Can we talk about these base-gaskets that are narrower than the piston? & Case-matching in general!

[Cerberus]

I'm curious if anybody could explain how/why Stihl made what, to my lay eyes, looks to be a VERY inefficient "crankcase to cylinder mating area", IE the match-up of the cylinder to the crankcase has this intentional "barring" of the sides of the cylinder beneath the transfers, even the metal base gaskets* have this 'side blockage' that makes it so you can't slide it over a piston.
(*these gaskets, the proud/protruding/bulging side faces downards correct? Want to say it was an OPE member's channel I was watching where they say the bump goes upwards, but the Stihl manual certainly seems to read the opposite!)

While I'm curious about why Stihl chose to make it that way, I'm also curious about "the negation of the blockage", I mean if one were so inclined to open their casing up and smooth-out that hard ridge, ditch the OEM base-gaskets, they could create FAR more "intake charge potential" for the cylinder (would, of course, need to port the transfers to realize this potential...am guessing that "hot-saw 660's" exploit the heck outta this area)

Thanks for any insight on this, oh also if you ground-off enough of your case wouldn't you "compensate" appropriately for the big-bore kits? My understanding was that their less-than-stellar performance is due to inadequate bottom-end displacement to match the larger cylinders, so if you've ground enough from that crankcase transfer area you've effectively equalized that mis-match (though I'm gonna guess that, generally speaking and 'for power', the goal would be smaller cylinder on larger crankcase, not vice versa!!)

Thanks again for any thoughts on these topics and happy holidays everyone!!!


//for fun....here's a picture of what I'll be doing "round 1 correcting" to:

///like my piston-ring compressor? ;D The handcuff zipties work a charm, not sure I'd even desire to try another type, they let you go hands-free and regular compressors do not

 

[Mastermind]

Now roll the crank 180 degrees and see how high the counterweights are in relation to the sealing flange.

Also.....on a BB kit.....the added displacement of the bore also increases the case volume. Everything under the piston is case volume......so.

The reason BB kit run like garbage is because they are garbage.

 

[Cerberus]

BTW I should mention that that ^ crankcase isn't just "2 halves that aren't matched to one another" properly, but the actual assembly has left a ~1/8mm or maybe 10-15thou misalignment between the case-halves.... Would anyone recommend loosening of the bolts, bang with hard-rubber mallet, and retighten bolts? Maybe it'd work! Guessing it's a hard 'No' but had to ask, I can't justify a pre-emptive full tear-apart so plan is "run it til it fails, then learn the cranks/crankcase" (my last area to learn on this platform, really!!)


[off-topic, but if anyone has good compression ratio #'s they can suggest I'd be very grateful, have a cylinder for one of my units and have it "about" where I want it but really want to decide based on compression ratio not on squish, as I've tapered the squish-band like 8* on this jug so squish# isn't going to be representative in the normal way not when the comb.chamber is like 75% of oem size!]

 

[Mastermind]

BTW I should mention that that ^ crankcase isn't just "2 halves that aren't matched to one another" properly, but the actual assembly has left a ~1/8mm or maybe 10-15thou misalignment between the case-halves.... Would anyone recommend loosening of the bolts, bang with hard-rubber mallet, and retighten bolts? Maybe it'd work! Guessing it's a hard 'No' but had to ask, I can't justify a pre-emptive full tear-apart so plan is "run it til it fails, then learn the cranks/crankcase" (my last area to learn on this platform, really!!)


[off-topic, but if anyone has good compression ratio #'s they can suggest I'd be very grateful, have a cylinder for one of my units and have it "about" where I want it but really want to decide based on compression ratio not on squish, as I've tapered the squish-band like 8* on this jug so squish# isn't going to be representative in the normal way not when the comb.chamber is like 75% of oem size!]

No.

There are dowel pins that hold the case halves solidly.

 

[Mastermind]

I've tapered the squish-band like 8* on this jug so squish

Then you have ruined it.

 

[Wonkydonkey]

I guess you need to build a China 660 and play around with it, all the ways you think you can improve on the stihl design.
As Randy said, the big bore are junk, that is partly due to a number of reasons Ie, transfer tunnels and maybe a bit of free porting,, the list is there somewhere, what other guys think but you will have to find them reasons what other people think..

when someone or some team designs something, there is a lot of work that goes into it. Just back read 2stroke motorcycle history.
And then also read 2stroke design etc.. there’s a good book in pdf on the net by a guy call “bell” as well as “Jenkins”
Have a read then come back. But I think the maths involved in those books will keep your brain cells occupied for a bit.

 

[Wonkydonkey]

I've tapered the squish-band like 8* on this jug so squish

Then you have ruined it.

You need to read about squash band velocity …

read them books

 

[Mastermind]

read them books

This.

 

[Cerberus]

Now roll the crank 180 degrees and see how high the counterweights are in relation to the sealing flange.

Also.....on a BB kit.....the added displacement of the bore also increases the case volume. Everything under the piston is case volume......so.

The reason BB kit run like garbage is because they are garbage.

