Big-Bore 56mm conversion questions!

[Larry B]

I had the Hyway big bore kit on my 660 clone. Made good power but drank fuel like crazy. Switched to a Cross MMWS kit and made slightly better power and uses less fuel.

 

[Mastermind]

I had the Hyway big bore kit on my 660 clone. Made good power but drank fuel like crazy. Switched to a Cross MMWS kit and made slightly better power and uses less fuel.

I used an OEM cylinder kit to start with on those. Cut the squish band and base, and used some epoxy to create the port height I wanted on the exhaust port. That's where it got twisted. They didn't get the port shape right on it even after sending it back for a couple of revisions. They did great on the upper transfers......but made the exhaust too flat.

 

[Larry B]

I am by no means an expert on saws but with the the MMWS kit with gasket delete advanced timing and modified muffler i am happy with it.

 

[Cerberus]

I used an OEM cylinder kit to start with on those. Cut the squish band and base, and used some epoxy to create the port height I wanted on the exhaust port. That's where it got twisted. They didn't get the port shape right on it even after sending it back for a couple of revisions. They did great on the upper transfers......but made the exhaust too flat.

Do you happen to know displacement #'s or compression-ratio gains from your machined jugs versus regulars? I've been trying displacement tests here and they're...not as simple as I'd expected they'd be (well, simple, but hard to trust precise #'s / get tight #'s upon repetition..)

I'm gonna throw out something crazy here -- what of reducing chamber displacement by a slightly protruding decomp plug? Don't get me wrong I have a pop-up coming and appropriate squish it's just a thought I had (because these big bore's chambers' ceilings are far thicker aluminum, ergo the decomp plug & spark plug sit recessed, I had to grind to make the plug sit appropriately but the decomp plug I just bought a longer one & cut-to-fit....but did leave the slightest protrusion (sanded/smooth of course!) because my thinking was "This can't cause any more turbulence than a 1mm recession at the hole, like usual, so may as well have it bulge out a lil instead of recess-in a lil, right?) But then you realize that the chamber is <10cc's, and that just 10 threads on the decomp plug is a whole CC (and it's fine enough threading) so that is "pop-up range" displacement alteration, no?

Thanks a ton for any thoughts (and for everything, obviously....not even 1yr ago I would've laughed at the idea that I would ever open & work-on an engine of any sort, now I currently have 2 saws opened on the 'bench'/table, thanks to your generosity man )

 

[Cerberus]

I am by no means an expert on saws but with the the MMWS kit with gasket delete advanced timing and modified muffler i am happy with it.

Do these MMWS jugs have higher transfer ceilings than OEM? Because otherwise my thought is as-usual: You really don't wanna drop the cylinder unless you've got the ability to raise your uppers' ceilings, otherwise your tiny intake-timing just got cut further..

(MM-- How close is a mass-production MMWS jug, to the one that you ported in my unit? FWIW I've probably spent >1hr studying & mapping that thing lol, learned so much -- how close do you consider "perfect" for skirt-overlap on the edges of an exhaust-port? The big bore ports are barely letting me get past 61% bore-width before the (wide!) skirts no longer cover...and am not spending $170 on a gorgeous Wiseco to resolve this!)

 

[Cerberus]

I did a 56mm jug using a 395XP piston. Complete waste of time and energy. The OEM 54mm top end wound up much stronger.

Did the 395xp piston "fit well enough"? I just checked and, shipped, they're <$15.....I know you're not suggesting it but I'm not even running it the same I'm using only bottom ring (so can cut the crown to fitment)
Would LOVE to hear that that 395xp slug will be "plug & play" on my 660's connecting rods/small end bearings/wrist pins, I know you think it's dumb but I just wanna try it's cheap & I'm learning TONS (am not just 'mcguyvering' I've already finished 2nd read of Jennings' book, am genuinely convinced the problems on these big bores can be overcome because they're just intake, freeport and compression -- all can be brought in-range if that piston skirt were like 20thou taller!!

 

[Nutball]

What about transfer tunnel size and curve? Is it the right shape & big enough on the big bore?

The problem with a longer skirt on a big bore is it won't fit the case. The piston will hit the case before the bottom of the stroke. Really the bottom of the port needs to be higher to stop the free porting.

