Someone asked me about the "press fit sizes" of these valve guides (and other things), so here's my research on it, based on Honda's 1970s production methods (which seem to have worked out pretty well?) :
The heads are made of a zinc-aluminum alloy. The zinc stiffens up the aluminum and (greatly) reduces its tendency to "migrate" when hot. For the sake of the numbers to follow below, we'll assume it is the MOST expansive aluminum alloy 7075 (which it is not, by far, due to that zinc aid), at 13 uinch/in/degree F (that is, .000013" per inch of diameter per degree F).
So, for a valve guide of the 750's OD, which is .4755" OD, at 300 degrees this would be:
(13 * .4755 * .000001 * 300) = .00186", or for most discussions, .002" of growth.
(I chose 300 degrees to work with because that's the temperature of the exhaust "flower" on the front of my 750's head after a hard hiway run at 75 MPH in 4th gear for several minutes on a 95 degree summer day, pulling off the road and leaning over right away to measure it with a high-accuracy temperature gun I used to have.)
For the bronze, this same expansion (using silicon bronze) is 9.0 u"/inch/degree, or
(9.0 * .4755 * .000001 * 300) = .00128", or a little over .001" (to help keep the numbers down).
(I'm an engineer: we love to play these silly number games...
)
Keep in mind: the hole in the head also SHRINKS a little bit from the full expansion number, as the metal around it grows somewhat: this is why rounding that number up from .00186" to .002" is valid: calcs show the real value would be .00189", so I'll call it .002" to throw in some extra "looseness", just in case... ).
So...at 300 degrees, the exhaust valve guide grows (much more quickly than the head) by .001" while the hole in the head might grow as much as .002". Therefore, the press fit must be MORE than the .001" difference in their growth rate. So, minimum press fit should be a little over .0010". Keep in mind: if both holes are EXACTLY the same size, this is still a press fit that will not slip, ever.
Now for the magic of 'the rest of the story' (thanks, Paul H.!)...
The head always carries the heat away much faster (exhaust side) than the heat coming into the guide via the valve stem, so the head hole is ALWAYS smaller than the guide OD, even if they started out at the same diameter. Honda demonstrates that they knew this by using a press fit of just .0002" on their cast iron guides for the exhaust side. You can easily check this out yourself if you have taken an old head apart: just measure the guide OD and the hole ID and you will find .0000" to .0004" (those are ten thousandths of an inch) interference fits there. In other words, if the guides are clean of crud, they just tap out with a hammer after the first stronger blow breaks them loose.
On the intake side: the guides from Honda are usually the same OD as on the exhaust, but the head holes are typically .0002" to .0004" smaller than on the exhaust side. Since the incoming fuel will cool down the intake guides (relative to the head temp) by about 50-80 degrees, it is necessary for this side to be a tighter fit.
But: the old cast iron guides only expand by 6 u"/inch/degree! (Compare that to the 50% more expansive bronze). Yet, these bikes work perfectly with these very light press fits with cast iron guides, so even if the same .0002" fit-up were used with bronze, it would work fine.
So...if you use bronze guides, make them the same OD as the cast iron guides for an easy installation with NO ID shrinkage, or if you are worried about "loose guides" use a minimal .001" press fit. This is light enough to be tapped in by hand using a piloted driver tool (use a .258" diameter inside these .260" ID guides) if you heat the head to, say, 250-275 degrees and spray a little CRC freeze spray (about 2-3 seconds) on the ID of the guide (pre-oil it, please...). In the end, you will find that after the head has cooled with the new guides in place (presuming the APE pre-sized guides, here...) that near the seat end of the guide it will be slightly narrower from the head squeezing it down afterward. I find that if I check the bore immediately after installation using an intake valve stem (0.2595" when new) that they drop right in: after about 2 hours of cooling they have to be honed or reamed a bit as they become less than .0005" clearance nearest the valve seat end. This takes about 3-5 minutes honing with an old, well-worn hone, or 2-3 seconds with a nice, new, sharp reamer in a drill at 350 RPM.
The APE guides are typically the same OD or +.0004" larger than the cast iron guides, based on the last 3 sets I just measured this week. This bodes well for their installation. Their ID is .260", which, if the guide did not shrink at all, would be a prefect spec fit with no post-installation honing. Usually, the exhaust guides need no further work, coming out in the Honda spec'd clearance range of .0012"-.0024" easily. Sometimes the intake sides close up more than .0004" after cooling, or else they, too, would be right on Honda's minimum clearance. So, a light hone on the intakes is often all the APE guides need to finish the job.
APE makes nice stuff!
