Topic: Pin2Hot's 73 CB750

[Pin2Hot]

So it looks like I've been wrong the entire time about what pistons I have.  These appear to be 0.75 over standard pistons:


So I have a bit of a dilemma.  When I gave the jugs to the machinist, I gave him the diameter for 2nd oversized pistons, 61.5mm.  However, I also gave him the pistons I'm using so he could make sure they fit.  The pistons do fit in the cylinders although it seems like they get hung up verrrry slightly towards the bottom of the cylinder.  Is this normal?  They could be just getting slightly crooked passing through.  Here are some slight scuffs from test fitting the pistons, should I take the jugs back to the machinist?



Also if anyone can help me identify the brand of piston, that'd be super helpful:

[Pin2Hot]

I took the pistons and cylinders to the machinist and he blessed the bores as being in great shape and within 0.001 of the skirts so I'm in good shape to assemble the top end.  He did notice that at the very edge of one piston skirt, it was dragging through the cylinder so he buffed down the high spot and allowed it to drop through the jug without any resistance. 

However, the final drive seal has finally worked its way out again so I ended up splitting the cases . . . for the 3rd time.  Luckily it wasn't that big of deal as I only had to take off the shifter mechanism but I'm not taking any chances this time letting that seal slip out again.

I ordered a new a factory seal and I scuffed up the mating surfaces where it sits to hopefully provide some bite on the seal to prevent it from sliding out.  I'm thinking I may have used too much liquid gasket on the seal which may have allowed it to squish its way out.  I don't plan on using any liquid gasket on this seal this time because there seems to be a ton of pressure surrounding the seal which, well, seals it up.  Hopefully this decision doesn't come back to bite me.  Famous last words.

Upper Seal Seat


Lower Seal Seat

[754]

If the case has been welded, the fit there can change....any evidence of weld in chain area ?

[Pin2Hot]

If the case has been welded, the fit there can change....any evidence of weld in chain area ?

No evidence of that, thankfully.  I'm putting the case back together tonight so we'll know soon enough!

[Pin2Hot]

The new seal is in and holding strong.  I've been measuring the seal to case depth and it hasn't moved, I think I'm in the clear.

Don't mind the orange clamp, that was insurance when letting the liquid gasket set overnight.


Pistons are on and the cylinders and remaining top-end shenanigans will go on this weekend.

[70CB750]

Following.

[Pin2Hot]

I've been struggling to find a set of ring compressors in a local auto parts store to finally get the cylinders installed.  There's nothing too special about the simple band type compressors that I'm willing to pay $50 bucks for so I printed one on my 3D printer.



I haven't used them for their intended purpose yet because I'm pretty sure the layers of the print may bind up on the rings themselves.  I have a couple options, use acetone to smooth out the layers which might work or find some super thin metal (like a soda can) to line the compressor which should let the rings slide nicely.  You can see that it compresses the rings well (below) but I'm worried that either plastic will scrape off and end up in the crank case or the aluminum can might be too soft as well.  Finding super thin steel to line this thing might be a challenge but it's still worth more than spending money on a tool I can make myself.

[koendd]

just use your fingers and patience.. thats what I did.
Tried all the tricks with soda cans etc nothing really worked well, except my fingers...

[greenjeans]

I think you're on to something with those clamps.   I've always just used std hose clamps.   What if you just sanded the inside of them smooth ?   Is the plastic too soft that grabs ?

[Pin2Hot]

I think you're on to something with those clamps.   I've always just used std hose clamps.   What if you just sanded the inside of them smooth ?   Is the plastic too soft that grabs ?

Thanks, hopefully it works out.  I'm pretty sure the rings would dig into the plastic even if it were sanded smooth.  One method for smoothing 3D printed parts is to suspend the part above a small amount of acetone as it is heated up so the vapors pass across the part.  It's very effective but I still think that even if it were smoothed by that method or by sanding, it'd still be too soft and the rings would grab.

I'm going to cut an aluminum can into a strip that lines the entire inside of the clamp and attach it where its squeezed tight.  I'll work on it after I generate the necessary beer cans watching the election tonight.

[Pin2Hot]

It worked!  I ended up sanding down the inside of the compressors I printed like greenjeans suggested and it worked very well.  I had my wife pinch them while I lowered the cylinders when she looked at me and said "just use rubber bands or something."  I knew I married her for a reason.  So I used zip ties to hold them in place while I worked the cylinders down.



Turns out though, the printed compressors only work for the inside rings as there's no way to remove them from the outside rings once the cylinders are down over the rings.  So I did those by hand, one a time.



HOWEVER, it would seem that I didn't do a good enough job deburring two of the rings, one on cyl 1 and the other on 3.  There's a vertical scratch on the inside of the wall but it seems to be rather superficial.  I can't feel it with my finger nor does it hang up on my fingernail when I drag it across the scratch.  Based on where the scratches are, I think they're on the oil ring gaps which I might be able to address without removing the entire cylinder.  Can I ignore this or should I address it?

[SOHC4 Cafe Racer Fan]

Not that you NEED it, but there is a Honda tool.
http://www.ebay.com/itm/NOS-HONDA-CB750-K0-CB750K1-PISTON-RING-COMPRESSOR-TOOL-954-3000000-/332020001616?hash=item4d4def2b50:g:B7YAAOSwKOJYGLRY&vxp=mtr

[Pin2Hot]

Did you use any lubricant on the cylinder walls while installing the rings? If not, do so now and cycle the crank. Marvel Mystery Oil is a good choice as its nice slick. If you can not feel the gouge, it may disappear with some cycles.