Luckily, you should be able to do some grinding on the case to let the piston skirt down in there, and not have to worry too much about magnesium shavings ruining the cylinder, just keep the bearings clean. A recent test by Red97 showed even large amounts of magnesium swarf won't noticeably hurt the top end, so I'm less shy about grinding on crank cases now, and I'm the kind that doesn't want a single spec of dirt or fuzz in the engine as I assemble it.

 

[Mastermind]

Would LOVE to hear that that 395xp slug will be "plug & play"

Not even close. Quite a lot of machine work needed.

 

[Cerberus]

At this point I would say avoid the Hyway as both jugs and pistons I have received from them are not serviceable in stock form. So far in my research the Farmertec stuff is better than the Hyway.

That's odd I have found Hyway (and heard only anecdotes that confirmed this) that Hyway > F.Tec any day, I have a hyway pop-up 56mm arriving Sat so can do a direct-compare (already have a 54mm and that thing smoked the 4 f.tec pistons I had on-hand! Same for the jug, from port edges & lack of imperfections to clean squish band & chamber, f.tec jugs are only for those looking to spend time w/ the dremel!!

If you really want one of these saws to perform. Get an oem top end for it. Have that top end ported by a pro.

But I actually enjoy this, on a personal/passion level...no amount of dissuasion, however strong or continued, is going to change that..

Also I'd argue the OEM top-ends are unnecessarily expensive especially if a pro is going to be re-working the whole thing, I'd only go OEM if doing true "plug&play" but I get that many porters won't touch non-OEM's.

Thanks Re the Husq piston not working out-- to your knowledge is there ANY 56mm piston that'd fit here besides the ones from the kits? I still think their obstacles can be overcome, but a better piston would make it easier!!
(if you know none,let me at least offer that Wiseco makes a 56mm for the 660 that's got skirting so wide you could go 70% on the exhasut port width, and cut your intake skirt wherever you want it, and set your exhaust skirt where you want too --- On that note, what's the recommended "Distance-til-Freeport"?? My build is so close to freeport, and the exhaust so close to exposing piston skirt sides, that I can't tell which of those 2 faux pas are causing it to spit fuel like crazy through the muff!!)

Thankfully my $30 got me two of these kits, and I have a 56mm Hyway pop-up, so am going to do a gentler build that focuses on at least 10thou per side of skirting on the exhaust and at least 15thou from freeport(unless I learn that that's not enough, at which point I'd need to run a 2nd gasket and give-up the idea of having a flat squish band lol!!) Knew I should've held-back when I found myself knowing I was near freeport, but still squishing high 20's, so "just went to low 20's" and can't get a perfect measure for freeport (unsure how) but am sure there's some :/

I had the Hyway big bore kit on my 660 clone. Made good power but drank fuel like crazy. Switched to a Cross MMWS kit and made slightly better power and uses less fuel.

I wish I'd understood what the MMWS kits were before buying my 54mm's I would've gotten them (MM I know you recomended them but they didn't come w/ pop-up's from my seller, and I didn't know it had your porting...thankfully that bad 54mm jug I have still gives me a real nice example of how you like your uppers (or how you approached the intake/exhaust/deck height/etc on the saw-- though I'm surprised you don't seem to touch the piston, maybe that isn't considered kosher for that kind of build though? I'm finding that piston porting&polishing is a hugely neglected area, blows my mind to see so much clunky, non-aerodynamic casting leftovers (from true seams to the general shaping of piston-windows) being untouched in "an engine porting", am having trouble getting my brain to "leave out"the piston as part of 'full job' (does the average porter tend to do work to some pistons? On the 660 those windows have to feed your transfer circuit so IMO you may as well flow them large enough to ensure no bottlenecking of the lowers or the tunnels to the uppers, nevermind that doing so reduces reciprocating mass of piston and increases aerodynamics and increases displacement of the bottom-end which is especially sought in the big-bore case (I can get an easy ~4-5cc's I think, from the lowers/piston/upper crankcase lip, but am left unable how to cleanly get a few more CC's to truly re-balance OEM spec top//bottom displacement balance (if anything we want to increase relative bottom end displacement, not top end, right?)

They did great on the upper transfers...

I am by no means an expert on saws but with the the MMWS kit with gasket delete advanced timing and modified muffler i am happy with it.

