1982 CB750F...Better Devil

trek97

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I found the suction cup on my new tool was too glossy and would slip on the valve.

I roughed it up w some emory paper and that did the trick.


 

Jimbonaut

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Thanks for the tip Trek - I'll be in the garage later if I don't swerve for a pint or two of stout instead. That could happen. It would not be the first time by any means.
 

doc_rot

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The Jimbonaut said:
Thanks for the info and tips guys. Nipped out last night and picked up the lapping compound and tool, and spent a while watching a few vids on how to do the job. Seems like doing it by hand (as opposed to using a drill with suction cup) is the preferred method, at least for a newbie. That way you get a better feel for the process, and can hear the changing sound a well-seated valve makes in its seat. Apparently - I'll find out one way or another.

I don't have the mechanics dye so I'll use a Sharpie on the seat. If I understand correctly - sharpie the seat, lap the valve, and once the sharpie is removed evenly from the seat then I'm in business?
pretty much.
 

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Jimbonaut

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Thanks Doc - I got a couple done last night with the guidance of a friend who’s an old hand at this kind of thing. Slow and steady definitely wins the race. I need to check the Clymers and see if it indicates how wide a band I want to see on the valve seating surface (on the valve itself) but I now know it’s not much, no more than 1mm wide I think.

It’s a very satisfying job that’s for sure - the sound of the grinding compound as it changes from grainy to smooth, knowing the improvements that will be felt once the bike’s back on the road and that engine’s roaring away - time well spent.


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Jimbonaut

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Valve lapping. On a long list of moto-related jobs I've enjoyed in the workshop that is right up there. It's just so damn satisfying. Here's a before -



Hooked up a brass wire wheel to the bench grinder, being careful not to brush the actual seating surface too much, or above the join where the valve meets the stem. They clean up pretty good -



Some grinding compound on the valve and its corresponding seat, a minute or so spinning the lapping tool - lifting the valve up and off the seat and rotating it through 180 degrees every now and then - and they come out looking like this -



That's what you're looking for - an even grey ring around the valve. Anyone unfamiliar with the valve geometry (as I was) - note that the seating surface is pretty narrow. There are a few angles to the valve, one of which is the actual seating surface (please correct me if that's wrong), and that's the angle you're lapping. The band isn't wide - from my understanding it's around 1mm. So softly softly catchee monkey kinda deal when you're lapping. I did it by hand - there are vids out there showing guys with drills but that all looks a bit aggressive particularly if it's your first time doing it. Lapping by hand gives a really good feel of the process, plus you can hear the grinding compound breaking down too, which is good.

Here's a before of the valve seat -



And here's an after -



I sharpie'd the valve and the seat after lapping - spinning the valve in its seat then shows any high/low spots where the sharpie has not been ground off. Again, that clean grey ring is a good indicator that your valve is lapped and seating well. Next - rebuild the valves and do the leak test, but that's for another day.

Also, I don't know what I was thinking. Looked at the cylinder block, looked at the gasket set I'd received in the mail and pulled it. I mean, shit. Got this far, may as well do it properly. Block came off without any drama -





- and I could then give the pistons a decent clean -



Decent enough anyway. I'm not going to remove the pistons I don't think, but all the rings are off and bagged - I'll check out the gaps but all being well they're within spec and re-useable. The block will get a hone - would it be a good idea to glass bead clean the block too? Seem to remember reading somewhere that media-cleaning blocks may or may not be a good idea.
 

Maritime

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In for a penny and all that. Looks good. I think you should end up with a good strong runner when done. As for media, the fins etc are ok, don't blast in the jugs, that's what the hone is for and you'll be fine.
 

adventurco

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The Jimbonaut said:
Decent enough anyway. I'm not going to remove the pistons I don't think, but all the rings are off and bagged - I'll check out the gaps but all being well they're within spec and re-useable. The block will get a hone - would it be a good idea to glass bead clean the block too? Seem to remember reading somewhere that media-cleaning blocks may or may not be a good idea.
Looks good Jim. I need to lap in the exhaust valve on the 50. Been a while since I've done one as my last couple projects have needed the seats cut ::)

I'd go ahead and bead the cylinder. I've done it a bunch. Trick is to clean the hell out of it with soapy water after your done blasting (don't want any leftover media in the bores), then blow it off with compressed air to dry it and lightly oil the bores so they don't flash rust.
 

