Piston skirt length

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pago cruiser

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I've soon to be in the market for a 750cc big cylinder kit from MikesXS - I think. Mikes has been out of stock for awhile, and Heiden Tuning as well. I don't know, but I assume that Mikes and Heiden use the same source? When I compared a few months ago, at least the pics were the same.

I called another vendor, and he indicated that the piston skirt height in Mikes 750 kit has been shortened. I know it would be lighter, and in performance engines, lighter pistons is a goal. But this is not really a performance engine, and a piston lightened with shorter skirts may have more inclination of piston rocking, at least when compared to a stock piston skirt. I did see that Mikes ad indicates that the skirts are "Mos2" treated. If the skirts were indeed shortened, AND rocking is a concern, (and I do NOT know), I don't know how a coating could compensate. Anti friction, yes. Anti rocking, I dunno.

Interestingly enough, Heiden's indicates that a "new design" for a 447 crank 750 kit is "comming (sic) soon". This has been on their site unchanged for the last month. But they do NOT list 9:1 pistons, only 10:1. But Mikes still shows 9:1. I live in Tucson, would prefer the 9:1 for longevity.

If I needed it right now (thankfully, I don't), the only way would be to buy Mikes Big Cylinder kit, then buy a 9.2:1 750 kit from 650 Central. This would also cost a bit more, as Mikes 750 kit is $700, while the 650 Big Cylinder is $380, and the 750 kit from 650 Central (Wiseco pistons) is $550, PLUS the sleeve installation and boring at $350, totaling $1280. Ouch!

Anybody happen to have a Mikes piston and a Wiseco piston (or a stock XS) BOTH in your garage/shop to compare?:confused:

TIA.
 
Last edited:
That sounds...informed. But... I'm not trying to design a piston; just trying to find, or verify, something that's not... ill designed, for lack of a better word.
So do you know if MikesXS skirts are short? That last sentence sure sounds odd...
 
After a deafening silence response... did a bit more online. Both folks are good enough to post pics. Here ya go:

Mikes:
MikesXSBigBoreKit.jpg


And Heiden Tuning 80mm (same 750CC sizes) 10:1 pistons:

HeidenTuning80MM.jpg


So, yes, Mikes skirts are shorter. Maybe 6mm or 8mm? So. How big a difference does it make? I gather Mikes sells quite a few of his full kits. Simply put, can I realistically get, oh, 25,000+ miles on these without issues?:umm:
 
You could sorta view those as oldschool (Wiseco) vs modern (Mikes coated). The oldschool skirt extension dips below the cylinder bottom at BDC, gets a little cooling and oiling. Modern piston relies on the coating. Two different design philosophies, each with their own advantages and compromises. Racing tricks, probably not important when considering a mild-mannered reliable streetbike...
 
Noticed that you're in Arizona. Hot weather, worse than here in Texas. Most piston and cylinder issues we ran into dealt with heat-related piston scuffing and cylinder scoring. I'd opt for the coated pistons to reduce that. Plus, the oil changes are very important to get that longevity. ..
 
Hey Two,
Thanks for the responses.
But I'm still confused...

I get the part where the longer skirt dipping below the cylinder at BDC get splashed with oil, assisting with cooling.

And I get the part where the coating (anti friction) would help with cooling.

But...coatings wear. Will an anti friction coating last 20,000 miles?

And if you were concerned with heat (and I am), it would seem hard to beat being splashed with oil a hundred times a second or so... which would lead to skirts as being preferred for longevity. I think.

This may just be academic, as I have seen racing pistons that are not much more than a dome with a wrist pin attached. But Mikes pistons still has a good length skirt, just less than the Wiseco's.

But at some point in skirt shortening, sh!t happens. Many racing motors (with those abbreviated skirts) are also good for 40 hours before a piston is replaced, with a whopping 80 hours between cylinder replacement... YIKES!

Yep, its usually hotter here in AZ than TX- 102 today; but not always. I travel to Big Spring several times a year for work - next week actually. But that stretch east of El Paso can seem like the Hiway to Hell...