If the mmws comes pre-machined as it sounds like, then it should be pretty darn boss plug&play.....how much did you shave from the key for timing advance? I currently have 10thou off mine on the big-bore build (with 25" chain and 8 pin sprocket) but have several more keys to test...
What'd you do for muffler? I'd love to see some really good muff-mod comparison videos, I think they would show that the average muff-mod is taken too-far, Jennings' book says that for muffler outlet-pipes (which I think is analogous to our "muffler exit port") is an area that, when too large, you get great gains dialing-in by tightening it, however when you go too-tight you don't start losing power at even remotely the same rate (this makes sense to me now, after having done my 2511t's exhaust system, sure it's better now - probably eats 1/5th more fuel, too -but that was strong OEM and the muffler outlet must be 1/3rd the size of a dime, it is hilarious, and if you saw the exhaust port, flanging & muffler internals it's even more hilariously choked-up...I think we're over-doing our mufflers at our own peril, the cylinder sucks charge from the muff right after BDC and we're mixing atmosphere with charge on so many of these 'high flow mods' :/

What about transfer tunnel size and curve? Is it the right shape & big enough on the big bore?

The problem with a longer skirt on a big bore is it won't fit the case. The piston will hit the case before the bottom of the stroke. Really the bottom of the port needs to be higher to stop the free porting.

Luckily, you should be able to do some grinding on the case to let the piston skirt down in there, and not have to worry too much about magnesium shavings ruining the cylinder, just keep the bearings clean. A recent test by Red97 showed even large amounts of magnesium swarf won't noticeably hurt the top end, so I'm less shy about grinding on crank cases now, and I'm the kind that doesn't want a single spec of dirt or fuzz in the engine as I assemble it.

That's really cool to hear Re cases, though I'm pretty OCD there -- I actually did the exhaust port flange on my 2511t w/o unmounting the jug, no need because exhasut port was like >2" deep and thus very very easy to properly "pack & seal" for the work, and then light & care allowed confidence in 100.0% cleanup (same for muffler, becvause there's a baffle wall inside there you have to open-up!)

Re 660's...I ordered the big bore because I had to port my case, the casting was crap so my thinking was "oh this is gonna fit a 56mm now!" and I check & see they're $30 shipped so I ordered...now it's a fun game seeing if I can make one work (work well, I mean the one I just built that is putting out charge from the muffler, it's not so bad that it's staying home, it's coming to a job Thur so I can push it a lil, and presumably by then or over the weekend I can finish this 2nd 56mm jug and the htyway pop up for it (got the 2nd one for free when I complained about the ring-of-death, 200thou unplated tops in these jugs, can only run bottom piston rings!)

Do you happen to have any pics of your work on 660 cases? It'd be massively appreciated!!

(Oh re transfers on 56mm setups, the transfer tunnels come deeper already (thinner walls though, like less aluminum behind the plating in-between lower & upper windows), and I don't have a right-angle grinder, but even the stock window-shape can be made to flow WAY more if you make the tunnel feeding it way larger, as well as making the uppers' roofs recessed further-back, it makes it so the charge's path is both wider & cleaner-flowing )

Not even close. Quite a lot of machine work needed.

I know I asked already but that 056 piston does look like it'd be good, has anyone ruled that one out yet? Because they're all 56mm bore, I can just deduce the rest of the measurements based off pictures, and the 056 looks good but I don't wanna buy something to test it (if I know it fits, then I'm happy to buy it and alter it as necessary, but it's gotta fit that connecting rod or it's gonna be beyond me!)

 

[Cerberus]

Since it's super important to me (am gonna be starting work on big bore jug #2 tonight..), I'm gonna ask again specifically & in another way:
Let's say that you've found yourself sitting 35thou squish clearance, and 20thou distance before-freeporting: What do you do? Leave the 35 squish and pray 20thou before-freeport is sufficient? Or is something like 10thou before-freeport OK, letting you drop things to get your squish to 25 and still have your 10thou distance-til-freeport?

