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

[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

[calj737]

If that included cutting new seats, that’s pretty reasonable.

[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

[calj737]

Burn oil, foul plugs, run smoky down the road. No pride in any of that-

[70CB750]

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

[754]

You can replace the guides yourself in under an hour.
 A guide driver tool is handy, easy to make on a lathe .
 Warming the head will help.

 My favorite trick , drill guide, then break off on the spring side, easy to do with a drill press and a bit of wood fixturing.
 The plus side, you can do it in a pinch without removing carbon from the port, because you are driving it in the port. Its the carbon remaining stuck to the guide after beadblasting, that wears and scars the bore,  and attempting to pick it out, makes it worse next time.
 BMW actually recommends this method in some manuals  but I discovered it on my own after  seeing a lot of scarred up guide oversize bores on HD heads.

[754]

 The trick to drilling and snapping off,  is the fact that most guides has either a groove for a ring  or an undercut beside the flange on the guide.
 So you drill just under that size and past the grrove, the tap the guide and head comes off.
 It works on most except hard steel guides  .
 There is few things better  (on engine repair) than driving a guide out , and having a scar free shiny bore..

[Jessamine]

Wait, you can swap them without recutting the valve seats? I'm definitely down for that if it's possible.

Thanks for the specific feedback, y'all. I'm gonna stick with this head (already fully disassembled and cleaned, and no other issues), but I will commit to swapping the guides.

1975 CB750F
1986 CN250
2005 KLR650

[754]

Not usually , but if the guides are as concentric as the old ones, its an easy cut.
 A good 3 angle cut is worth doing anyway.
 I think the shop you went to is pretty pricey, look around

[Jessamine]

Not usually , but if the guides are as concentric as the old ones, its an easy cut.
 A good 3 angle cut is worth doing anyway.
 I think the shop you went to is pretty pricey, look around

Okay, that's what I figured. I'll shop around for that labor. I'm still sticking to "if the intakes are perfect, there's no need to replace them".

Last questions for the night: are the intake and exhaust guides different? Is there a good source for just exhaust guides? And would I need to get the first oversize, or the standard? I think the ones in there now are stock and standard size.

CycleX has individual oversize (but not standard sized) ones at $13/ea-- most eBay sellers are asking $20+ per guide for NOS standard (no oversize)


1975 CB750F
1986 CN250
2005 KLR650

[754]

I am not sure you understand concentricity, and runout.
 If the guide bore and guide of are not in the same plane, it is out of concentric.
 Stock guides should be very good,  most after market are too. Sometimes cheap ones are not.
 
You can check your new guides sort of, put them on a clamped valve, place dial indicator on outside of guide, the roll it around one turn. Try that two places,  on the portion that sits in the head.
 If it's zero or very low run out,  then its very concentric.
 If the new and the old had really low runout, then the valve with new guide will sit in near the same spot, but because the old guides had a lot of play, the guide contact area is likely a stretched circle.
 So a bit of material has to come off the seat to get a good seal.

 Oversize guides generally are needed because the hole gets compromised. 
So depending on how you remove them std may be good,  some use .001 to be safe.
 If you did not need the oversize it Nat tighten up clearance and need resizing.. and that is a whole new can of worms..

 I hope that explains it somewhat.

[754]

 Most aftermarket guides are bronze and harder to ream than stock guides.
 How much are stock guides on say ebay ?

[Jessamine]

Oh oh oh-- so it's how the valve sits askew in the old worn guide that changes the angle of the seat over time, and so when you replace the guide the valve now sits at a different (proper) angle, which won't match up with the seat anymore, so you have to cut it anew to ensure concentricity? Is that right?

1975 CB750F
1986 CN250
2005 KLR650

[Jessamine]

$20/guide on eBay for NOS Honda. These look a bit different from mine though-- pretty sure mine are brass

https://www.ebay.com/itm/NOS-OEM-Honda-Exhaust-Valve-Guide-1972-77-77-CB750-PN-12027-300-330/222600882692?epid=1411801766&hash=item33d40bea04:g:0NQAAOSwlxhZggqz:sc:ShippingMethodStandard!19143!US!-1

1975 CB750F
1986 CN250
2005 KLR650

[754]

 Pretty much.
 In the case of other bikes, where a lot of people make stuff for them, some guides are cheap and not close to inline.
 That means a lot more grinding or cutting to so the valve job. Do that a few times and the seat is now lower.
 Not a huge problem on our bikes.. but I probably would not buy really cheap guides if they were out there and made in China.