I like your sig BTW...
 
Actually, I was implying that the coating would reduce scuffing. Of all the bookmarks I've collected, I can't find the ones on 'mos2 piston coatings'. Maybe you could do a google search on that. I was fairly impressed with the reports.

The heat-related piston scuffing problems we saw were primarily on the FRONT of the piston. The rear of the piston gets the oil from crank windage, when it's down. The front of the cylinder gets gets oil from the same windage. However, when the piston goes down, the skirts and cylinder are oiled, but when the piston goes back up, the oil has been scraped thin by the rings. AND, when the piston is shoved back up, its resistive inertia causes the angled rod to shove the piston forward against the cylinder front, which has been scrubbed of oil by the rings. This forward shoving can be several hundred pounds of force (longer rods reduce that), and with excess heat, thinned/hot oil, hot piston trying to expand and melt, ...etc., scuffing on the front can happen.

So, that's why I'm really interested in these modern coatings. We didn't have them back then, and reports from those (mos2 coatings) websites look very promising.
 
I think your over thinking it. Mike's kit works fine. If your worried about heat mod the side cover to plumb in an oil cooler. I have one off an 82 Yamaha XJ650 Turbo Seca. It very close to the size of the one Mike's sells. Item #70-0750.
Mine drops oil and engine temps around 45-50 degrees.
Leo
 
XSLeo - You're probably right, but as an old fart engineer I usually don't let go until I resolve the puzzle in my brain... And I viewed NOT having any info on the piston coating as odd...

Appreciate the info on the cooler. I'm already planning on one; lots of good info on the site. Did not know about the Seca cooler. Still have more to do...

Anyhow, I found this this evening; I love the Inet when it works... This type of info would not have been possible to find except by word of mouth 20 years ago... Guess I'm comfortable with the answer. So unless or until someone posts pics of Mikes or the Wiseco pistons in poor condition after 10,000 miles of use, guess there's no reason to fear the future... I'm just trying to stack the deck in my favor of getting 25,000 hard miles on one engine...

Here is the URL of the data:
http://www.partinfo.co.uk/files/SB2167 - PISTON COATINGS - SILK SCREEN PRINT.pdf

Mos2CoatingVSPistonSkirtLength.jpg


Thanks for the responses guys. Now I just have to wait until MikesXS gets some more hardware in stock. Next puzzle: How come he is out of stock on these more than in stock???
 
Well done, Pago. I have zero experience with coated pistons, but they sure look promising.

If my time machine wasn't broke, I could take you back to the '70s and show you boatloads of scuffed pistons, including Wisecos of the time. My 80mm pistons back then were factory Yamaha, cast, and show scuffing after only a few thousand miles...
 
More good (and funny...) stuff found here:
http://www.speedtalk.com/forum/viewtopic.php?f=1&t=6334

The most interesting is about 3 pages down, where a gent had both coated and un-coated pistons in the same engine, at the same time.

First time I have seen (although I'm a noob on this topic) coefficient of friction (Cf) values listed for motor oil and some coatings. The coatings win handily...

The only thing I have NOT seen is a comparison of how much skirt shortening, with the resulting reduced area, and subsequent increased friction on that reduced area, does the reduced friction coating compensate for? I'm assuming this is closely held data by piston manufacturer's... Sigh.

Unrelated question: When I posted this initially, I checked the box for an "engine" tag; but it does not show up in that section... Seems the only way I can find this post now is in my "Subscribed Threads". I can see no way to tag this (again) with an "engine" tag, which is where it belongs. Any idea how? This stuff might be useful to other inquiring minds...
 
Thanx for the link. It had the product links that I was looking for:

http://www.techlinecoatings.com
http://www.kgcoatings.com
http://www.dowcorning.com/applications/search/default.aspx?R=740EN
http://www.ellsworth.com/Product-Li...-Friction-Coating/Anti-Friction Coating/7155/

AND, I liked this post by piledriver:

"The coefficient of friction of motor oil (out of the bottle) typically runs ~0.15 or thereabouts. (steel on aluminum) and can approach .4 when old and tired or if it gets cooked.

