Topic: What head fits a 74 bottom end??

[CB750R]

hey guys, I'm waiting for my new pistons and stuff to come back from the machinist, and now I need a new head cuz my old set up stuck a valve, and mashed things up good!!  Whats the rules for mating different heads to blocks?? I know the 77F- 78S all had larger valves, but what about the 75F or 76F, are they fair game with K blocks?   How do I know exactly if the block I got off ebay is indeed a  k or a F setup??

Thanks
G

[scondon]

  Honda changed the oil passages at some point, maybe 74 or 75 I'm not sure. The passages down the cylinder studs were enlarged to fit dowels and rubber seals where the cylinder mates to the head. I would make sure to get a head that matches your cylinder in this respect as a mismatch may accelerate oil leaks. It's also a good idea to get your head and cylinder "decked" to avoid leaks on a mix-n-match engine.

   I don't know if there are differences between the K and F head blocks themselves but I think either should be a bolt on affair with no worries(excluding 77-78F).

[Geeto67]

hey guys, I'm waiting for my new pistons and stuff to come back from the machinist, and now I need a new head cuz my old set up stuck a valve, and mashed things up good!!  Whats the rules for mating different heads to blocks?? I know the 77F- 78S all had larger valves, but what about the 75F or 76F, are they fair game with K blocks?   How do I know exactly if the block I got off ebay is indeed a  k or a F setup??

Thanks
G

You can't mix  stock F head parts on a Stock K bottom end unless you like metal on metal contact or 6:1 compression. I forget which is which right now (brain fart) but one of the heads has a huge combustion chamber and it's block uses a pop up style piston while the other uses a dished or heron piston and a small combuston chamber in the head. Anyway, here is a link to the different engine mods over the years:

http://www.hondachopper.com/modifications.html

I think to be safe stick with a 73-76 K head since it looks like the head got improved oiling in 73 and the block was redesigned in 74 btu the head was not. 77/78 K and all F motors are a completely different animal when it comes to pistons and heads so stay away.

[scondon]

You're right Geeto. Those widened passages don't extend into the head. Obviously I should wait for the coffee to kick in before posting(haha). Please ignore my earlier comment about oil passages, CB750R. 

[Geeto67]

You're right Geeto. Those widened passages don't extend into the head. Obviously I should wait for the coffee to kick in before posting(haha). Please ignore my earlier comment about oil passages, CB750R. 

No worries mate, ideally you want the heads with the best oiling but I think a 72 head may work with the later block as well. I don't think a 69 or 70 head will but you probably wouldn't want one anyway since they are probably gonna cost ya more and you don't get the good oiling.

On the issue of decking the head and cylinder block, be careful. Some used parts may have been decked in their past life and while you are only removing a few thousandths, doing it from both engines may result in piston to valve contact. I know someone out there makes a copper head gasket for a cb750 (check with Wayne and Sons) but I forget who.

What is wrong with your existing head, cb750r, that you cannot use it? Most heads can be refurbushed for the cost of a used head core.

[CB750R]

last night, on my first ride since replacing head gasket, and cam chain, a valve got bent, and stuck down, so I'm in for some fun when I remove it, I know the spark plug was mangled, so I imagine the combustion chamber isn't lookin so hot either. 

Grant

[Geeto67]

Grant,

I read your other post and didn't put 2+2 together. Bent valve and an spark plug, sounds like your head and cyl block have been decked once before already. I wouldn't worry about the combustion chamber as much as I would worry about the piston and rings. Hitting a valve usually means the con rod is bent also (but not always) and the broken plug peices mean your rigns may be wasted since all that stuff falls down. Chances are you may have a mark dug into the combustion chamber where the valve hit it but remember debris travels dwon not up. I have reassembled engines with heads like that and not had any problem with them, but if you want you could get away with some squish work on the combustion chamber (and while you are at it some port work and other hp head goodies). If you only bent one valve the chances are either that valve's springs have failed, you cam chain slipped or broke (that one will be easy to spot hard to fix), your cam broke (happend to me once) or your head is decked crooked. Pull the head and take a look, it is always best to know what happened than try and guess what happened. Most of the damage to a head in a situation like that occurs to the moving and replaceable components. Every once in a while you will get a bent guide as well or a mark on the c. chamber but most of the time it is the valve, springs, retainers, and cam that take the hit.

[CB750R]

I have a set of 67mm pistons and freshly milled block to reassemble engine with!!! and I'm not too sure what year the block is for the new setup, since I got it on ebay.  So, as I said the head looked milled because there's evidence of where someone took a wire brush to the head, and it didn't get fully milled out. So, I'm very skeptical in using the existing head with the pistons and block I've paid a LOT for when I put it back together.  So thats why I'm trying to figure out the rules between heads and blocks, so I can go ebay a new head to match my sweet 900cc block and pistons!!

