Topic: CycleX pistons "for chopper or bobber".

[750four]

Got these from the link above 3 days after I ordered them.....
Finish and quality look good. has 65 stamped into the top and it has a heart shaped relief on one side. D and K head gasket. I wish I had the specs on an OEM piston. Just wondering if this piston will have a greater compression ratio? They look pretty flat on top just like OEM. Any one have an ideas?

[MCRider]

X4
Got these from the link above 3 days after I ordered them.....
Finish and quality look good. has 65 stamped into the top and it has a heart shaped relief on one side. D and K head gasket. I wish I had the specs on an OEM piston. Just wondering if this piston will have a greater compression ratio? They look pretty flat on top just like OEM. Any one have an ideas?

Even if the tops are as flat as stock, you get a nominal but not insignificant bump in CR from the fact that the cylinder capacity increases but the combustion chamber is the same.

More volume squeezed into the same combustion chamber = >CR.

[bobbed_750]

Just a question in the title "69-77 HONDA CB750K PISTON BIGBORE 65mm SET OF FOUR". I have a 78 im assuming all these motors were the same up to 78, I'm just not big on buying something I cant use.

[750four]

Your motors a bit different. Assuming it's an F2. Cycle x makes a kit for your year 750. The F2 jugs and head are different in a manner that will give you an unwanted lower compression ratio. They sale a taller piston.

[bobbed_750]

From reading around and running the numbers my motor is  F3. My engine number is cb750f-3103351, am i correct in saying F3. I have have said before in my previous post I am an infant when it comes to these bikes I have only had mine a couple weeks But i have pretty decent mechanical knowledge.

[750four]

Is that your bike in the avatar pic? Looks pretty sharp. That was just some info I've picked up on from this site. Not sure about the F3's, but I think 76 they changed the head and jugs, more like a smog motor. Bigger valves, less compression. I'm sure a search could find the difference in this forum. It's been covered. There's even a timeline somewhere. I'm not much help huh..lol 

[bobbed_750]

yeah the motor timeline with numbers from this forum is were i got that my motor is an F3. What do you mean when u say jugs? And yes thats my bike in the avatar

[750four]

jugs=cylinders .... or boobies 

[Jerry Rxman Griffin aka MuthaF'er]

Is that your bike in the avatar pic? Looks pretty sharp. That was just some info I've picked up on from this site. Not sure about the F3's, but I think 76 they changed the head and jugs, more like a smog motor. Bigger valves, less compression. I'm sure a search could find the difference in this forum. It's been covered. There's even a timeline somewhere. I'm not much help huh..lol 

The F did not make a "radical" change until 77 and 78. The early 75 and 76 F's had basically the same 69 - 78 K engine with a few changes to improve performance. It had the "normal" size valves. The 77 and 78 had the bigger valves and all the cylinder-up "improvements" that were necessary due to the larger valves but it was not down on compression. These were "improvements" to increase HP that turned into problems mainly due to the new valve angle necessary to keep those big valves from hitting each other.

[750four]

Oh, I had read on satanicmechanics site and time line that the cr was down in the F2 and on. And had also read that the cylinders where @ 1mm taller on the 76'  and on. And there are a few years that had a thicker sleeve( primo for boring 836). I've also read that the K series had a stronger bottom end and tranny. Guess you can't always look for the ol internet for answers. It's a great debate.

Here's another one: What pieces of the cb750 sohc engine are the greatest? K1 lower end, F2 head, ? cylinders, I bet 10 different answers from 10 different people. I've read that some of these parts interchange and I've read they won't.   

[scroggins5000]

Just a question in the title "69-77 HONDA CB750K PISTON BIGBORE 65mm SET OF FOUR". I have a 78 im assuming all these motors were the same up to 78, I'm just not big on buying something I cant use.

The years 69-76 are the same. 77(F2) and 78(F3) are the same. I have a 78 CB750 F3. I bought this set from dynoman: http://www.dynoman.net/bikepages/F2/pistons.html

[j4hfr34k]

I think the "for choppers or bobbers" might kind of sum it up. Sounds like Ken wanted a good piston for these style bikes. Not over done to keep costs down. Forged is better. But, sounds like these cast will work good too. I'm building a bobber, welded in hardtail and raked a bit too. Like most 95% of the Honda chopper and bobbers. And while I love hipo parts just like the next guy, I can't justify blowing 2 times the amount of money on parts I can't put to the test. I think these where intended for big bore, big cam, bigger attitude.

Thanks for all of your input Hondaman. You are a real asset to the forum. Even if I feel like I'm sitting at a table full of astronauts at dinner. (I read, I shake my head "yes", but I don't know what your talking about.... most of the time) lol

How about those ebay piston kits:  CB750K 65mm straight from Japan..?. CHEAP, but what are they??

