Topic: CycleX pistons "for chopper or bobber".

[HondaMan]

I bought a set of these 0.50mm oversized pistons for a low-cost rebuild I'm doing, and to evaluate them in general. Here's what I have found with them:
1. They are EMGO (i.e., Chinese) copies of the ART pistons.
2. They have some flash that must be removed before you use them, lest it end up inside your oil pump, which will cause significant pump damage. Pictures below: the flash is around the oil holes that are just below the oil rings.
3. The surface of the inside skirt ring is not finished: you must finish this yourself or it will end up like the flash in #2 above, and will pass through the oil pump.
4. The pistons are copies of the K0-K6 type pistons, with the "flat top" (barely domed) style crown. This is different from the F and K7/8 pistons, and will result in compression ratios of about 8.8:1 if used in the later heads and cylinders. When used in K0-K3 cylinders, they will yield about 9.0:1 compression, and will yield about 8.9:1 compression in the K4-K6 (taller) cylinders. This mimics the Honda ART pistons for the K0-K6 engines.

The pistons come with fitted wrist pins that seem to have a good fit of 0.0004" to 0.0008" clearance with the pistons. I measured the wrist pins' diameter and they are on the Honda spec.

[Old Scrambler]

Thanks for the info........Let us know how they do with fire in the hole.

[mick7504]

EMGO is the brand name of the Cyclex gaskets sets too.
Made in Taiwan and appear to do the job.

[timdhawk]

would the compression difference of using these in a 78 750k be that noticeable or have any performance issues for the average rider?

[HondaMan]

would the compression difference of using these in a 78 750k be that noticeable or have any performance issues for the average rider?

They are "flat dome" type from the earlier K engines, so they will drop the compression of those later engines to about 8.8:1. This will make the compression numbers (after ring break-in) to be about 105 PSI. On the upside, it will let you tune the engine to be like most modern cars, where you could add extra spark advance with stronger spring tension and run hotter sparkplugs, all to increase the low midrange torque. It will drop off the high-end HP a bit (about 8% or so, volumetrically speaking), but I think the overall increase you will get for boring the cylinders round again (which is on the order of 10% increase) will still net you an improvement.

If, at the same time, you advanced the cam about 4-5 degrees (by slotting the sprocket bolt holes on the cam sprocket, or by getting an advance sprocket from RC or someone), you would definitely feel a larger torque in the 3k-6k RPM range. In most cases, everyone rides right there, so adding all these things together could really wake up a tired 750. Drop   a tooth off the countersprocket (from stock gearing) and your Saturday night rides could really light up!

And, you could run cheap gas.

[bucky katt]

3.
4. The pistons are copies of the K0-K6 type pistons, with the "flat top" (barely domed) style crown. This is different from the F and K7/8 pistons, and will result in compression ratios of about 8.8:1 if used in the later heads and cylinders. When used in K0-K3 cylinders, they will yield about 9.0:1 compression, and will yield about 8.9:1 compression in the K4-K6 (taller) cylinders. This mimics the Honda ART pistons for the K0-K6 engines.

sorry for the hijack, just a question for you Mark. what is the height difference between thek4-k6 cylinders and the earlier ones? is it worth trying to hunt the earlier ones down for a mostly stock rebuild to get a little more compression or is the increase not worth the extra effort?

[HondaMan]

3.
4. The pistons are copies of the K0-K6 type pistons, with the "flat top" (barely domed) style crown. This is different from the F and K7/8 pistons, and will result in compression ratios of about 8.8:1 if used in the later heads and cylinders. When used in K0-K3 cylinders, they will yield about 9.0:1 compression, and will yield about 8.9:1 compression in the K4-K6 (taller) cylinders. This mimics the Honda ART pistons for the K0-K6 engines.

sorry for the hijack, just a question for you Mark. what is the height difference between thek4-k6 cylinders and the earlier ones? is it worth trying to hunt the earlier ones down for a mostly stock rebuild to get a little more compression or is the increase not worth the extra effort?

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. The "ears" on the advancers were just bent in a little further to accomplish this part: you can either bend them back out or cut into the ears and the weights with a grinder to restore the advance. The initial "F" marks were also advanced a little, which is the clue when you hold them next to the earlier versions. That was done to make it idle at about 1150 RPM while keeping the idle throttle opening in about the same place on the carb slides.

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.

[srook]

Hi Mark,
I think what everyone (especially me) really wants to know is, would you use them again?  Do you forsee any problems down the road with these pistons?  Finally could you post pictures of the pistons with flash removed?
Thanks
Scott

[smccloud]

Since its getting close to time to tear down my '72 750 to find the noise, I am also interested to know if you'd use the pistons again.  Specifically on a street bike that is at worst raced from stop light to stop light (I love beating restored cars with my raggy looking bike) and on some longer rides (i.e. memorial ride for my fiance's dad's cousin).

