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F Head on a K2 Engine

Jeff K2

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Aguanga, California
I am new to the world of SOHC 750, so please hold off on the laughter.

I have inherited a bunch of SOHC stuff that was originally modified to 65mm bore and used on 3/4 Midget cars.

I have also picked up a couple K model bikes. The first is a K2 that I have pulled the motor and removed the top end. My goal is to put the 65mm piston/cyl set in it. I have 2 F heads that have been ported and the chambers opened to allow the pistons to make the proper compression ratios (CB 350 + 1.00mm pistons that have been relieved for valves and coned). I also have 2 K heads that are in the same state of machining. I would like to use the F heads for the bigger valves in conjunction with my aftermarket cams to allow better flow.

My question is; Can I use the F heads on the K engines. If so, what do I need do to avert problems?

Thanks in advance,

Jeff K2
 
Yes the f will bolt on the k motor but the f head has a bigger combustion chamber than the k. They make f pistons because of the increase in chamber volume. All things being equal if you put an f head on a k you will reduce compression.
 
Yes the f will bolt on the k motor but the f head has a bigger combustion chamber than the k. They make f pistons because of the increase in chamber volume. All things being equal if you put an f head on a k you will reduce compression.

Thanks, Dirtdigger.

The pistons I am using are domed in their original form. They have been machined to create valve relief and coned to create a specific compression ratio (CR) for alcohol, much higher than gas burners. The F heads have been "fly cut" to increase the chamber size to avoid piston interference and adjust the CR. The cylinders I have at 64 to 65 mm bore have varying amounts of aluminum taken out of the bottom to acheive the proper CR with certain pistons. I am using a stock height cylinder. Per my measurements and calculations, I should be at 9.2 :1 with either the F or K head as they all have been reconfigured to a similar condition.

I have mocked-up the setup using a K head on a TQ Midget engine (no tranny) to be sure there were no problems with interference. The measured compression was 130 PSI under starter cranking. I am at 3600 feet elevation, so I am fairly confident the CR is close.

Is 9:1 good for this engine?

Are the stock cylinder studs good for 836 cc upgrade? I am concerned about the extra load on the studs.

Thanks again,

Jeff
 
Honestly I dont rebuild a stock 750 with the stock studs they all get heavy duty studs. 9:1 is pretty close to stock, most aftermarket 836 pistons are in the 10.25 or 10.5 :1 ratios. I wouldn't run more than 10.5 on a street motor with quality of gas. 9:1 is pretty low for too big of a performance camshaft as you will start losing bottom end when you start getting big with the camshaft. I like around 10.5 to 1 and run 91 octane. It all depends on what you want to do with the motor, if you want to increase power and add some performance parts you need to get that compression up a bit. You can have the cylinder and head milled some, I have them milled for use with MLS gaskets. Then you can adjust with different thickness head and base gaskets to get to your target compression ratio. What cam are you going to run?
 
Honestly I dont rebuild a stock 750 with the stock studs they all get heavy duty studs. 9:1 is pretty close to stock, most aftermarket 836 pistons are in the 10.25 or 10.5 :1 ratios. I wouldn't run more than 10.5 on a street motor with quality of gas. 9:1 is pretty low for too big of a performance camshaft as you will start losing bottom end when you start getting big with the camshaft. I like around 10.5 to 1 and run 91 octane. It all depends on what you want to do with the motor, if you want to increase power and add some performance parts you need to get that compression up a bit. You can have the cylinder and head milled some, I have them milled for use with MLS gaskets. Then you can adjust with different thickness head and base gaskets to get to your target compression ratio. What cam are you going to run?

Thanks again, Dirtdigger!

I am sorry to be such a pain. I have all of these parts without any documentation of configurations used to build the motors. Ask me about old Triumphs, I can answer, but this is all new to me even though it is very old.

I will replace the studs with the full diameter ones I have. I have milled cylinders at varying heights, so I will have to sort through them to get the right one for this set of pistons. I have Arias full copper head gaskets that are at 65mm to match the machined chambers and bores. I would like to use them.

The motor is going to be used for a general purpose CB750 Cafe type bike.

The Cam is an Andrews EP-13, I think. There are no marks on the cam other than the casting EP-13 and a K stamped on the end. I assume the the K denotes that it is for a K head. The duration was measured at 370 degrees and the lift at the lobe is 0.352 inch. The duration would not allow making diameter measurements to deduce the lift at the cam.

Given the K does mean it for a K head, will it work in a F head or should I use a K head? Both types cc out at the same chamber volume. Both types are ported in similar shapes and seem well done, to my inexpert eye.

Jeff
 
No problem thats what the sight is for. Copper gaskets are good for racing setups that are apart fairly often but on a street motor they will tend to seep and leak oil after a period of time. But they can be used.

Yes k cams and f cams can be swapped between the different heads. How was the duration measured, the 370 doesnt seem right for duration, it is way too big. The largest performance cam I can think of off hand has 325 duration. Could the 370 be valve lift? That is a common valve lift number for the 750. I have heard of the ep-13 camshaft I am trying to find some specs on it.
 
You can pm me if you need to get technical info once you start putting this bad boy back together. Would love to see some pictures of the pistons and the cylinder heads too! Take a photo of the markings on the camshaft.
 
You can pm me if you need to get technical info once you start putting this bad boy back together. Would love to see some pictures of the pistons and the cylinder heads too! Take a photo of the markings on the camshaft.

The duration was measured by using a dial indicator zeroed on the round. I then rotated the cam until I got 0.002" movement in each direction. I read the degrees in between, did the math.

In my looking for anything on these cams I found a site for CB750 cams that has cams that go to 268 at .050" , so I'm sure my duration is overstated.