(PS, since I'm not positive it's the freeporting causing the charge-loss out the muffler: If you're freeporting by, say 5thou, but your exhaust port is so wide that there's barely 2-3 thou of skirt clearance on each side to seal the exhaust port, is one of these "much more likely" to be the culprit? I obviously messed-up on the exhaust port width, I just don't know what the minimum overlap should be...same for freeporting, I thought "a tiny bit is OK" but now am not so sure! Thankfully I'm 99% sure I can get the 2nd jug setup so that squish is under 30ish, while still having no free-porting (and if I need more exhaust I can just raise the ceiling, this build had exhaust at 106!))

 

[Cerberus]

To be crystal clear Re Husqv 395 pistons-- MM are you saying they do not fit a 12mm wrist-pin? I know the crown-height, and skirt heights, will need machining -- but will they fit the wrist pin / small end bearing of the 660? I keep looking / measuring the photo from one for sale on ebay and it looks like that's a 12mm wrist pin (which'd mean the only work would be altering crown-height, and skirt heights, but I'm plenty happy with doing those....if it fits the 660 connecting rod's small end bearing & wrist pin and I'm getting to choose its height & skirt-lengths, that's good / ideal for me!!)
I'm confident the 56mm wiseco's would let a big bore smoke a 54mm (how could it not? What part of it is drawback enough to overcome the >10% displacement jump? Maybe the carb can't feed it properly or something?)

 

[Nutball]

That's really cool to hear Re cases, though I'm pretty OCD there -- I actually did the exhaust port flange on my 2511t w/o unmounting the jug, no need because exhasut port was like >2" deep and thus very very easy to properly "pack & seal" for the work, and then light & care allowed confidence in 100.0% cleanup (same for muffler, becvause there's a baffle wall inside there you have to open-up!)

The 2511t is aluminum, a lot different material than magnesium found on large crank cases.

Don't worry about free porting the bottom of the exhaust port unless it is much more than 1mm. It's the sides of the piston you want plenty of overlap past the sides of the exhaust and intake port.

 

[Cerberus]

The 2511t is aluminum, a lot different material than magnesium found on large crank cases.

Don't worry about free porting the bottom of the exhaust port unless it is much more than 1mm. It's the sides of the piston you want plenty of overlap past the sides of the exhaust and intake port.

That's cool to hear Re the 2511t but I actually flared/flowed my exhaust port's edge/flanging (last several mm's) while it was mounted! Setup was simple enough and that cylinder//muffler interface is just AWFUL, the exhaust port is awesome a long straight tube but then it hits this sharp 90deg angled, rough-edges bend that is actually a slight choke-point! You can open the whole area up, port matching the heat shield & gasket of course, about 20% (the muff mounting screws are your limiting 'edges' so to speak, where you start & work-from) Would love knowing how much that, plus the airbox mods (ie not having it under vacuum!) add to the already large #'s seen just by un-corking the muffler exit!!

Re freeporting, is it really that little a problem? I ended up w/ a different brand of 56mm, "NewJ's", and it actually had altered ports the intake was raised (annoying, as I like builidng my intake, so this just left it w/ a higher ceiling than I'd have wanted) but was nearly 1.5mm higher exhaust floor! Let me flatten like 1/3rd of the band, polish the remainder at OEM-angle, then removed around 10 from the base and came in:
squish 24.25thou;
exhaust- 99.5 (100, 101 even, with an exaggerated center-ceiling)
intake- 81
transfers- 126 (transfer tunnel enlarged, uppers' roofs "brought back" effectively making the uppers larger, and piston heavily ported to allow max flow into the flared lowers when piston's at BDC
And still have about 50thou before freeport!

My 1st 56mm build was spitting, it did have too-open a muffler but I know it was also the cylinder and figured it was the freeporting (was basically 5-10thou of freeport, just a hair), subsequently realized I didn't do my math right on the skirts and was barely closing the port (if at all lol), how much do you consider "minimum overlap"? Jennings says 2mm and am pretty sure Blair says the same but they're not really focused on our types of 2-strokes (sucks "learning 2-strokes" entirely in a paradigm of "expansion chambers are critical part of system" lol!!)

 

[cloneguy]

what jetting are u guys useing seems like ever big bore clone i do love timing and jetting u guys got vids of youer saws cuting

 

[Cerberus]

what jetting are u guys useing seems like ever big bore clone i do love timing and jetting u guys got vids of youer saws cuting

Welcome!!

