Topic: When did cb750f switch valve guides?

[rheilman]

Hi, considering buying a nice looking, relatively low mileage 77cb750f...
I believe that year would make it an f2. 

I know this was right around when Honda switched to valve guides that had problems with quick wear.  Sounds like the earlier 77 models were most suspect, does anybody know which serial numbers are likely to have the poor valves? 

I have also heard that Honda acknowledged the problem and offered to fix the affected bikes... Will they still fix them? (I know that's a pretty long shot, but maybe someone here has had experience...)

Any other peculiarities with these engines? I've heard they can be less reliable than other years, but don't know if that's just due to the valve guide issue..

Any thoughts or advice truly appreciated.   
Ray

[bruzer59]

  no honda will not be repairing a 36 year old motor,,, i too am in the same boat ,worring about when these guides will fail !!!!!!!!!

[Don R]

The switch to softer guides was in 76, mid year I think. 77 and 78 F have the reputation of valve guide problems. The severity depends on the oil used and maintainance.
If it's a nice running bike and doesn't smoke buy it.

[754]

I think you have to change them every time you open it up..

[HondaMan]

The most-affected engines were the F2/3/K8 and the late K7. Many of these also had smaller valve stems with their cast-iron guides. In particular, the black-painted engines, with their attendant 25 degree hotter oil, wore them out the fastest, often in less than 25k miles.

Today we can get bronze guides to replace them, which helps a bunch.

The K0-K2 engines, and very early K3 engines from the "old factory", had Stellite guides on both sides of the head. These are the legendary engines that you hear about with over 100,000 miles and no teardowns. Interestingly enough, there are occasional instances of Stellite guides showing up in the early F0 (1975 "F") bikes, too. I think OFreen might have gotten one of these, as he has more miles on his virgin 'F' than anyone I've even seen! 

I recently saw a head that was from a '75 "F" that sure enough had factory Stellite exhaust guides in it. At first I thought it was a K1 head until I looked closer. It has both Stellite AND valve seals! Even after 40 years, Honda still holds little surprises...

[rheilman]

Thanks for the info, especially the detailed stuff from Hondaman...

So is the general consensus that the 77 78 cb's are best to be avoided, or are they pretty solid other than the valve guides?  Sounds like there is a good chance that the one I'm looking at falls into the period of poor guide material...

Also heard that poor quality valve retaining clips contributed to the excessive wear of the guides on these heads... Is this true?  Can the life of the top end be extended by switching to better retainers without going in for the guides themselves?

Does the extra heat of the black engine make for extra trouble?  Am i going to be wishing that i had just held out for An "old factory" bike? 

Thanks for any advice or info,
Ray

[HondaMan]

Thanks for the info, especially the detailed stuff from Hondaman...

So is the general consensus that the 77 78 cb's are best to be avoided, or are they pretty solid other than the valve guides?  Sounds like there is a good chance that the one I'm looking at falls into the period of poor guide material...

Also heard that poor quality valve retaining clips contributed to the excessive wear of the guides on these heads... Is this true?  Can the life of the top end be extended by switching to better retainers without going in for the guides themselves?

Does the extra heat of the black engine make for extra trouble?  Am i going to be wishing that i had just held out for An "old factory" bike? 

Thanks for any advice or info,
Ray

For the guys who run the engines hard, the F2/3 valves with the smaller stems and clips can pull thru the top retainer plates after much abuse. More often, this happens with high-lift aggressive cams and their associated valve springs.

The black engines run about 20-25 degrees (F) hotter than the silver-painted ones (post-K2 engines) and about 35-40 degrees hotter than the K0-K2 (unpainted) ones. There was also a mistake (a Jinglish mis-translation) in the post-1975 Owner's Manuals that stated "use 10w40 oil, when it was SUPPOSED to say, "use AT LEAST 10w40 oil", implying to read the Service Manual which recommended 20w50 or 20w40 oil. This happened to all of the Hondas from about 1975 until 1978 when it was corrected, but the bikes really suffered from the too-light oils. The pre-1975 Owner's Manuals all showed the 10w40 for below 40 degrees ambient temperature, and 20w50 above that. This is one big reason the earlier bikes have fared better these 40 years.

[kghost]

Was it just the paint? Would have thought black would radiate heat as fast as anything else....

[Retro Rocket]

Was it just the paint? Would have thought black would radiate heat as fast as anything else....

Thats a good question Tim, everything i have been told about black engine paint {depending on the type of paint} was that it disperses heat better, thats why its a common factory engine finish, i have had some pretty reputable guys tell me that and its also been discussed here before... No offense Mark...

