Topic: First timer top-end rebuild gone wrong, yippee! Ugh...

[markreimer]

Ok. Picked up some high temp thread sealant and a brass set screw style plug that's 1/8 size but unsure yet if it's the proper thread pitch. I'll have to measure the switch I have and  test fit or something. Don't wanna ram the wrong thread in there and add salt to my wounds.


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[markreimer]

I tried sealing a brass tapered tube into the hole so I could still use a pressure switch, as seen in the photo (in the photo it's just a plug at the top). Sadly even though I was able to get the insert in a ways further than the pressure switch, it still leaks out the side of the weld. I think the taper just opens up the crack too much. It's going back to the welder tomorrow morning for a final attempt at that fix. If it still leaks I'll see if a bolt will work, but the hole is tapered so I fear a bolt will simply push it open again.
At the moment the only thing that DOESNT leak is the original switch, which fits quite loosely in the hole. It must not be tapered enough to open up the crack.




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[markreimer]

Even worse news! The second weld attempt failed. So I came home to remove the pressure switch and clean it up for some JB weld. It came out and brough all the threads with it. Now the hole is cracked and stripped.

My only remaining, and final option before scrapping this case, is threading something down deeper into the hole where the remaining good threads are and sealing it in there for good with more JB.

I heated and cleaned the outside, then applied high heat JB to the crack. I'm letting it cure under a heat blanket for three days. The brass tube I posted earlier does grab the lower threads and will hit the torque spec for the switch without blocking the oil flow, so I'm going to try that. If it still leaks on Saturday, I'm pulling the engine and looking for either a new bottom end or will try splitting the case and repairing the hole.


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[markreimer]

If you were just a tad closer...

I'd use a whiz wheel at this point and cut the boss back. Mill an aluminum plug, and weld it in.

You need to proceed a bit carefully if you don't split the cases. The threads and shards can enter the oil galleys now, and rinsing them is a priority lest you tear up a bearing. If you do split the cases, then its a much easier crack to fix. Any welder can fix it then with access to both sides and even plug weld it from underneath.

I feel (and am rooting) for you, but I did tell you its a tricky fix 

Yeah I hear ya. I'm just about resigned to the fact that I'll be learning to rebuild the bottom end this winter. 750's aren't that common around here. At least not common enough that I could bank on finding a donor motor to either outright replace this, or steal the bottom end from. If this 'fix' (if you can even call it that) doesn't work and I need to pull the cases apart, I would definitely go the route you suggested and plug that hole entirely. Though that got me wondering if it would be possible to plug it, drill and tap the plug, and thread a switch in. Hard to say now.

As for now the plan is the same - wait till Saturday, then open up the galley port on the right hand side and flush a litre or two of oil out just incase there is some filings and threads that made it into the galley yesterday (i'm fairly certain at least a bit must have). I hadn't considered a destroyed bearing, I was actually worried little filings would make their way up toward the head and plug those tiny orifices in the head, and destroy the new valve job I did last year. Either way, I'll open up the galley and kick it over a bunch to flush lots of oil through, then give'r #$%*.

[markreimer]

Could it be? Finally some success? Yup. JB weld seems to have done the trick.

I bought some high heat JB and went to town on the engine with a torch. I blasted it repeatedly till I couldn't see any more oil burning off. Then blasted more, cleaning with acetone between heat cycles. Also I roughed the surface a bit with a file.

I worked the JB in generously and wrapped it halfway around the pressure switch port.  Laid a heat blanket on top and let it cure for three days. Then I came back and screwed the brass tapered tube into the lower remaining threads with some permeated thread sealant, letting it sit for a day as well. It was able to meet torque spec despite missing upper threads.

Next I removed the oil galley plug on the right side. It also took repeated heat and persuasion, seems I must have over tightened it last time. Once off, I hi let the starter with the kill switch off and pumped a liter or two of oil out the side to flush out any little metal bits that might have made their way in there.