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=130376084990&viewitem=&sspagename=STRK%3AMEWAX%3AIT

I'll know soon: I ordered some of those. They are 811cc bore.

did you get them in curious of your thoughts

[HondaMan]

I think the "for choppers or bobbers" might kind of sum it up. Sounds like Ken wanted a good piston for these style bikes. Not over done to keep costs down. Forged is better. But, sounds like these cast will work good too. I'm building a bobber, welded in hardtail and raked a bit too. Like most 95% of the Honda chopper and bobbers. And while I love hipo parts just like the next guy, I can't justify blowing 2 times the amount of money on parts I can't put to the test. I think these where intended for big bore, big cam, bigger attitude.

Thanks for all of your input Hondaman. You are a real asset to the forum. Even if I feel like I'm sitting at a table full of astronauts at dinner. (I read, I shake my head "yes", but I don't know what your talking about.... most of the time) lol

How about those ebay piston kits:  CB750K 65mm straight from Japan..?. CHEAP, but what are they??

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=130376084990&viewitem=&sspagename=STRK%3AMEWAX%3AIT

I'll know soon: I ordered some of those. They are 811cc bore.

did you get them in curious of your thoughts

I got them, but they were mis-advertised. The post in eBay I bought was labelled "64mm" when in fact they are the same 65mm that you see above. This makes them 836cc, a disappointment (as I am looking for 811cc pistons...still...).

The top edge is very sharp, needs to be turned off before installing these, or it will get hot. The rest of it seems OK: they have microgrooved sides, like the ones from Z1, which will help cool them and ease break-in.

[j4hfr34k]

Thank you for the information hondaman, one of the reasons i like this forum so much so many "gurus" willing to give help and advice to the rest of us

[750four]

How much of the top edge would you shave off. Radius the top edge maybe a 1/16th"?

[HondaMan]

How much of the top edge would you shave off. Radius the top edge maybe a 1/16th"?

Put a 45 degree angle on it. Make the result 1mm wide (think metric!). Then, after boring, scribe the head inside the cylinders and make a corresponding quench band at the same angle in the head. (Do you have my book? It's in there, too.  ).

[N30R3L0AD3D]

I know this is an old thread, but I just bought a set of these 50 over's from cyclex. Anyone have any problems with these pistons? Are they good?

[HondaMan]

I know this is an old thread, but I just bought a set of these 50 over's from cyclex. Anyone have any problems with these pistons? Are they good?

Go after the sharp edges on them (if they are still like that) and smooth them down with some 220 grit sandpaper, and balance them all to the same weight, within a gram or so. After that they will work fine if you're not racing. I have 2 engines out there with them, they are doing good in commuter service.

[madmtnmotors]

The deck height increase was about 1mm. This was done (spoken unofficially by a Honda rep I knew well) to reduce compression a little when the U.S. was having the first gas crisis: octanes were dropping like a rock and premium was real hard to find. These bikes were rated to run on 93 octane (by today's numbers: 95 octane) back then, until the late K4 when the deck height change (quietly) occurred. That change lowered the compression ratio about 0.4 to 8.8:1 effective compression ratio (ECR), as the cam lost some duration, too, and the spark advancer lost about 2-3 degrees of spark advance.

Total changes at the late K4 were: extra idle spark advance, less overall spark advance, 1mm more deck height, 2-4 degrees less cam duration, which was taken off the closing side of the intake cam lobe. This early-closing cam effectively increased the ECR if the deck height had stayed the same: it would have been 9.2:1. If the deck height had changed by only 0.5mm, the ECR would have stayed at 9.0:1 like the original engines.

This all made the bike feel torquey-er at lower engine speeds (helpful for the new 55 MPH speed limit of the day), but sacrificed considerable high-end HP.

So decking (milling) the top of the cylinders by 1mm would bring the CR back to earlier (pre '74) spec when using the more readily available, low-top pistons, for the earlier motors? Any downside to doing this, other than making sure not to revert to higher crown pistons for later rebuilds? I've always felt the CR on my K8 was a little anemic, even since the 23,000 mile mark (when I bought it in 86', 40,000 miles ago). I, too, am rather unkind to my machines, but the CB continues to thrive on the neglect and abuse.

[HondaMan]

The deck height increase was about 1mm. This was done (spoken unofficially by a Honda rep I knew well) to reduce compression a little when the U.S. was having the first gas crisis: octanes were dropping like a rock and premium was real hard to find. These bikes were rated to run on 93 octane (by today's numbers: 95 octane) back then, until the late K4 when the deck height change (quietly) occurred. That change lowered the compression ratio about 0.4 to 8.8:1 effective compression ratio (ECR), as the cam lost some duration, too, and the spark advancer lost about 2-3 degrees of spark advance.