[Bailgang]

No offense from me either Mark but I'm curious too mainly because I've got a recently purchased 77F. Since I first started playing with engines as a kid I was always told that if you're going to paint a performance engine especially an air cooled engine then black should be the color of choice due to its ability to absorb the heat so it could be radiated away. Is there any other differences between the K and F motors that would explain the increased heat? Was Honda's decision to paint the F engine black based on aesthetic and or market reasons so the F motor could be easily noticed or was Honda aware of the increased heat and painted it black as a feeble attempt to help cool it down? I'm not trying to draw anyone into a debate here, I'm just trying to figure this out.

[Cougars750F0]

The most-affected engines were the F2/3/K8 and the late K7. Many of these also had smaller valve stems with their cast-iron guides. In particular, the black-painted engines, with their attendant 25 degree hotter oil, wore them out the fastest, often in less than 25k miles.

Today we can get bronze guides to replace them, which helps a bunch.

The K0-K2 engines, and very early K3 engines from the "old factory", had Stellite guides on both sides of the head. These are the legendary engines that you hear about with over 100,000 miles and no teardowns. Interestingly enough, there are occasional instances of Stellite guides showing up in the early F0 (1975 "F") bikes, too. I think OFreen might have gotten one of these, as he has more miles on his virgin 'F' than anyone I've even seen! 

I recently saw a head that was from a '75 "F" that sure enough had factory Stellite exhaust guides in it. At first I thought it was a K1 head until I looked closer. It has both Stellite AND valve seals! Even after 40 years, Honda still holds little surprises...

I have a 75 750F0 How does find out what type of value guides they have? and I looking into what type of upgrades to perform on my bike in the future. If the later F1 and F2  have thinner value stems should I leave the head the way it is? can I get a larger values or modify the head or should I be looking for a F3 head? So many questions   I will be buying your book!

[HondaMan]

Was it just the paint? Would have thought black would radiate heat as fast as anything else....

Thats a good question Tim, everything i have been told about black engine paint {depending on the type of paint} was that it disperses heat better, thats why its a common factory engine finish, i have had some pretty reputable guys tell me that and its also been discussed here before... No offense Mark...

No offense from me either Mark but I'm curious too mainly because I've got a recently purchased 77F. Since I first started playing with engines as a kid I was always told that if you're going to paint a performance engine especially an air cooled engine then black should be the color of choice due to its ability to absorb the heat so it could be radiated away. Is there any other differences between the K and F motors that would explain the increased heat? Was Honda's decision to paint the F engine black based on aesthetic and or market reasons so the F motor could be easily noticed or was Honda aware of the increased heat and painted it black as a feeble attempt to help cool it down? I'm not trying to draw anyone into a debate here, I'm just trying to figure this out.

No offense taken, guys.

Just stick a meat thermometer in your oil tank after a good hard run on the hiway and a putt about town afterward.

Oh, wait...I'm presuming you have more than one bike to check? Sorry... I get lots of them here, it's easy for me. Maybe I'll put those numbers into my next book, make an attempt to ride the same 'route' and then measure the oil temp? It would be a good 'real numbers' sort of test.

[Bailgang]

If at all possible Mark please make the comparison between identical bikes, for example a pair of K's with the only diff between the 2 is one painted aluminum the other flat or low gloss black or a pair of F's one painted aluminum the other black. Mark you could be our very own "myth buster" here.

[Retro Rocket]

I'll stick with what i have been told, I've worked with and used all types of paints including powder coating, electrostatic, all 2 pac paints and epoxy paints {i use epoxy paint every week}.. I recently rang a guy that has all modern and custom  powder coat finishes for some advice on different types of powder coatings and we discussed this very topic, he said that old powders were very thick and that alone would retain heat, i already knew that from my own experiences in the 1980's but after looking at the newer engine finishes he has and uses, and listening to what he was saying about the number of bike engines he does, i would use black with out a worry. The F2 Honda's were in a higher state of tune, that may account for some extra heat..  Black finishes are used on heat applications for their known dissipation   I'll add, its bloody hot here in summer and not much cooler in winter, i've never had a problem with a black engine...
This is an excerpt i copied a while back about black engine finishes...

Fact?  It's been said that black is actually the best color for dissipating heat from the engine. This statement is true, here's why:

The reason black paint dissipates heat more than any other color is that black is the most capably dissipative color for infra-red (heat) wavelengths. As we all know, back is very absorbent when energy rays (visible AND invisible) such as sunlight hit it. It is also able to cast off the most heat energy, all of its radiation being in the infra-red part of the energy wavelength spectrum. Single colors tend to focus their ability to both absorb AND dissipate in the wavelength of their color, and far less in the infra-red compared to black.

[phil71]

colors and heat only apply to the heat in white light. This may matter some on a sunny day, but it won't mean a thing at night. SO if black engines run hotter it's because the paint is thicker than the cloud silver on the older engines. but it's got nothing to do with color.