[HondaMan]

Hi Mark,
I think what everyone (especially me) really wants to know is, would you use them again?  Do you forsee any problems down the road with these pistons?  Finally could you post pictures of the pistons with flash removed?
Thanks
Scott

I don't forsee any problems with normal use. It's those guys like me who flog things often that may wish to step up a grade to the cam-ground, polished ones or the tightly controlled ART Honda variety pistons.

The one thing in particular that bothers me with these pistons isn't their cast finishing flaws (which can be cleaned up), but the 3-piece oil rings: I don't like those, never did. Still don't. I prefer the one-piece ring for what may be personal reasons: I can't argue with 88,000+ no-oil-usage performance from them, either (including racing time). And, I have not been nice to my 750, outside of giving it fresh oil more often than Honda's schedule. All of the 750s I fixed over the years that were using oil past the rings have ALL had the 3-piece rings. Conversely, not all high-mileage 750s I've been inside that worked OK had one-piece: many had 3-piece rings. I won't argue beyond that particular observation, though, or we'll have an oil thread on our hands. 

If I were building a bike for general use, and the rider was someone who was, say, a college kid, broke, needs something to ride, sure, I'd use them (and am about to...). If the rider was going RTW or blitzing alongside (or ahead of !) me in the Rockies, no. They will probably work fine in between those two boundaries, though.

[smccloud]

Hi Mark,
I think what everyone (especially me) really wants to know is, would you use them again?  Do you forsee any problems down the road with these pistons?  Finally could you post pictures of the pistons with flash removed?
Thanks
Scott

I don't forsee any problems with normal use. It's those guys like me who flog things often that may wish to step up a grade to the cam-ground, polished ones or the tightly controlled ART Honda variety pistons.

The one thing in particular that bothers me with these pistons isn't their cast finishing flaws (which can be cleaned up), but the 3-piece oil rings: I don't like those, never did. Still don't. I prefer the one-piece ring for what may be personal reasons: I can't argue with 88,000+ no-oil-usage performance from them, either (including racing time). And, I have not been nice to my 750, outside of giving it fresh oil more often than Honda's schedule. All of the 750s I fixed over the years that were using oil past the rings have ALL had the 3-piece rings. Conversely, not all high-mileage 750s I've been inside that worked OK had one-piece: many had 3-piece rings. I won't argue beyond that particular observation, though, or we'll have an oil thread on our hands. 

If I were building a bike for general use, and the rider was someone who was, say, a college kid, broke, needs something to ride, sure, I'd use them (and am about to...). If the rider was going RTW or blitzing alongside (or ahead of !) me in the Rockies, no. They will probably work fine in between those two boundaries, though.

I look at the 3 piece oil rings like this, they have been used in high performance car engines forever so they should be fine for someone like me that doesn't flog his bike all that often

[scroggins5000]

I had a tough time finding pistons and rings for my 78 750F. I was trying to stay with stock sizing, but after many unsuccessful searches I just went ahead and bought the 836 piston kit from www.dynoman.net. I chose this set over the CycleX piston kit, because the CycleX set seemed cheap and don't offer as high of compression. Do you know anything about dynoman pistons? I'd like to know if I made the right choice.

[MRieck]

I had a tough time finding pistons and rings for my 78 750F. I was trying to stay with stock sizing, but after many unsuccessful searches I just went ahead and bought the 836 piston kit from www.dynoman.net. I chose this set over the CycleX piston kit, because the CycleX set seemed cheap and don't offer as high of compression. Do you know anything about dynoman pistons? I'd like to know if I made the right choice.

They are forged JE pistons.

[HondaMan]

I had a tough time finding pistons and rings for my 78 750F. I was trying to stay with stock sizing, but after many unsuccessful searches I just went ahead and bought the 836 piston kit from www.dynoman.net. I chose this set over the CycleX piston kit, because the CycleX set seemed cheap and don't offer as high of compression. Do you know anything about dynoman pistons? I'd like to know if I made the right choice.

They are forged JE pistons.

Thanks, "M": good to know!

[cb750k77]

so let me get this right???

to maintain the correct CR.... i would need to install earlyer cyclenders??? to my 77k motor??? and the real question is will it fit correctly??

how do i tell what year the jug is?

[heyjones]

I've been eyeing these for my rebuild, too. $170 to replace all four pistons and rings seems too good to be true, based on the other prices I've seen.

[heyjones]

I found another Hondaman thread on these pistons:

http://www.z1enterprises.com/detail.aspx?ID=4074

Between these and the CycleX pistons, which did you prefer? Not much of a price difference at $30 either.

[HondaMan]

I found another Hondaman thread on these pistons:

http://www.z1enterprises.com/detail.aspx?ID=4074

Between these and the CycleX pistons, which did you prefer? Not much of a price difference at $30 either.

