Topic: One of these cases is not like the others +(now trashed piston)

[Garage_guy_chris]

Im just getting into the engine part of my restoration and i have been finding lots of weird and strange things along the way. I just caught the fact that someone has cut and ground the box away that goes around the starter. Why would someone do this.... Now i cant seal my starter.... thanks previous owner.... and all i assume to save 1/4 of a pound. Im about ready to drag the whole pile out to the curb...Im really starting to question just about everything on this supposed ready to run completely rebuilt 836cc engine. I was really hoping to not have to take this thing completely down. Now i think its adviseable to split the cases to check and inspect everything....who knows what else has been done.... Anyone have a K4 engine they wanna sell?

[754]

Look at the end of that plug, and see if it says R.C.

[Toxic]

or look at it this way, if they were paying that close attention to detail on an engine rebuild, maybe they knew what they were doing.

You're stressing out, relax put it in and run it, you will know soon enough if it was done right.

If you are really worried about this supposed ready to run engine, how about instead of tearing right down, pull the valve cover and check the condition in there.  Check valve clearances etc..

Just pulling all the engine covers and checking how things look might give you some peace of mind.

You can buy one of those inspection cameras for $99, stick it down the spark plug holes and see what you can determine.   I have been dying to find a reason to buy one. lol

[lucky]

Maybe you can find a new rebuilt engine online and it says, "certified" and is in a plastic
vacuum formed package and looks brand new.

[Garage_guy_chris]

I have taken another few looks at this mod and while im not so happy or pleased about it its unlikely to have any impact other than cosmetic. the issue im facing now is that someone has gone to great lengths to lighten this engine:
- Removed electric start
- Deleted starter clutch and machined alternator rotor, 
- Machined gear off of starter gear leaving just the hub,
- Ground and cleaned up rockers,
- Removed this starter box off.
- I have also found a odd cam im working on identifying

Im gonna have to go deeper to see how far this weight reduction goes, The more i think about it this bike must have been being built for racing of some type unfortunately i dont know any details of this. Im just hoping upon deeper inspection things can be returned to a street-able / tour-able engine. im hoping to not find a lightened crank and modded transmission

[754]

Why dont you get it rideable, then decide.. You should pull a plug and get a light in there and look around
Have not seen anything unstreetable so far, if you need starter stuff , I have stuff like that.
 Did you buy engine seperate , or in the bike..
 Most of us feel like lotto winners, when we find unexpected hop up goodies in a motor.

 Do you want to trade some of it for stock parts ?

[ekpent]

I'd be happy I think to have those problems. Costs money and time to make hypo mods and you may be pleasantly surprised if a faster 750 fits your scheme 

[Toxic]

Hi Chris

I'm not too far away from you, I have an unopened '77motor if you want to swap. I pulled the cam cover off to adjust valves and that is it.

[Garage_guy_chris]

Why dont you get it rideable, then decide.. You should pull a plug and get a light in there and look around
Have not seen anything unstreetable so far, if you need starter stuff , I have stuff like that.
 Did you buy engine seperate , or in the bike..
 Most of us feel like lotto winners, when we find unexpected hop up goodies in a motor.

 Do you want to trade some of it for stock parts ?

Unfortunately there is 6" of snow on the ground so i have months to tinker indoors.

Already ahead of you on the inspection cam



All i can tell is its not stock CB750 pistons. I was told they are 836cc

I'd be happy I think to have those problems. Costs money and time to make hypo mods and you may be pleasantly surprised if a faster 750 fits your scheme 

I want a mildly hopped up engine that's why i bought this bike.  The whole idea behind opening it and inspecting is to inspect as its been sitting something like 2-5 years,  i also want to figure out what else has been done to the engine,  make sure it was built properly and will have longevity on its side. it needs to be a streetable / tourable setup. Im pretty sure it was being setup by the PO for racing

also i figure knowing what parts are  in it so i can narrow down compression ratio, and fuel requirements, and finally the carb tuning to come just a little.

[754]

Looks like almost new.. Great..

