Topic: 78 Supersport build

[Bodain]

My task for this evening was to slowly mount the 78 K head on the 78 F cylinders.... I set about the task... Looking things over very carefully... Head is on and head bolts torqued... Now I'm looking at the cam towers... Hmmmm.... Now I see a SHOW STOPPER...

First image is 78 F. Second image is 78K  ... On the F head you will see 4 small o-rings just below the intake valves. These feed oil up thru the cam towers to the camshaft.

On the K head you will only see two in the center..The two outer holes are not present. My camshaft would have ground to a screeching halt very quickly.

Time for plan B.... At this point I don't know what it is. Seems to be look for a engine or complete K top end....


Shot at 2007-07-21



Shot at 2007-07-21

[MRieck]

 I said before...the rings ARE available from Honda under a different part #. I believe a K7 or 8 uses the same rings. I got them in stock bore for jtb so I KNOW they are available. You can search on the forum for the thread where I was asking about them. I wish I wrote down the part #.

[Bodain]

I said before...the rings ARE available from Honda under a different part #. I believe a K7 or 8 uses the same rings. I got them in stock bore for jtb so I KNOW they are available. You can search on the forum for the thread where I was asking about them. I wish I wrote down the part #.

I'm past the rings. I'm using stock 69-76 rings. I had the grooves on the piston machined out to fit. The rings are perferct.

[Bodain]

Well it turns out I was wrong. Thats never happened before!

When comparing the two heads side by side I observed the 78F had the 4 o-rings and the 78 K only had two.  I looked at it again... On the two outside holes. There is no center hole. I was thrown by the fact that the F head had o-rings on the outside and the K head didn't. This hole situation is infact identical between the two heads. So you use the four o-rings and the two outside holes are essentially stops.     Things are looking up....

K head is mounted on F cylinders. Installed the remaining top end... Did a valve adjustment....
Thing are looking very good...  Turn the engine over slowly.. There was an outside possibility the valves might hit the domed pistion, but I didn't really believe it...  NO PROBLEMS...

True I have domed pistons. Also true I have cutouts in the domed pistons for valve clearance for the larger 78 valves that are now smaller with the K head.

Time to put this engine back in the frame..... Yippeeee...

[Wheelhorse77]

Could always use playdoh or clay to measure piston to valve clearance.

[Lumbee]

...I'm anxious to see your results...I've always heard you can't use the domed pistons with the K head due to clearance.  Once its back in the bike kick it over hard and fast and listen for any knocking before you start it up...   

[Bodain]

...I'm anxious to see your results...I've always heard you can't use the domed pistons with the K head due to clearance.  Once its back in the bike kick it over hard and fast and listen for any knocking before you start it up...   

I've already done it.. There is no problem. I've read several threads here on it. I get the impression there are two scenerios for domed pistions.  77 F domed pistons with no cutout in the pistons for the valves. This would probably not work....   My 78 F has domed pistons with the cutout for valve clearance.

The first time I turned it very slowly.. The engine is now back in the bike and I've kicked it pretty hard looking for a sound or feel... It sounds and feels fine... The 78 F cylinder is a 1 to 1 match with all stud and oil passage holes in the 78 K head...  Then looking on top of both heads at the oil passages again... I believe we again have a one to one match...

I'm feeling pretty good about it. It will be a good while before I actually fire it up...

Another thing occurs to me. The 78 F head cumbustion chamber area is a good bit deeper than the K head. Compession will probably be different.

[TwoTired]

Another thing occurs to me. The 78 F head cumbustion chamber area is a good bit deeper than the K head. Compession will probably be different.

Yes, it seems pretty obvious that your compresion ratio will be much higher.  Unless you cc'd the head and pistons you won't know what the actual CR is, I suppose.

Expect to use premium fuel at a minimum. If the CR is up above 12:1 consider converting the carbs to run on alcohol, or hunt around for 145 octane aviation gas.  If there are any old warbird racers where you live, strike up a conversation for a possible fuel source.

Be watchfull of pinging and pray that detonation doesn't bend/break rods and/hole pistons.

If you can keep it together, it should make lots of power for a while.

Cheers,

[Bodain]

Another thing occurs to me. The 78 F head cumbustion chamber area is a good bit deeper than the K head. Compession will probably be different.

Yes, it seems pretty obvious that your compresion ratio will be much higher.  Unless you cc'd the head and pistons you won't know what the actual CR is, I suppose.