[HondaMan]

In October of 1972, following an accident (in September) with my then-fresh K2 that broke off some fins on the right side of the engine, I tore down the engine top and had the fins redrawn. Then I re-ringed the top end and decided to make it black like the R/C 1000cc cylinders, to be 'cool'.

The first thing I noticed on the very next ride was: my feet were nice and warm in the cool October riding. At first I didn't think much of it, as winter riding was next. Then came Spring, and my feet were soaking wet in my boots from the hot airflow, and I could really feel the heat on my pant legs. My (ex) wife also got a burn on her inside thigh thru her jeans when we went on a longer ride along the river one hot night in April '73: it came from the oil cap. During the previous summer, we had ridden to the Black Hills and back at triple-digit speeds and this had never happened.

So, I got a (wife's) meat thermometer and hopped on the bike, tore along my favorite "race" track (thru the woods and back roads) at illegal speeds for a while, then pulled over to check the oil temps. I nearly left my fingerprints on the oil cap in the attempt! The oil was almost 215 degrees in the tank.

I had been through SAE Automotive training in college before that, and learned that oil temps should ALWAYS be below 200 degrees for a happy engine. So, the next day I got some lacquer thinner, brushes, and aerosol sprayers for paint to spray with, and spent that weekend removing the black engine paint (and some from the frame...), leaving slight traces of it that still show today on the bottom fins. I repeated the oil temp test on approximately the same roads and speeds, and noticed immediately that the oil cap was just warm to the touch when I stopped at the same spot to test: the oil was 178 degrees.

Based on this 1973 experiment, I occasionally retest, because over the years I have found it to be a good indicator of the ability of the oil to lube well (lower temps), or when to remove my Vetter lowers for the summer (190 degree oil after commuting, with Castrol 20w50), and which oils to avoid altogether (10w40 anything, for one example).

Also based on observation: Honda added the "oil cooler plate" to the F2/F3 engines that they made black, but left them off on the same engine (K7/8) with smaller valves, painted silver (even though larger intake valves make for cooler running engines, generally). Anyone who has had the chance to ride both of those bikes will readily notice the increased heat coming off the F2/3 engines, I would say.

[PeWe]

Hondaman, very interesting experiences!

What about 20W-40? Cold start viscosity OR at higher degrees [210° F or 100° C] most important, engine operating temp? 50 needed to not harm the engine?
I have 20W-40 home. My K6 engine is silver painted, head  and cylinder glass beaded. Sweden is not that warm country either, riding days around 60-75F (15-25C)

[Retro Rocket]

I think you will find that paint has come a long way since the 1970's....

[PeWe]

I think you will find that paint has come a long way since the 1970's....

Yes, but it might have an insulating factor.....

[Retro Rocket]

I think you will find that paint has come a long way since the 1970's....

Yes, but it might have an insulating factor.....

Go back and read whats already been said, any thick coating will insulate, thin engine coatings do nothing or next to nothing otherwise all motorcycle manufacturers would not paint their engines, at least a 3rd of the 40 odd bikes i have owned have had black engines from the factory, I have also painted a few myself. Besides, you totally missed my point, modern paints have virtually nothing in common with paints used in the early 70's.  It is also a fact that some coatings are designed and used to dissipate heat...

[rheilman]

Very interesting discussion here.  I'm sure that the modern coatings are quite a bit different than what was used in the 70s, and Hondaman's experiments seem pretty telling. 

I guess the root question I have is still the same:  are the F2/F3 750s considered to be pretty reliable in general?  Or do the valve guide issues and hotter running temps make these bikes something to be avoided? 

[Retro Rocket]

Put new guides in it and you should be good to go, they have more performance than earlier engines and with modern replacement guides and seals you should be just fine....   Some of the HIPO guys love these engines....

[70CB750]

I have jugs of the engine off now, based on the discussion here, should I consider to go silver?  I always put function above looks and if it makes her run cooler, I am all for it.

[phil71]

One more time.. the color isn't as important as how thick it is.. so if you are planning to put silver OVER the original black, you're just making things worse.

[Retro Rocket]

The color is slightly important....

Physics theory (corrected):  Heat (electromagnetic energy) radiation
rate, per unit of surface area, is proportional to the fourth power of
the surface temperature.  Heat radiation rate, per unit of surface
area, is directly proportional to the ?emissivity? characteristic of
the surface.  Emissivity = 1.0 (approx.) for a dull black surface;
emissivity = 0.3 (approx.) for a smooth, shiny copper surface.

Hence the rate at which heat is radiated from an engine block with a
dull black surface might be roughly estimated as 3x the rate at which
heat is radiated from an engine block with a shiny metal surface.