Total changes at the late K4 were: extra idle spark advance, less overall spark advance, 1mm more deck height, 2-4 degrees less cam duration, which was taken off the closing side of the intake cam lobe. This early-closing cam effectively increased the ECR if the deck height had stayed the same: it would have been 9.2:1. If the deck height had changed by only 0.5mm, the ECR would have stayed at 9.0:1 like the original engines.

This all made the bike feel torquey-er at lower engine speeds (helpful for the new 55 MPH speed limit of the day), but sacrificed considerable high-end HP.

So decking (milling) the top of the cylinders by 1mm would bring the CR back to earlier (pre '74) spec when using the more readily available, low-top pistons, for the earlier motors? Any downside to doing this, other than making sure not to revert to higher crown pistons for later rebuilds? I've always felt the CR on my K8 was a little anemic, even since the 23,000 mile mark (when I bought it in 86', 40,000 miles ago). I, too, am rather unkind to my machines, but the CB continues to thrive on the neglect and abuse.

Well, you're dealing with a different engine, there. The post-1975 engines (F0 and later) all had the taller cylinders, but also had taller pistons, and with domes on them, so there isn't usually room to deck the cylinders. They also have a quench band driver on the edges of the crowns. The CR on the F0-F3 engines is quoted variously as 9.2:1 through 9.5:1, depending on which manuals you believe, and which cam is in use. The F2/F3 cams had the most duration (not by a lot when compared to the F1, though, which had more than the pre-"F" engines). The K7/K8 engines had smaller intake valves with essentially the F0 cam, which resulted in the highest ECR of all of them at 9.5:1. The biggest issue with the K7/K8 and F2/F3 engines is in the exhaust valve guides: they wear out quickly and this causes compression drop due to erratic exhaust valve seating (and often burned exhaust valves). When replacing them, use Kibblewhite or API guides, which are better materials, for longer life and better heat transfer to the head.

[N30R3L0AD3D]

I definitely don't plan on racing it.. I have a 1015 motor for that purpose only...

But, I am putting a rather mild street cam in it (its the 360 valve lift and 270 degree duration cam).

I tore the motor down, and decided to do a complete rebuild including replacing all lower end crank and rod bearings. And I will warn you, they are NOT cheap. As far as the head goes, I am porting and polishing it, and will be throwing new valve guides in. I am also having a 2 angle valve job. I'd do 3, but apparently nobody in town here knows what it is or has the right equipment.

I'll let you know how the build goes HondaMan, and thanks for the heads up on the pistons.

[HondaMan]

I definitely don't plan on racing it.. I have a 1015 motor for that purpose only...

But, I am putting a rather mild street cam in it (its the 360 valve lift and 270 degree duration cam).

I tore the motor down, and decided to do a complete rebuild including replacing all lower end crank and rod bearings. And I will warn you, they are NOT cheap. As far as the head goes, I am porting and polishing it, and will be throwing new valve guides in. I am also having a 2 angle valve job. I'd do 3, but apparently nobody in town here knows what it is or has the right equipment.

I'll let you know how the build goes HondaMan, and thanks for the heads up on the pistons.

Where are you? Here in the Denver area, Ridge Reamer does a nice 3-angle valve job that I'd bet even MRieck would nod to (but, I think his is 'smoother" and breathes even better...).

[N30R3L0AD3D]

I live in the Northern Michigan area.

[HondaMan]

I definitely don't plan on racing it.. I have a 1015 motor for that purpose only...

But, I am putting a rather mild street cam in it (its the 360 valve lift and 270 degree duration cam).

I tore the motor down, and decided to do a complete rebuild including replacing all lower end crank and rod bearings. And I will warn you, they are NOT cheap. As far as the head goes, I am porting and polishing it, and will be throwing new valve guides in. I am also having a 2 angle valve job. I'd do 3, but apparently nobody in town here knows what it is or has the right equipment.

I'll let you know how the build goes HondaMan, and thanks for the heads up on the pistons.

270 degrees! The CR750 had 215 and 225 degree cams.

[MCRider]

I definitely don't plan on racing it.. I have a 1015 motor for that purpose only...

But, I am putting a rather mild street cam in it (its the 360 valve lift and 270 degree duration cam).

I tore the motor down, and decided to do a complete rebuild including replacing all lower end crank and rod bearings. And I will warn you, they are NOT cheap. As far as the head goes, I am porting and polishing it, and will be throwing new valve guides in. I am also having a 2 angle valve job. I'd do 3, but apparently nobody in town here knows what it is or has the right equipment.

I'll let you know how the build goes HondaMan, and thanks for the heads up on the pistons.

270 degrees! The CR750 had 215 and 225 degree cams.

Its gotta be a difference in measuring technique. The timing card for my puny-butt Webcam 41 says lift is .360:  "advertised duration =270°": duration at .050" = 237.

The duration I measured at 50thou was 225. Though I am hardly an expert measurer.

The drag race people I hung with back in those days always chided CR cams (well Yoshimura cams actually, which should be close?) as wimpy.