I did, just regular motor oil though.  Here's a pic of the same cylinder before I wiped it clean to get a better photo of the scratch.  Like a fine red wine, legs and all ...

[Pin2Hot]

It almost looks like the end gap is too tight from that picture. Its interesting that the oil wasn't wiped with the rings on the downstroke?

I measured the ring gap on all cylinders and didn't find any that were too tight and all were below the max service limit but I'm going to check the oil rings tonight just to be sure.  I had applied oil to the entire cylinder so I think what you're seeing is the residual left up top.

[Pin2Hot]

Wellllllllll, it would see as though the gaps are too tight on the oil rings.  I pretty much took the manual to say that any gap less than 0.028 was good.  The gap on the oil ring is less than 0.004 which is what was causing the scuffing.  I'll run the hone through the cylinders very lightly to clean that up, gap the rings and try again.  In the meantime, I'm printing another little tool to help with assembly.

[Pin2Hot]

I looked around here and on the intert00bs and found a couple clever ways to grind rings and ended up making this little jig:



It worked super well and I was able to gap all of my rings to 0.010 which I've read is a safe number.  I ran the hone through the cylinders very lightly just one more time to remove any surface scratches and when I assembled the cylinders, NO MORE SCUFFING!!

Here's where I am now as I wait for new cylinder stud nuts to arrive.  Of all the things I've kept track of, labeled and stored, I can't find TWO of them so I have to wait to torque the head down.  It's actually just sitting on top of the cylinders here because I just wanted to see what it looked like.  I'll be assembling the top end, timing and setting up the PowerArc ignition.

 

[Phoenix]

To my tired old eyes, it looks like you left the two o rings out between the jugs and the engine case.  Hope it's my eyesight.

[Pin2Hot]

To my tired old eyes, it looks like you left the two o rings out between the jugs and the engine case.  Hope it's my eyesight.

They're there, if you look at post #41, second photo, you can see the orange o-rings sitting beneath the gasket.  I haven't yet tightened anything down nor put any gasket sealer on the gasket between the jugs and case but I'll make sure those o-rings are still there when I do.  Thanks!

[Pin2Hot]

When you do torque the head, let the motor sit overnight. Release in reverse order all the nuts, re-torque in stages. This will finalize the compression of the gasket and stretch your studs giving you a final, correct torque value on the head.

Excellent advice, I'll do exactly that.

[Pin2Hot]

Torqued and letting it rest overnight.  It's very satisfying to see it so "complete" considering it had been sitting in various bins for literally the past 5 years.

[Pin2Hot]

I have an engine, fellas.  Feels so good to see tons of empty parts bags littering my workbench.  I was worried I'd have a tough time timing the engine but it was actually really straight forward.  I guess I just had nightmares of valves crashing into pistons.  Now it's time to finish polishing the covers and get this thing in the frame!  Thanks for all the inspiration.

[Pin2Hot]

I hammered in some new bearing races from all-balls to get the front end put together and all of the sudden, I have something resembling an actual motorcycle.



I'll have to push the tapered bearing back off of the steering stem for paint but I wanted to start mocking up the bike to get the frame ready for modification.  Here's a general idea of what I'm going for:



I'm also trying to decide on the headlight set up.  I really dig the different sized headlights but may also go with a single light up front.

5 3/4" high beam and a 90mm low beam projector w/ LED halo


Single 5 3/4" high/low combo w/ LED halo


It's a lot of fun finally seeing this thing taking shape.  The next major items are modifying the swingarm to fit and chopping off the back of the frame.

[SOHC4 Cafe Racer Fan]

I like the double light mock-up.

[70CB750]

Making good progress

[Pin2Hot]

Edit - 6/21/17
This is not the best approach to this modification.  See later posts (page 7) to see why.  Heed the warning in calj737's post below, he's right.

I spent the past couple nights working on the swingarm.  I'm using a 1980 DOHC swingarm to extend the rear end out a bit, mainly for looks.  I was sure to capture all of my math to get this thing to fit so that it may help others in the future.  I measured the swingarm gap in the frame using the stock arm to get an accurate measurement.

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I measured the depth of the DOHC dust seal and bearing retainers to subtract that from the frame gap.  That gave me the new overall width that the DOHC arm needed to be which was 228mm.  I needed to chop off 11.7mm. of metal from the drum brake side of the DOHC swingarm.  This side is obvious because it has the most meat on it.  I used a zip tie to mark where I needed to cut and used an angle grinder to do the work.

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I pushed in new greased up roller bearings as well as the retainers.  Luckily the width of the new roller bearing and retainers were juuuust enough on the modified side to not have to machine out any more material in the swingarm.

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So the one thing that's obviously missing here is the hardened spacer that I have yet to cut to size.  I also haven't decided if I want to drill out the frame to accept the wider swingarm pivot or machine a spacer/shim so I can use the stock 73 pivot bolt with the hardened spacer.  I think the latter is where I'm leaning.

As of now, I just slid the stock pivot bolt through the new swingarm to see how things turned out and I couldn't be happier.

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