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Old 07-07-2013, 08:57 AM   #1
TheRadBaron OP
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Joined: Jul 2010
Location: Central Illinois
Oddometer: 403
New valve guides. Too much intereference fit?

I'm rebuilding the motor of my '78 Yamaha XS650 and it's pretty much done accept for the cylinder head. I found that one of my exhaust valve guides was scored up and the rest were on the loose side, so I decided to replace all four while I had the head off. I'm an experienced mechanic on these old Japanese bikes but I've never changed guides before. I did a lot of research on here and other forums about what's involved in the process and got comfortable with the idea. I have a greybeard ex-racer and mechanic buddy who has the experience and the tools to ream the new guides and do a valve job.
I found a really good deal on a complete set of all four guides, NOS Yamaha in 1st oversize. I bought them figuring that they'd work since the Yamaha manual states that OS guides should always be used. I know that it's possible to use standard guides in a lot of cases, but I liked the idea of the OS ones and the price was right.

I beadblasted the old guides to clean them up and knocked them out with a proper drift after heating the head in the oven. They came out without any trouble. Using an expanding bore gauge and transferring the measurement to a micrometer give the diameters of the valve guide bores in the head as being about 15.03mm for the exhaust bores and 15.01mm for the intakes. My new guides measure 15.09mm for the exhaust and 15.15mm for the intake (weird since I though 1st oversize should be 15.1mm). Anyway, this gives me an interference fit of 0.07mm for the exhaust and 0.14mm for the intake.
The manual specs a 0.04mm interference fit, so I suspect that this might be too much. Especially for the intakes.

What route should I take here? I thought about using a brake cylinder hone to open up the bores in the head slightly, but the last thing I want to do is get the bores off axis or something. I know it would also be possible to turn down the diameters of the guides slightly, but I don't have axis to a lathe and I'd rather not have to pay a shop to do it. Anyone have any tips or insight? Thanks.
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Old 07-07-2013, 09:42 AM   #2
Langanobob
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Joined: Dec 2006
Location: Reno
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It's hard for me to get an accurate measurement with expanding bore gages and a mike - you have two steps and two opportunities for error. If it were my engine, the first thing I'd do is take the head and guides to a good machine shop and ask them to measure the interference fit. I'd also confirm what the first OS Yamaha guide OD is supposed to be to make sure your new guides are in spec. Not likely they aren't but worth confirming.

If it does turn out you have too much interference, then I don't have a great solution. If I didn't have a lathe as a last resort I'd probably chuck the guides in a drill press and carefully remove material with emery cloth. This takes finesse and careful and frequent measuring but can be done.

Bob
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Old 07-07-2013, 11:53 AM   #3
Stan_R80/7
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Location: VA
Oddometer: 841
I would measure the diameter of the old guides that were removed. Then compare the new guide to the old. Is the oversize for the valve stem (bore) or head or both? Could you have received incorrect guides?

Another way to measure the bore (often done with measuring headspace in firearms) is to cast a low melting point metal that shrinks, such as Cerosafe: http://www.brownells.com/gunsmith-to...y-prod384.aspx then measure the OD of the removed casting.

Chilling the guides (in a freezer or with liquid nitrogen) and heating the head (~ 400F) should allow for easy installation of the new guides. If not, the interference fit is too much. The manufacturer should state very clearly the interference fit dimension - although 0.04 mm (0.0016") is not much of an interference fit.

Also, someone with a metal lathe can make new valve guides or turn the oversize ones down. Making a new head is much more expensive. So, I recommend working on the inexpensive parts and leave the expensive ones alone. Good luck!
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Old 07-07-2013, 12:16 PM   #4
Beezer
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.07mm... thats about .0027".... should be able to make that work, but kinda at the edge. .14mm is almost 6 thou.... that will probably NOT work on a small head. I would not try to force anything over about 2 1/2 thou. I used to do aircraft cylinders & .0025 is about right for intakes, .0045 for exhausts, but they are twice the size. heat the head to 600, freeze the guides (rubbed with light oil) and don't wait around when hitting them home. I have a converted slide hammer to drive with. if they are cast iron guides be sure to get the biggest temp difference you can. if too tight they will crack when you drive them. bronze guides... no problem. taper & smooth the combustion end of the guide if you can. if they don't go right in, figure out why.

any time the guides are replaced the seats will need to be trued to the stem axis... it will always be off slightly. if you decide to trim off the guides, do it in a lathe with a collet head or you'll have big problems getting centered.

Neway seat cutters rock...


Beezer screwed with this post 07-07-2013 at 12:27 PM
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