Topic: Valve guides: something to watch out for...

[HondaMan]

I have my F2, sucking oil down the outside of the guides puffing smoke, so I need to sort this. Long story.
I have a set of aftermarket guides but they weren't used as the OD is at least 0.01mm smaller than old OEM guides I have here, they virtually drop in. Not happy. So, my question is can anybody tell me the differences between the K series (Yamiya style) guides and the F2 guides? they have a different part number. Yamiya don't specifically supply F2 bits.
Are they different length, material, circlip position? Finished ID is the same, Valve stem seal is the same part no (xx-300-xx). Could I use Yamiya guides? (as the slight OS would be a good fit in my head (cylinder that is....) Thanks

I have seen where the F2/3 guides wear quickly, due to several circumstances I won't go into here, but which were highly relevant to this issue. Then the guides got changed, often in a typical 1980s bike shop where the old ones were beat out, new ones pounded in, reamed until the valves would meet the seats (which was usually seriously clearanced in the guides then) and sent back out the door to soon smoke some more. Happily, that particular shop is gone from this area, went out of business.

But, the practice continued, and the F2/3 and K7/8 engines locally were the most affected by it. This action leaves the guide's holes oversized, sounding much like yours. I still have one of those heads here (F2) and a set of Kibblewhite's bronze oversized guides for it, which looks like it will fix things up OK: the bike owner just scrapped the rest of his bike after he sent me the head with the guides removed, oversized holes in it from the multiple guide installations, and decided to get another bike instead.

I never put it back together as it had no owner, but Kibblewhite's guides might save your day?

[HondaMan]

Back to the resolution of the original head in this post:

the 2nd set of Yamiya guides arrived, and 2 of them were as oversized as most of them in the first set were, a little more than 0.0020" larger at 0.0022" OS. The other 6 were uniform at 0.0018" larger than the ones originally removed from the head (which were OEM). Since the holes had already been exercised by the replacements prior, these new ones pressed into the heated head just fine. The sizing went OK with only minor material removal, with the guide in #3 intake showing an untouched segment near the middle of the bore, a little bit toward the upper side above the head where the hone found nothing to touch. So, the ID was not perfect in that finished bore, like it is in all of the APE guides I have previously installed.

I leave you to draw your own conclusions about these guides: I will not use them again in my work. I have hundreds of good-running CB750s in my history, all which were done as I describe often, with not a single failure.

[brianc]

Thanks Hondaman. All the info here is great, the OD photos confirm mine are well undersize. You gets what you pays for is the moral here.
What is going to help at this stage is to know how ANY K7/K8 F2/F3 exhaust guide differs from the usual K series. Maybe is it the overall length or specifically the circlip to top of guide dimension, to allow for the different cam? If they are just different length, I have a good lathe here just itching to make swarf. Thanks
Edit... the top of my guide is 16mm from the spring seat surface and the overall length of the guide is 46.5mm if that helps.

[PeWe]

Ask forum member MRieck about valve guides to F2 head.

[brianc]

OK, for what it's worth I purchased a set of guides for the K series 750 so I could compare to the F2 750 guides.
Good on the OD, about 0.04mm larger which is a good thing.
They are approx 2.1 mm longer (from the circlip position down). And 0.3mm higher (circlip position up).
So, I've machined 2mm off the bottom. redid the taper a bit and left the top.
I assembled the cam bearing, cam, rocker and valve to make sure of the retainer to original guide clearance at full lift (standard cam).
It came in at about 6.6mm, so the extra 0.3mm isn't a problem.
My man has redone the guide fitment and he's happy with the tighter fit.
See how it goes now!

[HondaMan]

The fit is the important part. I don't know (even now) why Honda changed the guides used in the F2/3 heads. They were cheap, simple cast iron and shorter than the previous guides of similar iron, which will fit without troubles: I've done quite a few of those. There wasn't enough difference in those 'new' guides to affect performance (flow in the ports), and the guides in the K7/8 heads of the same era were the same ones used in the earlier engines. My guess is that Honda made a separate team of guys to "tweak" the F2/3's design to try to perk up the F2's flagging sales as they also brought out the K7 to meet the requests of the American public for touring-like machines: the Gold Wing wasn't cutting it all that well then because it was so much bigger and heavier. The F2/3 guides, especially the exhaust side, wore quickly when the extra-hot engines were run with the 10w40 oils that American Honda was trying to force Honda Japan to use in the bikes - because Kawasaki was doing it since their Z1.

Of course, Kawasaki had DOHC heads for almost zero side-load to the valve guides and their crankshafts only lasted about 10k miles, but we won't discuss that...