Cf for Mos2 at 300,000 PSI is ~.05 (gets better with pressure)
Cf for WS2 at typical pressures is ~.04 or less.

Either are chemically/physically stable >600C. Oil ceases being oil long before that.
(MoS2 works better at extreme pressure, WS2 generally kicks everything elses ass everywhere else, and does OK at extreme pressures)
Under some conditions either can form thin films with a Cf of .005 or less.
Motor oil, by itself, is not all that. Not even remotely.
Piledriver; Van Alstyne, Texas"

Oldschool books I have on friction claim that the coefficient of friction of oil-lubricated pistons is more closely tied to the differential surface velocities than the materials in use. But that doesn't apply when the thin-film breaks down.

I had a can of assembly dri-coat moly that I used on racing camshafts back in the early '70s, but it was mysteriously *borrowed*, never to return. Want to get some of this modern stuff. I coated my custom XS cam with this, and after several thousand miles it's still on there...
 
Thanks Two. Nothing like hiding in plain sight...:doh:

I just lucked upon that site; about the 4th page down in a google search site. It started out a bit dated (2007!), but was resurrected a few years ago. Great data in there. Glad you listed some of the links. I'm seeing more and more dead links on average when looking for technical stuff. A shame that even with the Inet, collective knowledge still disappears.

The post about the guys oven was deja vu. In 72 I had one of the first Z1's. After about 10,000 miles on it, I stripped it down and rebuilt with Yoshimura 1000cc overbore, cams, ported, stainless valves, bronze bushings, 4 into 1, etc. I used Kal Guard (now KG Coatings) black (aka Gun Cote) on the engine, baked in the oven in the house we were renting. That oven stank for months...when we moved out the house, it STILL had some residual stink when turned up high... A little late, but apologies, Mr Landlord...:oops:

Re: Tags
It appears there is a minor disconnect between tags you can use when starting a post, and tags that display in "The Garage". I used "engine building" when creating the first post, which seemed reasonable. But "engine building" is not listed as a tag in The Garage; only "engine".

I checked as a part of this post, and there are a total of (3) posts with "engine building"; the most recent about a year ago. :shrug:

I've added the "engine" tag so this will (I think) appear in a place more frequented, and findable.
 
Interesting. Sounds like we hail from a similar timeframe, just opposite sides of the fence. In '72 we were doing cb-750 to 1100 Action Fours kits. DADDYGCYCLES and others here also from that time.

I guess you know "The halflife of an engineer's knowledge is 3 years". Fortunately not here.

Next year I may start a thread "Geritol, and why I need it"...
 
If you or any one else is intrested have just pulled my 750 kitted engine down due to g/box failure, The engine was fully rebuilt 15,000+ kms ago rephrased 277 mike's 750 kit etc. The 750 kit seems to have lasted very well, the moss coating is worn but still there miner bore scuff marks butt can still see hone marks hoping to just re hone & new rings could proably get away with out these but for pease of mind & price of a set of rings WTF. The motor was still running great but was a bit rattly, not a death rattle just a bit rattly. I think I have found the source the cam chain was running to the right side of the rear guide & hitting the metal bracket not sure why yet but will try to keep you posted then there is the issue of the rear guide fouling on the big bore kit to check out, hope fully all will be revealed & put to bed as I get all back together take care
 
i can't answer your ? about skirt length. I will say this, will all the complaints about the quality of "some" of Mikes parts, I would be inclined to trust those "old school" Wiseco pistons over Mikes kit pistons. Nothing wrong with coatings, they have their value. Pretty sure mos2 is just moly. They are probably more worth while in a 2 stroke, IMHO.I didn't read all of the above responses.

Have you thought about 700cc's. That allows use of the oem cylinder.
 

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