I fired the bike up and crossed my fingers. No leaks at idle. Progress! Took it out on the road and brought it up to redline once warmed up. I rode it hard to get the engine hot and oil pressure high. Thus far no leaks still. Looks like I can actually ride for the remaining 4-6 days of fall hah..

I know it's not a good permanent fix, but at least it seems to run fine now and I can use the bike while looking for a new engine or lower cases. I considered splitting them and welding the thing shut permanently, but honestly I'd rather just fine a new engine and out my rebuilt top end on it. I was getting a real notchy shift from first to second anyway, and the case already had a repaired hole from a thrown chain. A new lower could be nice.




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[markreimer]

Time to give it a good wash. All that oil leaking had made it look like a dogs breakfast. But for the first time all summer it's not leaving oil puddles behind.


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[markreimer]

Well I'm sad to say I gotta resurrect this thread from the dead.

Winter is finally over and I took my 750 out of the garage, time to start it up for the first time since November. Popped the battery in, checked the oil, ready to go. Tried kicking it over but the kick felt oddly difficult. Tried the starter and it fired up after a few seconds. A quick ride later and I saw oil seeping out of my JB weld "Fix".

I turned the tapered bolt another 1/4 turn and it seems to have stopped, but clearly this isn't a reliable solution. Time to do a proper repair here.

I bought a spare '74 engine back in December but I haven't really looked at it closely since I bought it. I pulled it out and decided to give it a look, see what would be required to combine my newly bored block and rebuilt head to the bottom-half of the spare.

As I pulled the various covers off, imagine my surprise to discover everything had been harvested before I bought it! Remove the clutch cover....surprise, no clutch! Remove the points cover...hey, there's nothing here! No starter, shift lever, cam chain tension, and mission some exhaust spigots. I mean, it's fine, I have them on my current engine. If anything I know to pay much closer attention when I buy a motor. I paid for what I thought was a complete engine..lol.

I don't know how to test the bottom-half of an engine before doing all this work - any suggestions?

I turned the sprocket by hand and could shift through all the gears. Since the clutch wasn't in there, it was easy to turn by hand.

I removed the valve cover to see what condition the valve train was in. It all looks quite good. The lobes look good on the cam, though I didn't disassemble everything yet and check the bearing surfaces.

My thought is to take this spare lower and put my existing clutch, starter, ignition into it, and then transplant the entire current top end onto the bottom. They are both '74 engines so it should be easy.

The top end has cycle-x 2nd oversize pistons in it. Can I re-use the wrist pins (not the clips, I know!) they came with? There is about 400km on that top end.

All I need to buy are the top end gaskets. I already have all the lower half ones.

Really wish I didn't sell that frame kit....

[markreimer]

Dredging my old thread up from the dead because I finally have a date and location to work on the engine repair. I've got the 750 up and running nicely, and the oil galley isn't leaking after a 4-5km ride, so I'm going to ride it over to a friends garage on the 21st.

Friday night I'll pull the engine. 

Saturday I'll disassemble the current engine as well as the donor engine, then re-assemble one 'good' engine using the donor bottom end and the current refreshed top-end. If I'm sneaky, I can do it without even removing the pistons from the block, and avoid the risk of snapping a ring during instal. I've also got a Pamco ignition to instal and some new carb boots. While everything is apart, I'll pull the carb jets to confirm everything is clean and clear. I've also just replaced the plug wires and caps, so when the engine is back in all the ignition stuff should be fresh and good.

Sunday I'll re-torque the cylinder studs, pop the engine back in, hook it all up and HOPEFULLY ride it back home, just before the snow flies!

This will be my second time pulling the block, third time with the head, so I have a good idea of what's all involved. One thing I'm thinking is that the donor bottom has no oil filter housing on it, and the cam chain guide is removed. I'ms ure some junk has fallen in there somewhere. What's a good material to wash anything out? Diesel? I pulled the oil pan and everything looks sparkly and clean..

Any other general tips of stuff to look for? Both engines are 1974. I'll be swapping the clutch, starter, new ignition, block/head from the current engine.