[BPellerine]

you may have a fast motor hiding in there wish I had the same kind of luck!bill

[bjbuchanan]

I agree with 754, that is really clean inside. I bet you could run it as is, motors with that that attention to detail will probably be worked over nicely.

Hopefully it isn't a peaky cam tho. Valve guide seals should be R&R'd tho because sitting will dry them out, that is if you are going to go in...

[754]

 If the cam is a regrind, without welding, I would suspect it to have mostly lift, not bigger duration. Appears in the pic to not have a fat lobe...

[Garage_guy_chris]

I pulled the cam it looks good, gonna measure it up see what kinda lift and duration it has my initial impression is that its very close to stock.

Now the plot thickens ...I decided to pull the head and cylinders.

Initially things looked really good i found 64mm Yoshimura 812 pistons in it ( i was told it was a 65mm yoshimura 836)



Until I got to cylinder #4
Front of piston



Back of piston



I also found a couple other things...
-1 piece oil rings on #1, #2, #4
-3 piece oil rings on #3 (you can see at left side of last photo)
-Broken oil ring on 2
-all pistons have some scouring front and back beyond what i would consider normal wear.
-2 outer front cylinder studs rusted / corroded severely at base and are going to need replaced no idea how these didnt snap

So now its looking like im gonna be into it alot deeper than i wanted to be im not sure I can afford to fix this now either. Before i do anything i think im splitting the cases and check the bottom end / transmission as im not leaving anything to chance at this point.

My thoughts / idea's:

1. - order up a cruzin image (ebay) cast flat top 836 kit for $120
    - Bore the existing 64mm 812cc to 65mm 836cc
    - Maybe add a new mild cam

2. - Find a replacement Yoshimura 812 (64mm) piston for #4
    - Find replacement rings for all 4 bores
    - Clean up existing cylinders / rebore new cylinders

3. - Get a used stock bore cylinder
    - Order up a Cycle X +0.50mm  or +1.00mm $199
    - Find a stock cam

4. - Get a good used set of cylinders / pistons $???

5. - Find a whole engine and start over

[Jerry Rxman Griffin aka MuthaF'er]

If we open these up we usually expect to spend some money. Often times we find surprises both good and bad. That green base gasket was clue #1 before it came apart.

Is the rod loose on #4? The rings are intact so something is allowing the piston to rock like perhaps bearings.

The Yoshi 812 kit uses Honda CB350 twin rings 1968+ 3 piece oil rings.

If the cam was a regrind I think you'd find welded on layers on the lobes

I'm thinking your cam is an early perhaps sandcast engine cam. Hotter than most/all other stock cams. I don't have one to measure however. Bill Benton might have one to measure and he might have a PopY 812 piston too.

That stud is bad news. Good luck on getting it out.

Oh well, it's winter. What else do you have to do??

I'm thinking your easiest way out and probably least expensive is the 836 kit but I sure hate to see a 3/4 good 812 kit go to waste. 

[bjbuchanan]

3 months ago I was at the same point as you with the 812 dilemna. I unfortunately had three broken pistons tho, but my math means I have one extra at home you could use if you would like to go for the 812 rebuild and save machining work

Considerably better skirt wear than that sucker there, PM me if interested

[lrutt]

That definitely looks like:
- piston to bore clearance was not right ....or
- intake leak making that cyl run LEAN.

How does the bore look? will it clean up with a 3 stone hone?

[Garage_guy_chris]

That definitely looks like:
- piston to bore clearance was not right ....or
- intake leak making that cyl run LEAN.

How does the bore look? will it clean up with a 3 stone hone?

I might just be interested in that 1 good piston you may have. figure out what you might want for it i will have to pull out the micrometers see how the bad #4 bore looks. I have one other idea im gonna run across the forum see what that concensus is.

[bjbuchanan]

That definitely looks like:
- piston to bore clearance was not right ....or
- intake leak making that cyl run LEAN.

How does the bore look? will it clean up with a 3 stone hone?

I might just be interested in that 1 good piston you may have. figure out what you might want for it i will have to pull out the micrometers see how the bad #4 bore looks. I have one other idea im gonna run across the forum see what that concensus is.

Sounds good, pm me so this doesn't jack the thread.