I would be happy to send some photos of the stuff. Being new to this site, I do not know how to attach them to my posts. Is it the "Insert Image" icon?
 
most cams have a small "ramp" that removes the valve clearance so that the opening is timed correctly. a dial indicator on this style cam will show a much wider duration then it actually has. wether this cam has that i cannot say.
 
Jeff, yes there is an insert image icon, its right about the box that you type the text. There is also the Gallery that people post there own photo albums for everyone else to check out there stuff.
 
CB pistons.JPGPhotos are attached.

I have rechecked the heavy studs I have. I only have the 12 longer studs. The 4 shorter are stock. I noticed that is the way some kits are sold. CB Pistons 2.jpgCB Pistons 3.jpgDSCF0023.jpgDSCF0024.jpgDSCF0025.jpg
 
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The kits from APE and Cycle X are heavy duty on all the studs but I think you will be ok with yours. By the shape of those lobes that is a very aggressive cam. It may end up being to big for the size of motor you are building and for a streetable motor. Try it you can always swap in a smaller cam no problem. Those are some heavy looking pistons. What are you going to run for connecting rods?
 
Thank. I was hoping they were OK.

I think the cam is designed for mid range torque, although they seem a bit lumpy as compared to some of the other cams. This stuff is all from TQ Midget inventory. I believe the goal for them is to get the middle RPM to push hard. I have 8 cams and one ought to work some time, some way.

I will compare the weight of these pistons with the stockers I have and let you know. I have others that have lightening holes and shorter skirts. My intent is to run stock rods. I do not intend to have a high rpm machine. Again, I am thinking the cam and my needs are at mid range RPM. Just a torquey old bike. I have set of MC rods that I may put in the bike I will build for my son, after I get the right combination of parts sorted out on this one. Mine is simply an experiment to how these work. I have seen a ton of aftermarket rods. Do the stock rods fail often?

Thanks again,

Jeff
 
At a minimum on the stock rods put new bolts in. The stock rods are ok with the 812 836 motors. They bend at higher rpm and the bolts break more often than anything. I have run shot peened stock rods but I am running Cycle x super rods in my stuff now. The lighter the piston the longer your rods will live. I have a post going about a motor I am building, 970cc, still waiting on my cylinder and pistons to show up so I can weigh them. It has come to a stand still now as I need money for a good head. I also have a set of RC Golden rods in my stash but I am not going to be running them in anything.
 
Hi Dirtdigger,

The weight of 65mm piston is 45.4 grams heavier than the stock one with pins, rings and keepers. I found a source that cited a bare stock piston at 159 grams. I have a well used set that have holes in the skirt, so I will put one of them on the balance to see what they weigh as compared to the stock one.

I think a 25+% increase in weight is not good for the forces in the rods, nevermind the balance. Now I need to figure out where to lighten the pistons...

The MC rods I have are very similar to your Golden RC rods. They are aluminum, but not anodized gold. I have read bad things and good things about them. I do not know whether they are good or not. Why are you not using the Golden RC rods?

Thanks again,
Jeff
 
Aluminum rods are really not a good thing for a street motor. Aluminum fatigues faster then steel does and it will brake without warning. Aluminum also stretches more than steel at rpm so you would need to increase piston to head and piston to valve clearance. I have had a discussion with Russ Collins Jr about the use of the Golden Rods and he says in a street motor it is not worth the risk. The main reason for their aluminum rods is they are more forgiving on the crankshaft in nitro motors as they compress and take the violent hit as opposed to the crankshaft. I sold them a set of my Golden Rods for use in their restoration of his dads old nitro drag bikes, very interesting talking with him.

About the pistons, those are very heavy. My new 970 pistons weigh 169 grams. I wish I would have weighed the Wiseco 836 pistons I have in my current motor but I didn't. The balance aspect really isn't an issue on an inline 4 motor such as the 750 because you have two going up and two going down at the same time and the crank is always balanced in that aspect. You will be fine with the balance. I would be concerned with the rods if you are going to turn redline often but if you run at moderate rpm most of the time you should be ok but you are getting iffy. At the very least shot peen the rods and have them magnafluxed for cracks and put new rod bolts in them.

They used to put holes in the skirts in the old days to lighten the pistons but they did find that it allowed too much oil on the cylinders and on extended running it would overwhelm the oil rings with oil and you would start burning oil. RC Engineering did this but early on but did stop the practice.
 
I have wieghed the used piston and it is 29 grams heavier (roughly 18% over stock weight). It has some carbon on the dome, so it must have worked in the TQ for some time, however the underside of the crown is only a light gold to aluminum color. I do not know what rods were used with them, so I am thinking the motor had beefy rods. I have one bent Carrillo rod, so I think he ran better rods than stock. No documentation really sucks for me.

The used pistons are ART with offset valve pockets and 6 each 3/8" holes drilled through the skirt 0.200" below each oil return hole on the intake side and in between holes on the exhaust side. The new pistons are also ART, but have thicker/longer skirts, slightly higher dome and symmetric valve pockets. By the way I do have a set of these that have been run hard as the undersides of the crowns are black on that set.
 
Carrillo's are and were a popular rod for the miget guys. The Carrillo is the best rod out there for the 750 but also the most expensive.
 
Thanks again, Dirtdigger!

Yeah, I learned about fatigue limits in college. Aluminum does not have one, the strength goes to zero. The lesson was learned by the Electra jet, at the loss of too many souls.

I intend this build to verify the general characteristics of the motor, not running at or near redline. After sorting out what I am doing, how it works for my purposes, I will redo it and one for my son. Being retired has benefits.

The holes would explain the carbon on the top without the color on the bottom. Alcohol is generally not prone to sooting at reasonable mix rations. Thanks for the history. I will look for other opportunities for lightening the set. The pistons are very beefy.

Jeff
 
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