical looking to 300 K pistons but I've seen 2 different domes in both 9.0 & 9.2:1 engines. Either piston will work!

I wouldn't worry about that stud but I will say, like KSmith alluded to, the studs stretch and will cause head leakage. How tight were the nuts when you removed them?! Mine has been apart 3 or 4 times and this current time I had some fairly loose nuts! I am going with APE's studs!

Have you checked engine serial numbers to ID them? I'd do that to be sure what BOTH engines are. Also rely on the serial numbers when you buy parts as changes are made during model runs just like the rings.

If I was doing this I would buy one of the Japanaese 836 kits on EBay for $110!! You could spend more than that on Honda rings! They are pretty decent as reported by Terry. Search that thread. Maybe $100-$150 for boring. $80 for heavy duty APE studs. Gaskets and seals. If the 392 F head is good I'd definitely use that!  

[Tugboat]

Thanks fellas - The the nuts on the cylinder studs were pretty tight, and the condition of the outer bolts would suggest that it's never been taken apart before. But either way, replace the studs?

Any comments about the JB Weld fix? I've never used it at all before so mainly I'm just wondering how well it holds up on engines.

[cookindaddy]

Yours looks nicer than the similar "repair" done on my engine. Not sure if it was JB weld or not, but mine leaked oil, until I replaced the engine. One of my fingers is pointing to the "repair", the other is pointing to the chain cover mounting screw that the departing chain tore out.

You did a MUCH better job cleaning your engine before the pictures than I did!

How long do you plan to keep the bike after you do all this work on it?

Good luck with the rebuild!

[CycleRanger]

Boy does that look familiar!   

http://forums.sohc4.net/index.php?topic=60825.0

I tried adding more JB weld to stop the leak but it didn't work. I ended up finding a replacement engine for a good price.

[Tugboat]

So is it possible then to use my K top-end on the F bottom-end? The JB Weld fix looks okay to me but having never run this motor I'm worried that I'll spend all this time and money only to have to break it all down again to weld the case. I plan on keeping this bike for a long time, so I want to do what's best (within my budget & time availability).

Or, I guess on the other hand, is it possible to somehow remove that side of the case without completely taking apart the rest of the engine? Yeah, that's wishful thinking