I always start at 10-20thou and if being honest I've mixed-up which of my units has the 10, and the 20, thou keys (am like 99% sure the 20thou-removed key is in my "felling build" with the 56mm head, and the 10thou is in the 54mm bucking-build)

Would be curious for elaboration on jetting, have seen some videos but the carbs (the entire intake tract actually) has never seemed a weak-point, save for ensuring the airbox is setup for high-flow (and filtered while doing so!), one of my units has a proper Walbro the other has 3rd Gen F.Tec chinesium carb which, while it had the "won't return all-the-way down to Idle RPM when you let off the trigger" problem that many have, it went away real quickly and never came back (and both units have always allowed fast-idle once I figured it out, had thought they both "didn't allow it" til I got a feel for setting the switch, you gotta be realllly precise&deliberate but both of my chinesium levers let me use fast-idle!)

Very curious Re jetting, my big bore seems just fine it hardly needed that much H&L enrichment at all (although I guess that's to be expected, right? Because bigger displacement is sipmly increasing total flow thruogh the carb, not asking for altered air//fuel ratio)

I THINK the 395xp piston is a fit for the 56mm 660 top-ends, will order one the moment I can confirm this lol, I've measured and it certainly fixes the short-skirts problem (took me 3 56mm jugs to get 1 with good #'s, but *all* could've worked if I'd had longer piston-skirts to work with!)

 

[qurotro]

To be crystal clear Re Husqv 395 pistons-- MM are you saying they do not fit a 12mm wrist-pin? I know the crown-height, and skirt heights, will need machining -- but will they fit the wrist pin / small end bearing of the 660? I keep looking / measuring the photo from one for sale on ebay and it looks like that's a 12mm wrist pin (which'd mean the only work would be altering crown-height, and skirt heights, but I'm plenty happy with doing those....if it fits the 660 connecting rod's small end bearing & wrist pin and I'm getting to choose its height & skirt-lengths, that's good / ideal for me!!)
I'm confident the 56mm wiseco's would let a big bore smoke a 54mm (how could it not? What part of it is drawback enough to overcome the >10% displacement jump? Maybe the carb can't feed it properly or something?)

When using 395 piston you need a 395 wrist pin bearing.

 

[Cerberus]

When using 395 piston you need a 395 wrist pin bearing.

To be crystal clear, you're saying if I simply order the gudgeon/wristpin bearing as well, then the 395 piston (and its included wristpin) will properly fit my 660 con.rod?


Will be ordering asap, am so uncertain why this wouldn't already be "a norm", from the pics it seems that at-worst I'll have to reduce the crown height just a touch maybe (doesn't seem it, but no biggie if so, not for those skirts!) Had heard "they make those skirts short so they don't protrude into the 54mm case" but they certainly do exit the jug, can post pics if anyone doesn't believe but they are only 73thou shorter than OEM and OEM sees the skirts exit the jug >73thou..... Nevermind how easy some basic edge-grinding of the crankcase is anyways (was what I'd expected when initially ordering my 1st big-bore kit, was surprised/saddened learning the whole short-piston nonsense, took 3 damn kits to make 1 work....soooo much grinding/waste...and learning!!)

 

[qurotro]

To be crystal clear, you're saying if I simply order the gudgeon/wristpin bearing as well, then the 395 piston (and its included wristpin) will properly fit my 660 con.rod?


Will be ordering asap, am so uncertain why this wouldn't already be "a norm", from the pics it seems that at-worst I'll have to reduce the crown height just a touch maybe (doesn't seem it, but no biggie if so, not for those skirts!) Had heard "they make those skirts short so they don't protrude into the 54mm case" but they certainly do exit the jug, can post pics if anyone doesn't believe but they are only 73thou shorter than OEM and OEM sees the skirts exit the jug >73thou..... Nevermind how easy some basic edge-grinding of the crankcase is anyways (was what I'd expected when initially ordering my 1st big-bore kit, was surprised/saddened learning the whole short-piston nonsense, took 3 damn kits to make 1 work....soooo much grinding/waste...and learning!!)

395 and 660 have the same size small end. But the 395 use bigger wrist pin thus thinner needle in the bearing.

 

[Mastermind]

I've built big bore 660s with the 395 piston. Could never make one run as well as a 54mm oem top end.