Topic: Ill-Advised Super Sport (CB750-F0)

[SOHC4 Cafe Racer Fan]

At a minimum rehone your cylinders.  They should not be smooth but have a uniform cross-hatch.  You can probably avoid an overbore.

There's still some cross-hatching visible, but they are getting pretty glossy. I was mostly impressed by the lack of scoring. I've honed moped cylinders before, with a power drill honing attachment... do I have to get more technical with these?

1975 CB750F
1986 CN250
2005 KLR650

You should be able to hone with a power drill and attachment.

[754]

She  checking for bent valves on a together motor  just make an air adaptor to the plug hole.
 If at TDC on compression you should hear it if a valve is bent..

[Jessamine]

Actually, I tore the head down because it failed the compression testing. Right now it's sitting dismantled, waiting on a new set of valves and seals. As far as I can tell, against all odds, the valve guides are undamaged. But yeah, nil compression on 1 and 4, and only 3 was retaining fluid properly when I flipped the head to check. And the old valve seals fell into tiny pieces as I pulled them off the guides, so it was definitely their time to go.

[754]

Well at least check guides for cracks..

[Jessamine]

Well at least check guides for cracks..

I mean, visually they're perfect. So unless it's something I'd only be able to see by pulling them and inspecting the outsides of the guides, they're not cracked. I'll be checking deflection with the new valves to make sure they're within tolerance before I commit to reassembly, of course.

1975 CB750F
1986 CN250
2005 KLR650

[754]

I am pretty sure if they are damaged, slipping a new straight valve in, will have at least slight resistance.

[70CB750]

If you can keep old guides, that would be an easy out.  Just lap in new valves and put it back together. 

[Jessamine]

Alright, went to town measuring and this is what I found out, using the truest of the old valves (#3's) for measuring:

Intake valve guides are all perfect.

Exhaust valve guides are all evenly worn, with 0.011" deflection across all axes. Manual says the service limit before new valve guides should be installed is 0.004".

Is this enough of an issue that I need to shell out the money and to take it to a machine shop to get new valve guides installed and have the reaming/seat cutting done? I'm poor and stubborn about having other people do work on my bikes.

Also:
What is the stem size I should be seeing? My valves are measuring at 6.60mm Intake and 6.55mm Exhaust.

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

My $0.02 ok?

I would either lap in new valves - all eight and put it back together.

Or

Replace all 8 guides, new valves, cut seats.

Replacing only exhaust guides seem like a bad idea to me.

Some people send heads to MRieck for rebuilding, he is the best.

[Jessamine]

I sent a quote request to a shop in Upper Darby that does head work and I'll see what they come back with. It's all about how much money this would put me out, honestly.

Would I just be ordering one size up on the guides, if they need to be oversized when replacing? Is that something the machine shop would do, or is that on me if I've got all the other parts ordered?

[754]

At this point I would look around  for a low miles head on ebay, should be around 100 bux.
If you are lucky you get away with lapping.

 Not sure how the manual says to check valve to guide clearance, but if it's worn, it should be front to back, not sideways. If valve is sticking out to far, like say upside down, the clearance would be higher.. I have to look at the book see how they hold it to check.
 In many bikes, there are oversized guide, but I suspect not for these. They are usually needed due to rough guide removal.. time is money.
 I know a way you can get them out clean without buggering the hole oversized I can run you thru that In  a future post.

[Jessamine]

At this point I would look around  for a low miles head on ebay, should be around 100 bux.
If you are lucky you get away with lapping.

 Not sure how the manual says to check valve to guide clearance, but if it's worn, it should be front to back, not sideways. If valve is sticking out to far, like say upside down, the clearance would be higher.. I have to look at the book see how they hold it to check.
 In many bikes, there are oversized guide, but I suspect not for these. They are usually needed due to rough guide removal.. time is money.
 I know a way you can get them out clean without buggering the hole oversized I can run you thru that In  a future post.

I've been keeping an eye on eBay. I've only seen mystery mile K heads (no F) without many details in meh condition.

And I was measuring the deflection as indicated in the manual-- valve most of the way inserted from the cylinder side, just pulled out 3/4" or so above the seat, pressing on the edges of the valve face to check for play in the bore of the guide.

1975 CB750F
1986 CN250
2005 KLR650

[754]

77, and 78 750 K heads should work  ...winter is coming dont get in a rush..

[Stev-o]

I usually agree with Frank but not this time.  Buying a used head is risky at best, unless it is from a reliable source.  Most sellers on ebay would not be able to honestly tell you if the head is within specs.

Its been my experience on my last 2 engine rebuilds that getting a proper valve job is the key to having a good running motor [and not having a huge hole in the case helps too!]

[SOHC4 Cafe Racer Fan]

Stay away from MMC on ebay.  His parts can be sketchy.  Go with an F or 77-78K head -- I would ask someone reliable like Bill Benton if he has one in good shape.

[754]

I meant look for a head that is like from a motor with say 10,000 miles . This time ov year you got time.
 Sounds like she is sticking to a budget .
 Buy I g a 410 head for 140 to 200 , then doing a valve job m May be a good head but not a budget head, he'll she can just put guides in hers and buy a few. valves.

 Like we say in drag racing ...3200 for billet heads, doesn't sound bad if you say it real fast..

[Jessamine]

I ride year-round though
I don't intend to rush, but I'm not going to wait months for the right part to show up.

1975 CB750F
1986 CN250
2005 KLR650

[754]

Never hurts to look, low mile engines sometimes take a heavy hit to the cases..  broken cases motor do get parted out.

[70CB750]

Check with Bill what he has.

[Jessamine]

Got a quote on guide installation from a local machine shop-- $200. Ouch.

1975 CB750F
1986 CN250
2005 KLR650

[Stev-o]

Got a quote on guide installation from a local machine shop-- $200.

I wouldnt go back there

[754]

Ok maybe try this, put an ad up non here for a used checked head.
 At least you know what you are getting.

[Jessamine]

To clarify, It'd be $200 for the installation of 4 guides I provide, and cutting 4 seats to match. Nothing else.

1975 CB750F
1986 CN250
2005 KLR650

[Jessamine]

Back to my original question:
the guides are out of spec, yes, but how much of an issue are exhaust valves with a little too much deflection? What's the downside/risk?

They all seem evenly worn regardless of the valves' condition, so I think it's safe to conclude this is typical wear and tear over time from being ridden, rather than acute damage from the cam sprocket coming loose.

If, worst case, it's going to steal a tiny bit of compression at high RPMs, I'm not going to invest more time/money to get the valve guides perfect. I take pride in my work and I enjoy riding fast bikes, but I'm not building a racing machine.

If it's risking further component damage or will make the engine unreliable, then I'd be concerned enough to invest the extra time and money, even if I have to wait to finish it.

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

I said it before. Replacing half of the guides is a half ass job. Your opinion may vary