Well, being the consumate Engineer, my answer to that one is, "It depends". 
I'm not nice to my 750, never really have been. I've forced it to push a Vetter as hard as the bike would go when touring, I leave stoplights and signs hard enough to wear a flat spot on the rear tire in one year of riding (so then I have to go to the hills to make it round again), and I often ride in heavy city traffic in summer with the Vetter lowers in place, because it might rain. That cooks it but good, even boiling the fuel in the carbs on occasion. Even then, I'll leave the stoplight and go for gold, mostly to cool ME off. 
And then, I'll go out once in a while to see how fast it will still go (with the Vetter).

So...I want pistons that can take that. This means they must have very even casting, good materials, and good surface finish. ART pistons have done well, distorting only about 0.002" in the skirt area, and not burning oil, even with 80,000+ miles on the last 0.25mm set.

If I build a low-budget bike, it won't get the extra-power head and such things if the pistons are not top-notch, at least not from me. Over the years, I have seen some marginal pistons appear that worked OK, which I take as a tribute to the engine's design and the rider's moderation, but always on stock engines with stock heads and cams. Headers won't make enough difference to matter, either. And, I have built some of these engines, usually for riders I know aren't going to the dragstrip during the summer, and don't exhibit the riding experience (or craziness) to go out and flog a bike that will pass the ton quickly. In those situations, I am sure the EMGO pistons will work flawlessly. Just be sure to remove the flashing from the edges and holes.

If you're going racing with them, you might want to have the EMGO units tested at least: but then, it would be cheaper to buy some of the other ones out there. Serious racers magnaflux the rods and have the pistons closely examined with density (X-ray and similar testing was common when I did it) checking to catch any possible voids or weak spots: not cheap! It can prevent a fractured skirt, though, which is worth the cost. 

[heyjones]

I just figure the extra $7.50 per piston for a nicer part wasn't a bad deal! Also, not to thread jack, but I'm taking my top end apart per the instructions in your book and I've hit a snag with the rocker towers...

http://forums.sohc4.net/index.php?topic=64103.msg771675#msg771675

[750four]

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

[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.

[750four]

I recieved a response from the seller today. States they are cast from geniune Honda parts. From Taiwan. I thought 65mm is a 836 kit?

[GammaFlat]

I recieved a response from the seller today. States they are cast from geniune Honda parts. From Taiwan. I thought 65mm is a 836 kit?

I think I'm in love.  ..and there's nothing like a cheap date

pi R² * stroke * # of cyls

pi R (radius - 1/2 the diameter) ² * stroke(63 - stroke of a 750) * # of cyls(4 - assuming you're running on all 4 = displacement

 ...         3.141593 * (65/2)² * 63 * 4 = 836.213 - using 65 as the bore here.. yee ha! an 836

as a proof.. (stock numbers)
             3.141593 * (61/2)² * 63 * 4 = 736.461  - stock bore.. wadda ya know

[HondaMan]

I recieved a response from the seller today. States they are cast from geniune Honda parts. From Taiwan. I thought 65mm is a 836 kit?

I think I'm in love.  ..and there's nothing like a cheap date

pi R² * stroke * # of cyls

pi R (radius - 1/2 the diameter) ² * stroke(63 - stroke of a 750) * # of cyls(4 - assuming you're running on all 4 = displacement

 ...         3.141593 * (65/2)² * 63 * 4 = 836.213 - using 65 as the bore here.. yee ha! an 836

as a proof.. (stock numbers)
             3.141593 * (61/2)² * 63 * 4 = 736.461  - stock bore.. wadda ya know

Oops, this is me:     <<<
The ones I ordered are 64mm bore, not 65.
Sorry! The 65 are the 836cc size, GM has that right.
When I ordered, it said "10 or more [sets] are available". I was hoping to order a second set soon, but haven't seen the ad return on eBay yet. The pistons should be along shortly. The 811 seen to be getting harder to find these days. Z1 suggested they might try importing some soon, hope they do. I've always felt like ebay is kind of "hit or miss" on some of the sellers with Thailand return addresses. I've done OK with Honda 90 and Honda 70 parts from these guys, though, so we'll see!

[GammaFlat]

The ones I ordered are 64mm bore, not 65.
Sorry! The 65 are the 836cc size, GM has that right.
When I ordered, it said "10 or more [sets] are available". I was hoping to order a second set soon, but haven't seen the ad return on eBay yet. The pistons should be along shortly. The 811 seen to be getting harder to find these days. Z1 suggested they might try importing some soon, hope they do. I've always felt like ebay is kind of "hit or miss" on some of the sellers with Thailand return addresses. I've done OK with Honda 90 and Honda 70 parts from these guys, though, so we'll see!

I'm really anxious to get your take on the quality - is it the same vendor as the 64mm guys?