New valve guide install. Too much interference fit?

[TheRadBaron]

I'm rebuilding the motor of my '78 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.

 

[TwoManyXS1Bs]

In the old days, few shops heated the heads before driving guides out, took too much shop time. This opened-up the holes, so old documents seem geared toward the older shop methods: Beat out the old, beat in the new. If you're doing this correctly, by heating the head and gently removing/installing, you can probably use stock size guides. There's a couple good threads here on this, posted within the last 3 months, I just can't find them...

 

[jd750ace]

If you heat the head to around 240 degrees and deep freeze the guides before installing you can probably make that 0.07MM That's just under .003" of interference. Not enough that you can't work it with differential expansion. We do close to that all the time with aluminum plugs for repairs, and we don't heat the base part, we just throw the plug in liquid nitrogen. The coefficient of linear expansion works both directions, so a sub-zero part going into a 240 degree head gives you around 260 degrees of spread (assuming you have a deep freezer available, not the one in your fridge, they don't get as cold) If you don't have one, you can get way down there with dry ice in a Styrofoam cup with denatured alcohol. It won't freeze the alcohol, it just gets a little thick and syrup like. Works very well for shrinking parts. It will boil for a while when you add the alcohol. Let it stabilize before introducing the part (it will boil again for a while. When it stops boiling, the part is as cold as it can get. We generally attach a long hemostat to the part. Use gloves and goggles. Freeze burns hurt, and the slurry alcohol can freeze and permanently damage your cornea in a single splash.
$0.02