 If yours are yellow color likely bronze and been changed before.

[Jessamine]

Pretty much.
 In the case of other bikes, where a lot of people make stuff for them, some guides are cheap and not close to inline.
 That means a lot more grinding or cutting to so the valve job. Do that a few times and the seat is now lower.
 Not a huge problem on our bikes.. but I probably would not buy really cheap guides if they were out there and made in China.

 If yours are yellow color likely bronze and been changed before.

Actually, now that my partner and I are finishing up the deep cleaning on the head, I think they just appeared yellowed because of old oil residue. Looks like iron underneath-- Hooray!

Put in an offer on four NOS guides on eBay for $60 instead of the asked $80. We'll see what the seller comes back with.

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

You can replace the guides yourself in under an hour.
 A guide driver tool is handy, easy to make on a lathe .
 Warming the head will help.

 My favorite trick , drill guide, then break off on the spring side, easy to do with a drill press and a bit of wood fixturing.
 The plus side, you can do it in a pinch without removing carbon from the port, because you are driving it in the port. Its the carbon remaining stuck to the guide after beadblasting, that wears and scars the bore,  and attempting to pick it out, makes it worse next time.
 BMW actually recommends this method in some manuals  but I discovered it on my own after  seeing a lot of scarred up guide oversize bores on HD heads.

I did not like the idea of hammering guides in.  Much easier to drive them using a bolt, adapter and a washer:

http://forums.sohc4.net/index.php/topic,115305.msg1452240.html#msg1452240

[Jessamine]

Good news/bad new update... The new exhaust valves fit way more snug in the guides than the old ones-- they must have been worn along the stems somehow. These new ones measure 0.06mm thicker at the stems than the old ones, which puts their deflection well within tolerance.

Good news is I don't have to swap the guides anymore, so I don't have to pay for machining on the head.

Bad news is I already bought a set of NOS exhaust valve guides on eBay. Whooops. Guess I'll be throwing those up in the classifieds board later.

1975 CB750F
1986 CN250
2005 KLR650

[Jessamine]

Long day of prepping the head today, and (now that I know the guides are good) lapping in the new valves! I picked up a cheap variable rotary tool from Harbor Freight and used a set of brass wire wheels extensively, and a stainless steel one sparingly, on the exhaust ports to clear out the carbon deposits.

Once I got some of the gunk off I discovered there were two crush gaskets on every port, a wafer-thin copper one underneath and a nigh-indestructible layered steel one atop it. It took a lot of careful chiseling to separate them from the head, but I think the port openings cleaned up nicely in the end. 

The valves were very satisfying to lap in (first time!) And seem to have a tight fit in the guides and a good seal. It took extensive compressed air, careful scrubbing, and multiple baths in a Simple Green solution to get all the grit out of the ports. The Permatex compound I picked up was a little runnier than anticipated, but it seemed to do the job well.

I'm super excited to reassemble everything, and will update when it's back together!

1975 CB750F
1986 CN250
2005 KLR650

[70CB750]

You go, girl!

[Jessamine]

Quick question--
I'm finishing up my reinstall of the head and camshaft, and I cannot for the life of me get the timing marks to line up properly! TDC on 1-4 appears to come about 5-10° late (left) of the T mark on the plate. And with my camshaft notch at 12 o'clock, and index marks lined up on the camshaft holder, I can't get the timing and index marks to line up, even by moving the cam chain on the sprocket one tooth at a time. Anyone have a guess as to why?

Best setting I can coax out of it appears to set it right between the F and the 1 (photo attached) when the camshaft notch is at 12 o'clock. One tooth over results nearly in alignment with the spring retainer pin left of the T mark.



1975 CB750F
1986 CN250
2005 KLR650

[Don R]

 Is the tensioner in place and adjusted? Slack in the chain may cause this. Hang in there, half the fun is learning the in's and out's of these old beauties. When it's finally running good remembering all this will make your smile bigger.  I had an F0 that turned out to have a nasty chassis twist. It had been in a bad wreck and I switched to a K0. Later I got an F1, it was probably my favorite but it tested me a lot, all of the jets had been drilled and the float levels were as high as possible, I still miss it but need to convert it into money to pay for a Sandcast formerly owned by one of the local Honda Guru's.