I'll have to pop the cyilnder to check, I have to anyways because it's been 4-5 tanks since rebuilt top-end and, inspecting piston through exhaust-port, I see some quicker-than-desirable wearing (nothing too-bad, but still means it'd kill it eventually, if I understand this properly!) So will have it apart and 180'd shortly ;D

Still has 'friction' once piston goes up to the exhaust-port's middle and higher (IE most of the rotation, except the BDC area and like 50* either side of it), and squish is 1-1.5thou tighter on rear/intake side of piston than front (was same on the OEM build it came with) Don't want to take it apart but I can see some wear to those small horizontal grooves in the piston on Blue, and that's only at 4.5 tanks on that top-end, so have to take it apart & check!

I know there's a lot of garbage parts....though to be fair the only failure so far that I can blame on china is the clutch spring...and I know this wouldn't be a viable approach normally but my situation isn't normal, someone (;D ) generously gave me a unit to start with, loaded with over half a grand of OEM's and a pro 32" b&c setup, it changes the math for someone like me, someone who is OK with fixing this when the case fails (I WANT to pro-actively work on it, I just can't justify learning case-splitting & cranks for this....tried jsutifying it but keep concluding "wait til it fails, buy better hardware & rebuild")

Nearing the 2mo mark now and have always had at least (1) unit at the ready, typically both are up&fine though, and know - and am eager for - more involved work on this platform (frickin' LOVE this platform, almost forgot about my amazing cs590 lol )

 

[Cerberus]

(as much as I'd like to think "Oh since I'm grinding anyways, let's get rid of this sharp edge at the crankcase//lower transfer junction, make a smooth ramp into the cylinder-wall", I cannot help thinking that the big thick metal gasket, in this case of dodgy crankcase-matching, that the thick metal gasket will help "even-out" things in a way that just Motoseal will not!)

 

[Wonkydonkey]

[URL]https://www.dropbox.com/transfer/cap_pid_ft:AAAAAL-ZTXxCjVh9mgSiPcch1hUHKXQs3SB_BAYMAga--L4QomN43lo[/URL]

 

[Wonkydonkey]

https://www.dropbox.com/s/ivjsrus3lgie6xt/Two-stroke Tuner's Handbook.pdf?dl=0

 

[Ronie]

I've tapered the squish-band like 8*

I curious of the reason you tapered the squish band.

 

[Wonkydonkey]

I think those two links I’ve added are the same book by the same guy. Just different pdf's

however I,m uploading a file 2stroke design by Gordon Blair
I,ll add the link when it’s uploaded

 

[Cerberus]

Then you have ruined it.

I've only eyeballed that, will get a hard # (hopefully a good one LOL)

I saw 7-7.5deg on a 660 hot saw thread (arboristsite, gonna try finding link now) so figured it was fine -- I literally put the taper there AFTER removing it, I was working the band upwards and it began developing a taper - I thought "oh no", corrected it (over-corrected, actually), but then thought "some non-zero slope must be preferable", was thinking of how pre-detonation occurs and how the squish band 'feeds' and 'gets fed' by the chamber and some non-zero slope seemed superior to flat....so I re-sloped inwards :/

What is the max height you'd remove from a band, or is it cylinder by cylinder based on plating? Finding it impossible to eyeball, so using my finger as much as my eyes to feel plating at the top (so far all's well on that front thankfully)

If taper can be non-zero (IE if 3* is ideal), and if my cylinder still allows the band be raised, I *can* fix this!

If not, a $25 (shipped) f.tec cylinder is wasted ;P (haven't touched the ports on it yet, this is my 'extra/play' cylinder)

 

[Wonkydonkey]

Here's the 2stroke design link by Blair

https://www.dropbox.com/s/t7rwujmvmxjceiz/2StrokeDesign.pdf?dl=0

 

[Cerberus]

I've tapered the squish-band like 8* on this jug so squish


You need to read about squash band velocity …

read them books

Eagerly, am at a loss for good materials (was about to begin en masse reading of old 660 threads...)
Any specific recommendations? Would ideally prefer something that's not a $100 textbook but if that's a 'gold standard' / must-have then so be it.

[ninja edit-- if those links are what I think they are, thanks a TON Gotta get my privacy filters off, or maybe it's set up an account there, to access them but the title is promising ;D Thanks a ton, had been meaning to find a reputable book, IMO it's dangerous to just buy a book and presume it's all to be taken as-fact w/o knowing the author is reputable!!]

 

[Wonkydonkey]

Here's something to play with

http://www.dragonfly75.com/moto/sqVelocity.html

edit and the link in the link above about it from the man himself
http://www.dragonfly75.com/moto/SquishVelocity.html

 

[Cerberus]

I curious of the reason you tapered the squish band.

An artifact of how I was sanding the band, I removed 16thou total and about halfway there noticed I was getting taper, I eliminated it then reintroduced it LOL so screwed myself it seems (unless I can just raise the band&deck again and eliminate some of the taper....am nearly positive non-zero taper is optimal....and pretty sure only 16thou off the band means i can go further!!

Hopefully I can rectify it...if not I'll need to order a new f.tec cylinder to play with (this is not off either saw, I ordered a $25 or 30 f.tec top end kit to mess with! EDIT: and a flywheel puller & woodruff keys, for further playing!! Kinda looking at it like "the Blue saw will break its bottom end sooner than later, likely, so may as well do the experiments on him" lol)

 

[Wonkydonkey]

So tell me if you can download those links to the files in my Dropbox…

well can anyone else tell me as well. Just so I know I’ve done it correctly