Here's some sample numbers from valve guides I recently installed (bronze type):
Valve guides measurements were taken at the middle of the installed portion of the shank, on an 85 degree summer day, as the 'baseline' value.
Left in freezer for 30 minutes: shrank OD .0003" to almost .0004" (this freezer is -20 degrees F).
Left in freezer just 10 minutes: same as above
Freeze-spray applied for 2-3 second blast: shrank .0001-.0002" tops.
Head holes (for the guides) were measured in the shade at 85 degrees, then the head was set in the hot Colorado sun for 30 minutes (too hot to touch with bare hands afterward).
Holes grew about .0001" to slightly over that, but not .0002".
Put head in oven at 250 degrees for 15 minutes: Holes grew .0003" on average.
So...if you were installing cold, frozen valve guides into a hot, oven-baked head, the extra clearance gained would be about .0004" to .0007" overall. While this doesn't sound like much, when installing a brass guide it is enough difference to prevent the guide (soft bronze) from distorting. If these guides are simply driven into the head holes like we use to do with Honda's cast-iron ones, they tend to distort (misalign) and require more machine work to make the valve seats seal like they should.
If you are installing steering head bearings, they will shrink a little more than these numbers because they are larger. And, it does help to install the races into the steering head, especially if you have the head sitting out in the summer sun. Grease 'em first, in any case.