[markreimer]

Productive day... pulled the engine using hondaman's method. MUCH better, had it out within a minute of pulling the bolts.

Tore down the cracked motor as well as the donor motor. Started cleaning the gasket surfaces in prep for rebuilding the donor bottom end with the current top end.

Donor end cam and lifters look pretty knackered. The lobs aren't even the same shape across the cam. But the tower bearing faces are in much better shape than my current engine, so I may poach them but use my current lifters.

So tomorrow, clean and rebuild a good engine!






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[markreimer]

Oh also, the donor motor had scuffed pistons too, kinda seems like it had an oil supply issue, though the cam looks good so idk? Anyway, my current top end clearly hadn't even broken in yet. Cylinder cross hatch is very clearly visible still, and there is minimal carbon on the piston tops, just a little black film. Although all around the edges they are clean. Isn't that a common blow by indicator? These are new 2 oversize pistons and rings with an overbore, all the clearances were confirmed (ring gap, piston to bore, etc)


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[70CB750]

Scuffed pistons are normal.  Just polish them - lengthwise only.


Nice shop, btw 

[markreimer]

Well #$%*.

Had the whole swap finished except for the cam towers, three of the four bolts pulled the threads out. Yes I used a torque wrench. Guess it's heli coil time or somethinf


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[markreimer]

Heli coil success! Managed to find some locally on a Friday at 8pm, not an easy task. Got the motor buttoned up, starter in, clutch in, pamco installed but not wired up yet, and the engine in and rest of the bike assembled. 1:30am and time for a rest.

Tomorrow I'll wire up the Pamco, prime the oil pump and fill the bike with oil, then giver I guess!




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[markreimer]

Success! Wired up the pamco using the brake pedal switch for a hot lead, keeping the wiring behind the air box like it was originally. Then came time to fill it with oil...

Had a brutal time getting the oil pressure to build. First tried pouring oil down the right side galley, didn't help. I had oil st the filter but nowhere else. I dropped the pan and it was empty! So I removed and cleaned the pressure release valve, got some oil flowing, filled the pan with oil and bolted it up. I had pressure within half a dozen kicks. Time to start it up!

It very nearly started on the first kick! After a couple more kicks it started up and settled into a steady idle. I rode it around and after five minutes it started to stumble and run horribly. Oh no! Oh wait, I'm a dumbass and my pant leg caught the choke lever... pushed it down and hey whaddaya know, it runs well again! I've got my bike back!!

I'm going to ride it for a short bit and change the oil again. I'm nervous about metal flakes from drilling the heli coils, and my general dirty workspace creeping in where it shouldn't. I did my best to keep everything clean but who knows. Best to just flush it a few times and inspect the filter.




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[markreimer]

Just a quick 'one week post rebuild' update. I got back from my first real highway ride and am very pleased.

The bike is pulling much better than it ever has before. I found an open stretch of country road and had it up to 110mp/h before I backed off. I think it had a little more, but I was riding into a stiff head/cross wind and started to feel unsettled. I've never been able to get it past 100 before, and that was only possible with no wind (or a tail wind) in full tuck and a long straight road to build up speed.

I can easily pass semi's with a twist of the throttle, no need to down shift. It's shifting smoothly, though I did get two false neutrals, which I attribute to the new Sidi Adventure boots I've started wearing, which pretty much eliminate my ability to feel the shifter when downshifting. When i'm more 'deliberate' with my shifting, everything is fine.

Anyway, I've put in about 150 miles, no leaks or anything else unwanted. Thanks everyone for your help and input!

[70CB750]

Ecellent!

[markreimer]

Got around to my first post rebuild oil change today. As expected there was substantial metal shavings and other misc grit in the filter housing and oil pan. It seems the filter and pump screen are doing their jobs though as it was all caught there. The rest of the oil looked pretty clean. I changed the oil and filter, set the timing and also installed an oil pressure gauge. Turned the air/fuel screws out 1/8 turn as it's been bogging off idle. Now it's running better than ever and I've got good oil pressure. Still no leaks.


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