Jimbonaut

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Cool - I'll get the block honed and beaded. I remember the cylinder on the KLR flash rusting almost immediately after cleaning it, but wiping it down with some oil stopped it from going any further. If I buy/borrow the honing tool is it a pretty easy job?
 

adventurco

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The Jimbonaut said:
Cool - I'll get the block honed and beaded. I remember the cylinder on the KLR flash rusting almost immediately after cleaning it, but wiping it down with some oil stopped it from going any further. If I buy/borrow the honing tool is it a pretty easy job?
Yeah, you just want to put a cross hatch on the cylinder walls to break the glaze. Couple of the old in/outs pretty quick should do it.
 

doc_rot

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make sure you clean the piston ring grooves really well. Carbon is highly abrasive and if there are loose bits in there it will break your rings down quickly. Nice work.
 

Jimbonaut

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Cheers Doc - I've been cramming a q-tip with some carb cleaner into the grooves to clean them up as much as possible. Any other methods I'm ears - the pistons won't be coming off the con rods though I don't think.
 

adventurco

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The Jimbonaut said:
Cheers Doc - I've been cramming a q-tip with some carb cleaner into the grooves to clean them up as much as possible. Any other methods I'm ears - the pistons won't be coming off the con rods though I don't think.
What I do when I'm re-ringing is take an old ring (one from each groove) and break it into a small 1-2" piece, file the end down so its not going to gouge the piston and run that along the groove to clean out any carbon.
 

Jimbonaut

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That would do it, but I'm hoping the rings are in good shape and that I can reuse them. Bad idea?
 

adventurco

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The Jimbonaut said:
That would do it, but I'm hoping the rings are in good shape and that I can reuse them. Bad idea?
Not necessarily, the end gap needs to be checked and if they're out of spec you'll need to replace them. Really all the piston and cylinder clearances should be measured while you have the whole thing apart. I pull a cylinder off I will usually re-ring and hone.
 

Maritime

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Only need to re-ring if they measure out of spec on any one of the dimensions. Gap, thickness etc. and those have ranges so if your close to the good side of the range, keep em, if you are close to the almost worn side but still good, consider changing em. And of course if out of spec all together new is a must.
 

doc_rot

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all sound advice.

I had to rebuild a top end twice last year because I assumed the pistons rings would be good to reuse because it had decent compression before tear down. It smoked a bit after the rebuild, despite having great compression and it was leaking less than 1% on all cylinders.

My point is; you don't know its good unless you check. Its not much more effort once you're in there. If you don't want to buy the tool to measure the cylinder bore properly, get cozy with your local machine shop. My experience has been if you go in chat it up a bit and ask some informed questions they will typically go out of their way to give you a hand with your project as so few people work on their own engines in this fashion anymore.
 

Jimbonaut

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Don’t get me wrong guys - I’m absolutely going to be checking the rings, pistons and cylinders. And if anything is out of spec then it’ll be getting replaced/machined for sure.

I’ll see if I can borrow the required tools, and Doc you’re right - I get a lot of help from a few local shops for exactly that reason it seems. I ask a ton of questions, and the pros I know don’t seem to mind taking the time to help me out.

Viton valve seals showed up yesterday - now that they’re here I’ll give the head a good clean again, reinstall the valves, check the shim clearances and get everything ready for paint.


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Jimbonaut

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By a rather circuitous and very fortunate manner I scored a set of cams from a 900F yesterday - I've read that they are a straight swap into the head and give some pretty noticeable power gains (while at the same time a bit of a drop off in the low end). I'm trying to understand how that happens however. Is it because the lobes are machined differently to keep the valves open/closed for longer/shorter? Or are the lobes angled differently to each other on the cam so the valves open and close at different times? Interested to find out.

Also borrowed a tool for measuring the inside of the cylinders, and a valve spring compressor -



It's a beautiful day here in Montreal...this weather holds up, some snow starts to melt and our riding season officially starts in 2 weeks. Ambitious, but hey. Dusting off Rhonda and the KLR for a blat around town sounds gooood.
 

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