Topic: Few questions on installing valve guides

[70CB750]

Precondition:  The head is clean, old guides are out, I will be installing bronze APE VGH750 guides, already have the tool to install.  Outsourcing is not possible.


I searched over and over the forum and web and still have few questions:

- Preheat the head.  There is a school of thoughts to skip the preheat, but since I will be using drifter and hammer, I think preheat is a must.  350F seems to be the maximum safe temperature. 

- Prefreeze guides. Unlike the oven, the regular kitchen freezer is not capable to make a difference in the guide size.  Scratch the prefreeze.

- Smooth guides.  I would buy that, polishing guides with what?  Scotchpad?  Buffer and paste? 

- Clean and polish guide holes.  With scotchpad again??

- Use copper anti-seize before you drive guides in.  Agreed, can only help.  Does it have to be copper?  How about regular anti-seize like the one for spark plugs?

- Measure old guides and compare with new guides.  Will do.

- Bronze guides may need honing.  Honing with what?  6.5mm flex hone brush?


Anything missing?  Comments, opinions?

[flybox1]

will be doing this myself, shortly.
subscribed.

[edwardmorris]

Hoping I won't need to do it, but always good to be prepared, subscribed.

[HondaMan]

I heat the heads to about 250, as 350 is too much for me to carry to the garage before my gloves get too hot(!).
Then I oil up and freeze the guides with freeze spray (CRC brand, works in 10 seconds), set it in place, tap it in. Don't stop! If it gets stiff, finish the trip and you can hone it later, as the sticky ones mushroom a tiny bit just above the spring deck, pinching in the guide right there.

After that, I run a .259 reamer thru instead of a hone, as this makes a much straighter hole. Then I use the hone to finish it to size, only takes a max of 10-15 seconds. If you try to hone the crunched ones, they don't come out straight as the hone can't cut a straight side.

[tomkimberly]

If you can get some dry ice, it gets cold enough to work, make sure you wear gloves when handling dry ice.

Tom

[70CB750]

Thank you, Mark, makes sense.  Does the cutting part of the reamer need to be longer than the guide? 

.259" translates to 6.58mm, means you have 8 hundreds play on the valve stem when all is said and done, assuming the valve stems are dead on    Seems like a lot, from what I remember max/min play on valve stems is something like 0.060/0.025mm.

Sorry for all these questions, I am attempting to get it right the first time 

[70CB750]

More found in the archives, more food for thoughts:

MRieck:

--- Quote from: Hockers Choppers on May 21, 2007, 08:54:06 PM ---What kind of tolerance is TOO tight? I'm working on my cb750 (again) Last guy reamed out the guides to .261, way too loose( smokes at high rpms ) , book said .260, I've been told .259 and hone. If it goes in and moves is it not too tight? Where do you get the right size of guide hone?

--- End quote ---
Jesse, the intake guide is honed to .001 to .0012 clearance. The exhaust .0015 to .0017. This is for aftermarket bronze guides. My machinist hones them on a Sunnen horizontal hone with the appropriate small bore hone(s). I have seen vertical hones which are basically a hand held drill with a couple of U joints that allow the hone to "float".  Don't ream those aftermarket guides...hone them. It leaves a better (oil retentive) finish on the ID as well.

Intake clearance: 0.0254 - 0.0304mm
Exhaust clearance: 0.0381 - 0.043mm

[HondaMan]

Thank you, Mark, makes sense.  Does the cutting part of the reamer need to be longer than the guide? 

.259" translates to 6.58mm, means you have 8 hundreds play on the valve stem when all is said and done, assuming the valve stems are dead on    Seems like a lot, from what I remember max/min play on valve stems is something like 0.060/0.025mm.

Sorry for all these questions, I am attempting to get it right the first time 

Mostly these valve stems measure 6.58 (.2586"-.2590") on the exhaust and 6.60 (.2598"-.2599") on the intakes, once they have worn off their OEM finish: if they are less than this, they won't be less AND STRAIGHT because of some odd wear pattern. Honda calls for .0005" to .0012" clearance intake and .0016"-.0024" exhaust, so using the .259" reamer is the 'safe' bet, followed by the hone.

The manufacturing target for Honda appeared to be that they could use just one reamer size (.260") to fit both guides, which was pretty typical of their techniques in those days. As the reamer wore down from .260" they accepted it until the clearance was only .0005" for the intakes: my K2 (with K1 engine leftovers) was made to these real tight tolerances, and I'd have to say it really liked it!

Once the clearance exceeds .003", the valves start running hotter and hotter for lack of contact to cool them off into the head. This makes them wear out real fast, once they pass the .003" point. After about .005" they start letting oil get sucked by the intakes and make a mess!

[70CB750]

Thank you again, and thank you for your patience 

[lucky]

Reamers should be run at ONLY 100 RPM!!  If possible.
There are slow speed reamers and high speed reamers.
Spiral and straight.

[70CB750]

Thank you, lucky.

I dont like to hammer things, so I made this washer/adapter for pressing in the guides. I tried it with one of the old ones and it worked. So here it is, pressing guides in with 4"of 1/4" bolt, aluminum adapter and some washers.

[70CB750]

The head is cold, guides are oiled, the adapter sits in the valve seat and is softer that the valve seat. Guides slide in with little bit of effort, slowly but surely.

[70CB750]

See, it did not hurt one bit, miss, and you were so afraid

[750K]

Haha, clever. I'll remember that one, reminds me of a home made bushing tool I made for an old car.

[70CB750]

Fine thread 1/4" would work better, but it was still easy enough.