[Geeto67]

I have a set of 67mm pistons and freshly milled block to reassemble engine with!!! and I'm not too sure what year the block is for the new setup, since I got it on ebay.  So, as I said the head looked milled because there's evidence of where someone took a wire brush to the head, and it didn't get fully milled out. So, I'm very skeptical in using the existing head with the pistons and block I've paid a LOT for when I put it back together.  So thats why I'm trying to figure out the rules between heads and blocks, so I can go ebay a new head to match my sweet 900cc block and pistons!!

900cc...Well there goes your street driveability, hope you don't get stuck in traffic. One thing you can do if you are worried about it is a little trick car engine builders taught me to check valve clearance. Get some clay (not playdough) and work a flat little pancake of clay on the top of all the pistons. Then assemble the engine on the bench (set the vavle lash) and turn it over by hand. After you have brought every piston to TDC pull the head and check the clay this should allow you to see how much clearance you actually have by the indentation in the clay. if you have an interference problem you'll know it quick. Otherwise jsut do what the junkyard hot rodders do, throw a second gasket in the engine and cross your fingers. 

[CB750R]

Thanks for that tip, I'm definatly going to give that a spin when I bolt up the new puppies, I'm going to run an oil cooler to help tame scavenge some heat from the block, but why do you say I'll loose all streetability??

[Geeto67]

Thanks for that tip, I'm definatly going to give that a spin when I bolt up the new puppies, I'm going to run an oil cooler to help tame scavenge some heat from the block, but why do you say I'll loose all streetability??

Maybe I am overstating it a bit, but a 900cc kit can be a handful depending on compression and whose kit you use. 8.5:1 - 9.5:1 compression is a very good comp range to be in for the street 10:1 is ok and 11:1 is not really a good compression ratio.  I think at 836cc you are pretty much at the limit of what the stock carbs can support so you will need to move up to a set of CRs or mukuni's for the 900 (sudco can recommend something). 

With the bigger bore your engine will build heat faster than stock and will run hotter so oil management is definatly something to look at. I hope you plain on running a big oil cooler since an oil cooler is something most shops still recomend for a stock cb750 for the sake of longevity. Still if you get stuck in traffic a bored out cb750 can build heat like a mofo. That is the biggest thing in a street bike is it's ability to sit in traffic. Considering a stock cb750 will overheat in midtown manhattan traffic a 900cc will roast you good. Plus it is uncomfortable with all that heat coming back at you, espically heating up your legs.

 I assume if you are going that big you will want to put an aftermarket cam in the engine. I  am not even sure if a stock cam will work with the 900cc setup (don't see why not) but it will be tame if it does.

I don't blame you for wanting to build a big bore cb750, I have an 811cc that I love and one day I am thinking I wanna go bigger, but considering I commute on a stock cb750 I wouldn't dare touch that bike for fear of losing it's practically. I don't want to talk you out of it because a 900cc SOHC weekend rocket sounds cool but just keep in mid you are changing the bike's functionality and shortening it's longevity. If the bike is a weekend toy (and occasional weekday use) then go for it, if the bike is meant to replace your car then I would think twice about how I could maximize the oil cooling in that engine.

[CB750R]

Thanks for the input,

I live with a 100km's before the next town north, and 80kms before the next town south, on a two lane twisty mtn highway, and I hate the city witha passion, and will not be heading close to one!!!  This baby is going to be my little backcountry burner, and hopefully take me on some nice trips, across Britsish Columbia, and maybe down the left coast.   I'm tearing my engine down this weekend, to see what I need in head work, but I hope to get her running with stock cam this summer then do the head up with cam/springs/porting in the winter, we'll see.  My ultimate goal is shaming some hog owners who think thier stock bike with straight pipes is "badass" 

heheee

[Clyde]

75/76 F/F1 heads are the same as all K heads as far as combustion volume goes. The extra compression (9.2 vs 9.0) is a result of slightly different pistons. The K7/K8 heads/pistons are the same as the F/F1
The later F2/F3 (77/78) heads are different as they have bigger valves and the combustion chamber enlarged to fit them in. Be careful using F2 pistons with any other type of heads. If you use 77/78F heads with K/ F1 pistons you will run a very low compression ratio as mentioned above
I have mixed and matched F1 and K0-K8 heads without any problem. Some you need o-rings and dowels for the seals near the studs whereas the very early ones do not. In this case I have shortened the dowels.

[CB750R]

Thank you Very much Clyde for your answer now I can go ebay some of those 75-76 heads online at the moment.

