1982 CB750F...Better Devil

Jimbonaut

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Got the call from the machine shop, head's now looking spiffy after a glass bead clean. They also repaired the buggered spark plug thread, and cleaned up the others which, to be fair, were grateful for the attention -



Not sure what they're called but the little wells that the valves sit in look brand new -



- and that can only be a good thing. Waiting on my Viton valve seals to show, then this puppy gets rebuilt. Also discovered that the overhead cam chain tensioner (the bowed plastic part) has a huge crack in it. So turns out pulling the head really has been a good idea.

Fucking hell it's cold in my garage. Quite frankly a witch's tit sounds cozy compared to my garage. Shnuggling up in a witch's cleavage sounds absolutely horrific whatever the situation, but definitely warmer than the absolute zero that is my garage. Fuck.
 

Jimbonaut

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irk miller said:
It doesn't look very cold.
No? I guess that's something then. At least I'm not transmitting the deep freeze through the wonders of the internet.
 

Jimbonaut

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It's fucking madness Mike is what it is. "Divide it east to west, it'll be brilliant" said no Canadian ever.
 

Jimbonaut

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I thank god for lots of things this time of year - thermal underwear, bourbon, eventual snowplows...but this morning, receiving photos of a mate's holiday villa in sunny Costa Rica via WhatsApp, shit. I'm not thanking god for WhatsApp today.

Now that the head's all pretty it's time to reinstall the valves. This'll be the first time I've done this so before I get started does anyone have any tips or must do's (or do nots)? The valves have all been numbered so I know where they all go - first thing I gotta do is lap them. I'll pick up some paste today - are they all pretty much the same? I can get this from Canadian Tire -



- and also the little suction cup tool. Anything I should know, I'm absolutely all ears.
 

Maritime

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Do not over do it. Honda valves have a coating to harden them and if you grind it all off they will wear exponentially faster. I would install them and if they seat without leaking, don't even lap them. if they seep a little, lap only those ones.
 

Jimbonaut

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Maritime said:
Do not over do it. Honda valves have a coating to harden them and if you grind it all off they will wear exponentially faster. I would install them and if they seat without leaking, don't even lap them. if they seep a little, lap only those ones.
"Seat without leaking" - from what I've learnt, this can be checked by installing the valves into the guides (no seals, springs etc), filling the combustion chamber with water, press down on all four valves, and then blow air into the intake/exhaust port. Any bubbles will show where the valve needs to be lapped more. Is this the (or at least, a) way to check the seats?
 

adventurco

Nick Ol' Eye
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I've always done isopropyl in the ports, and see if they seep. I do it before I take the valve springs and keepers off.
 

irk miller

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^ That's for testing leaks on the valve seats. For the guides, I stick my finger over the seal and pull the valve out. It should make a *pop* off your finger.
 

Jimbonaut

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irk miller said:
^ That's for testing leaks on the valve seats. For the guides, I stick my finger over the seal and pull the valve out. It should make a *pop* off your finger.
Actually Irk that's what I'm testing for - leaks on the valve seats. The head has been cleaned (as have the seats) but the valves, not so much. They're pretty cruddy. So my understanding is that lapping the valves will ensure a better seal in the seat. To check for good seating I've been told the process I mentioned earlier is a good one. Would you guys agree?
 

Jimbonaut

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Also, ordered an Athena gasket kit from vintagecb750.com last night, and found out that they're based here in Canada. Bingo. No duties. Colossal win.
 

irk miller

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The Jimbonaut said:
Actually Irk that's what I'm testing for - leaks on the valve seats. The head has been cleaned (as have the seats) but the valves, not so much. They're pretty cruddy. So my understanding is that lapping the valves will ensure a better seal in the seat. To check for good seating I've been told the process I mentioned earlier is a good one. Would you guys agree?
I've always tested leaks on the seats the way Nick described- with springs and everything installed and 90% isopropyl in the ports, then checking for seepage.
 

Jimbonaut

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Right on. Thing is with that method, wouldn't it mean installing everything, finding a leak, then taking it all apart again? The air-blow method seems a bit more immediate in terms of results, and it doesn't involve installing everything.
 

adventurco

Nick Ol' Eye
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The Jimbonaut said:
Right on. Thing is with that method, wouldn't it mean installing everything, finding a leak, then taking it all apart again? The air-blow method seems a bit more immediate in terms of results, and it doesn't involve installing everything.
Yep, that's why I check for leaking valves before I disassemble ;)

Never heard of the method you're talking about but it may work?
 

Jimbonaut

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advCo said:
Never heard of the method you're talking about but it may work?
It better! ;D

Wife's step father gave me a Makita air compressor which he bought for some framing project or other which never happened. It's not got a huge capacity but it definitely blows air - bought a small air gun and will put this method to the test. I'll see if I can rig up some kind of video too.
 

doc_rot

Oh the usual... I bowl, I drive around...
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A tried and true way to check the seat while youre lapping is to use machinist dye on the seat. as you lap the valve it will wear off showing the high spots, lap until you have an even ring. You can also use a sharpie if you don’t have dye
 

Nybz

Member
I found the air gun to not be accurate enough to check for leaks.
After replacing pitted valves faces and lapping only the one that were leaking, I had installed all valves, springs and keepers, filled the combustion face wth water and I blew about 40psi air up in the ports and saw some bubbles in pretty much all of the valves...some a lot and some not so much.

After I filled the faces with alcohol and left them for a few hours and only found one or two valves that were leaking and needed alittle bit more lapping.

Make sure you get allllll that lapping compound cleaned out...trust me you don’t want that running in your cylinders.

Also with springs and everything installed try tapping the end of the valve with a soft hammer to open and close the valve a few times before testing for leaks. If anything is under there, it can give you trouble.


Sent from my iPhone using DO THE TON
 

Jimbonaut

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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?
 

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