Expect to use premium fuel at a minimum. If the CR is up above 12:1 consider converting the carbs to run on alcohol, or hunt around for 145 octane aviation gas.  If there are any old warbird racers where you live, strike up a conversation for a possible fuel source.

Be watchfull of pinging and pray that detonation doesn't bend/break rods and/hole pistons.

If you can keep it together, it should make lots of power for a while.

Cheers,

Your probably right. Still it's fun stuff. Plan B is in the works. I'll grab the next pure K unit that comes along. I don't really want deal with the oddities of the F2 F3 engine. At least I'll have all the aluminum engine parts all shined up and ready to slap on the K engine. <Grin>

[scondon]

  Sorry I'm just now getting back to this thread, Bodain. My main concern with having you read that other thread was to point out that the oil drains needed to be addressed if you were to use the K8 head. If they are not adressed then oil will pool around the four inner valve stem seals. My guess is that this would cause premature oil breakdown due to puddled oil sitting in the head and overheating, and possible stem seal failure resulting in smoky exhaust.

  It's all been theory up to this point as no one has reported trying exactly what you are now doing. I'm curious what the resulting compression ends up being and whether there are any clearance problems when the engine runs at speed. Keep us posted.

 

[Bodain]

Well it would be a good idea to run a compression test. This could be interesting....

New rings with a hone on a engine with 20,000 miles. I didn't do anything the valves...I did my standard leak test.. I pour some gasoline into intake and exhaust ports and let it sit for a few minutes. If the seats are bad I should see some gas leaking past the valves. I didn't find any leakage.

Standard compression should be about 140 to 150 I think...

The test reveals about 110 PSI across all four cylinders. Hmmmm    Not at all what I expected... Something is wrong. This is too low.... Oh the engine would probably run with 110 psi.. Given the history...I would have expected a higher number...

This project is going on ice for awhile while I reflect on it... I have some things I need to do on the Yamaha RZ to get it road worthy...

[Wheelhorse77]

What do you plan to do with your F cylinder head?

[Bodain]

The F head is actually usable with one exception. The aluminum insert on the intake of one cylinder is absent. So for now I'll probably hang onto it in hopes of finding that larger intake insert. The ones on a K are smaller and will not work.

[Partsbiker]

In an effort to reduce detonation you could always run an external oil passage and install a small oil cooler...the cooler the engine the less chance of detonation... might also let you run lower grades of gas too....also consider timing... retard if you end up pinging.

[MRieck]

The F head is actually usable with one exception. The aluminum insert on the intake of one cylinder is absent. So for now I'll probably hang onto it in hopes of finding that larger intake insert. The ones on a K are smaller and will not work.

Do you mean the threaded spigot when you say "insert"?

[Bodain]

The F head is actually usable with one exception. The aluminum insert on the intake of one cylinder is absent. So for now I'll probably hang onto it in hopes of finding that larger intake insert. The ones on a K are smaller and will not work.

Do you mean the threaded spigot when you say "insert"?

Yep... T-bone from SOHC sent me two of them from a K engine... Thats when I really found out the K spigot is much smaller that the 78 f spigot.



BUT.... Things are suddenly looking up... <GRIN>  They always do.. It's all timing... I just came into contact with a 78 Supersport for sale.  Seems it's none running with only 15000... I think I'll take my compression tester over their and take a look. Possibly free if I work another bike for him. If this item pans out then I will have lots of 78 items ...

[MRieck]

The F head is actually usable with one exception. The aluminum insert on the intake of one cylinder is absent. So for now I'll probably hang onto it in hopes of finding that larger intake insert. The ones on a K are smaller and will not work.

Do you mean the threaded spigot when you say "insert"?

Yep... T-bone from SOHC sent me two of them from a K engine... Thats when I really found out the K spigot is much smaller that the 78 f spigot.



BUT.... Things are suddenly looking up... <GRIN>  They always do.. It's all timing... I just came into contact with a 78 Supersport for sale.  Seems it's none running with only 15000... I think I'll take my compression tester over their and take a look. Possibly free if I work another bike for him. If this item pans out then I will have lots of 78 items ...

If not let me know...I can remove one from a late F head.

[Hockers Choppers]

STOP!!!!!  You can't use the K head on the F cylinder!!! The oil only drains on 4 studs on the F cylinder and 8 on the K so the oil from 4 valves will puddle in the top of the head!!! I think Terry from OZ found this out the hard way! Sorry I just saw this tonight, I could have stopped you sooner! Have to use the K head with K cylinder!!!!!!