[MRieck]

I have a set of 67mm pistons and freshly milled block to reassemble engine with!!! and I'm not too sure what year the block is for the new setup, since I got it on ebay.  So, as I said the head looked milled because there's evidence of where someone took a wire brush to the head, and it didn't get fully milled out. So, I'm very skeptical in using the existing head with the pistons and block I've paid a LOT for when I put it back together.  So thats why I'm trying to figure out the rules between heads and blocks, so I can go ebay a new head to match my sweet 900cc block and pistons!!

900cc...Well there goes your street driveability, hope you don't get stuck in traffic. One thing you can do if you are worried about it is a little trick car engine builders taught me to check valve clearance. Get some clay (not playdough) and work a flat little pancake of clay on the top of all the pistons. Then assemble the engine on the bench (set the vavle lash) and turn it over by hand. After you have brought every piston to TDC pull the head and check the clay this should allow you to see how much clearance you actually have by the indentation in the clay. if you have an interference problem you'll know it quick. Otherwise jsut do what the junkyard hot rodders do, throw a second gasket in the engine and cross your fingers. 

I run a 915 with a good amount of compression and don't see over 225 degrees in summer heat. On the highway it is tough to get it over 180. That's with an Earl's cooler and a digital oil temp gauge.

[CB750R]

Which Earls cooler model do you use? I have an adapter ring and cooler set up I won on ebay, but the cooler is a little ratbagged, so I figured I step it up and get a new one, and did you use earls braided tubing? if so what legths did you order? and lastly what did you use as a thermistat for the cooler, or do you run it all the time? I live in Canada, in a ski resort, so right now mornings are like 5deg C but days are 32+, so I think a thermistat may be essential.

[MRieck]

75/76 F/F1 heads are the same as all K heads as far as combustion volume goes. The extra compression (9.2 vs 9.0) is a result of slightly different pistons. The K7/K8 heads/pistons are the same as the F/F1
The later F2/F3 (77/78) heads are different as they have bigger valves and the combustion chamber enlarged to fit them in. Be careful using F2 pistons with any other type of heads. If you use 77/78F heads with K/ F1 pistons you will run a very low compression ratio as mentioned above
I have mixed and matched F1 and K0-K8 heads without any problem. Some you need o-rings and dowels for the seals near the studs whereas the very early ones do not. In this case I have shortened the dowels.

The intake ports are 2mm larger on the F2/F3 as well.

[MRieck]

Which Earls cooler model do you use? I have an adapter ring and cooler set up I won on ebay, but the cooler is a little ratbagged, so I figured I step it up and get a new one, and did you use earls braided tubing? if so what legths did you order? and lastly what did you use as a thermistat for the cooler, or do you run it all the time? I live in Canada, in a ski resort, so right now mornings are like 5deg C but days are 32+, so I think a thermistat may be essential.

Earls unit #21306. 3 feet of -6 AutoFlex line, 2, -6 straight aluminum Auto Fit ends, 2, -6 45 degree aluminum Auto Fit ends and 2, 90 degree pipe thread/AN adaptors -6 to 1/4".

[Geeto67]

I run a 915 with a good amount of compression and don't see over 225 degrees in summer heat. On the highway it is tough to get it over 180. That's with an Earl's cooler and a digital oil temp gauge.

Did you run the digital oil temp gauge in line with the cooler? That sounds like a pretty neat setup. Honda overbuilt the SOHC 750 with a huge margin of reliability in some areas. As long as you are moving along the bike should be fine. It is where you are stuck in traffic (surronded by hot cars driving the immediate area up by 10 degrees) going less than 10mph and the bike is idling along that you will wish you werent anywhere near a big bore kit. NY traffic is notoriously bad and I have friends who have overheated stock cb750s in traffic just mulling along at a speed too fast to push but too slow to get air circulating. I have overheated my 75 stocker just going across town in the middle of the day. These old aircooled bikes are not meant for modern traffic (espically in NY) and when you bore them bigger you only use up your safety margin. Of course if you live somewhere where you don't deal with modern traffic then hey go big. It is the use the bike is going to see that will determine whether it is a smart idea to cross over into 900cc. Since my idea if streetability means surviving manhattan traffic I would be skeptical about using a 900cc SOHC.

[MRieck]

The 225 degrees I saw was in Boston expressway stop and go traffic....and it was hot. No doubt it is tough in NYC traffic- got stuck in a big jam many years back coming from the Poconos secondary to a Latin parade or some crap BUT found the greatest Italian restaurant just on the end of Mulberry and Chinatown....another story I guess.  I always thought about mounting a small DC fan behind the cooler in case of the "stop and go". If you put an even  larger Earls cooler( like on my FJ) with a spin on adaptor I doubt you would not have any problems no matter what. You would have about 5 quarts of oil in the system to start with. Remember the VW Bug, air cooled and they withstood stop and go.