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piston problem ...

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pvincent04

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So I need to change my pistons on my cb750k4 1974, ordered new ones from vintage750.com, the new pistons don't have the same dome shape as the old ones. The old are tagged Honda, when I look at other sites or used pistons, they seem to have the flat shape like my new ones.

So If I install the new ones, I will loose compression, were the old pistons previously changed for more performance with other 61mm pistons from honda that are more conic ?? And if I change the pistons to the new ones I will basically go back to the original power and compression ?
 
Couple more pictures of the old piston might help. and markings on the underside? There should be a cast number by the wrist pin hole on the outside of the piston also. Are you sure the motor is a K motor? Hard to tell from the picture but that looks like an F2 piston.
 
Here's the close up of the old piston, you can't see any cast numbers but you can see the honda, I've checked and the serial numbers for the engine is that of a K4 1974

FullSizeRender (3).jpgFullSizeRender (3).jpg
 
How about on the other side of the piston? Normally one side says honda the other side will be a 410 or a 392 or similar number. 410 would be a F2 piston. Maybe a photo of the valve reliefs in the dome? See if you can see some marks on the cylinder head also, should be some on the top of the head in between the center two intake ports, should also be a 410 or 392 or similar type number. I am trying to determine if someone swapped a later top end on to the motor or if they just put different pistons in it. The new ones you got are correct for the K motor...if it has a K cylinder head then you will be at stock compression assuming no one has done any machine work to the chamber. If some one swapped F pistons into this motor it would have increased compression quite a bit.
 
If it has a 410 then it is probably a F2 piston. Now you will have to determine if any work was done to the head or if it is still stock. If it is stock then you will be going back to stock compression with your new pistons. Its possible that the F2 piston had been put in on purpose to raise compression, but it is also possible that someone in the past didn't know what they were doing and put the wrong pistons in. I would go with the stock pistons because those F2 pistons would get compression into the 11's. I dont know the dome volume on the F2 piston so I cant do the calculation to tell you for sure.
 
Someone may very well have put later model K or F cylinders and pistons on your early cases. If this is true, they may very well have used the later model K or F head as well...


Post a picture of the top of the head and the bottom of the cylinders and I can tell you at a glance whether they are later model F components.
 
Looks like a f series cylinder by the picture. Need a shot of the cylinder head that isn't so close. One of the top and one of the bottom. Is there a 410 number cast on the top of the head?
 
theres a picture of the underside of my f2 head I posted a couple of days ago u can see a definite difference to your head I imagine that's for the larger valves in the f2, u can see it on the thread I put on for the f2 head and barrel id
 
can't see any numbers... my engine is one hour from me so can't take more pics ... I'll try to go back another time, if what you say is right I'll have to return the new pistons I have and get new ones, I haven't been able to find F2 pistons as of now ... much easyer to find for the K
 
pvincent04 said:
Actually, if I compare that picture from your thread to mine, you can see mine is a F2 no ?
Click to expand...


Those are indeed F2 cylinders. Refer back to the other thread and look at the pictures to determine whether you have an F or K head. If you have both an F head and F cylinders then you will have to follow assembly procedures for the top end of an F motor or you could end up with either oil leaks or intrusion of the elements into the crankcase. If you have a K head then you would need to determine whether it is an early or late K head. An early K head has less clearance for domed pistons than the later K (and F) heads.
 
I have cc'ed quite a few of the 750 heads and K heads vary about 2cc between all the years. The f2 head is about another 4 to 5cc bigger yet. So swapping early and late k heads is definitely do able. Also note that the f1 head is not the same as the f2 and f3 head. the f1 head is just like the late K but still the chambers are small compared to the f2. I am betting you have a F2 head looking at how smooth the chamber is...so you will have to return the pistons or find a k top end. Its going to be up to you because piston and rings for the f2 are definitely harder to find.
 
Last edited:
So I found the pistons for the cb750 1977 on cmsnl, the first is for the supersport and the second just for the k model 1977, so I'm guessing from the picture it's the supersport ? The one that is more domed

13101410003-piston_medium13101410010-01_4b9b.jpgpiston-std_medium13101392020-01_d67d.jpg
 

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