372 Cylinder head.

[wcorey]

Where needed, one practical benefit I can see with the angled squish (with equally angled piston) is that as it reduces the chamber volume at the same time serves to minimize the distance you cut past the top of the bore plating.

 

[Darryll]

On the subject of compression...

I went through a long and painful journey with a somewhat aggressively ported 394 where it started out fairly well behaved and progressively devolved over maybe 10 tanks into an untune-able be-otch.
Wouldn't make it through a cut without significantly leaning out but when richened up a bit would be blubbering and wouldn't clean up under load. Was at first using it as a test mule for a dyno project so most of the runtime after some initial break in was in a series of many short runs so it took a while to notice the issue. It also was very quick/easy to start so the odd fact that it was building significant compression as it broke in was escaping me.
After many carb swaps, jet drilling, timing changes, etc I'd get frustrated and shelve it for a year at a time. I finally realized it was getting difficult to pull over to the point where I couldn't do a reasonable compression check because I couldn't pull it through more than one cycle or so with no ignition. It started out around 220-230psi if memory serves and at the end was pulling over 280 on like one and a half compression cycles. Was getting concerned about ruining the recoil and/or snapping the end of the crank off.

So evidently it was overheating so significantly that at the end it would lean out just on short piss revs. Surprisingly it didn't have detonation issues even on my usual 87 ron/mon mix...
I didn't want to alter the port timing or squish clearance so ended up grinding out the chamber to I think around 200psi, tuning issue disappeared but seemed to then suck the life out of it. WTF...
Again I shelved it in frustration and it's been sitting there a few years now...

Great read. I was laughing the whole way through it. It seems we are all effected by these simple 2 strokes in the same way. I have a 50mm 372 cylinder that over the past 6 years has gone on and off the saw. It started out ok then went drastically down hill then last year I realized my mistake and improved it, then recently I raised the compression to where it should be and now it running better than stock again with an Exhaust at 90. I have blow down at 25 and I'm now thinking maybe that's to much, maybe I can reduce that and so it goes on. I'm sure in the end I will stuff it completely.

That compression sounds crazy why is it not detonating I wonder? I assume you had cut it with a lathe....did you leave the square edge or radius it out? I wish now I did a compression check on the head in this thread before it lost some plating. I had not experienced detonating before so at first I thought I had a bad spark plug so I changed that and it was exactly the same. I then suspected detonating and pulled the head when I got home and sure enough there was plenty of evidence. Mine was carbureting ok but a bit finicky down low. I did have to run it quite rich on top.

 

[wcorey]

That compression sounds crazy why is it not detonating I wonder? I assume you had cut it with a lathe....did you leave the square edge or radius it out?

No clue on detonation, I've yet to have an issue on a saw. I've often heard that small displacements with minimal squish clearance just aren't particularly prone to it unless extreme comp ratios or of course bad fuel. And even ported saws really aren't very high output for the displacement.

Typically turn bases on the lathe and mandrel cut the bands, for me it's been the most efficient method. Never liked the prospect of having a spinning cylinder getting away from me with a boring bar in there. I've also tried it on the mill but found it didn't save any time there and still not as 'safe' as the manual method, so...
Band had a moderate radius, actually more than usual for me, most often I just break the edge a bit.
Long ago read a paper by some guys doing very detailed and documented porting/tuning work on a very high output small displacement race bike and one of the findings was that a sharp transition from band to chamber gave noticeable gains. Of course totally different animal i'm sure, piped and no doubt with a tight, peaky power band.
Somehow I guess that stuck with me though.

 

[Darryll]

No clue on detonation, I've yet to have an issue on a saw. I've often heard that small displacements with minimal squish clearance just aren't particularly prone to it unless extreme comp ratios or of course bad fuel. And even ported saws really aren't very high output for the displacement.

Typically turn bases on the lathe and mandrel cut the bands, for me it's been the most efficient method. Never liked the prospect of having a spinning cylinder getting away from me with a boring bar in there. I've also tried it on the mill but found it didn't save any time there and still not as 'safe' as the manual method, so...
Band had a moderate radius, actually more than usual for me, most often I just break the edge a bit.
Long ago read a paper by some guys doing very detailed and documented porting/tuning work on a very high output small displacement race bike and one of the findings was that a sharp transition from band to chamber gave noticeable gains. Of course totally different animal i'm sure, piped and no doubt with a tight, peaky power band.
Somehow I guess that stuck with me though.

Very similar to mine.

 

[Darryll]

No clue on detonation, I've yet to have an issue on a saw. I've often heard that small displacements with minimal squish clearance just aren't particularly prone to it unless extreme comp ratios or of course bad fuel. And even ported saws really aren't very high output for the displacement.

Typically turn bases on the lathe and mandrel cut the bands, for me it's been the most efficient method. Never liked the prospect of having a spinning cylinder getting away from me with a boring bar in there. I've also tried it on the mill but found it didn't save any time there and still not as 'safe' as the manual method, so...
Band had a moderate radius, actually more than usual for me, most often I just break the edge a bit.
Long ago read a paper by some guys doing very detailed and documented porting/tuning work on a very high output small displacement race bike and one of the findings was that a sharp transition from band to chamber gave noticeable gains. Of course totally different animal i'm sure, piped and no doubt with a tight, peaky power band.
Somehow I guess that stuck with me though.

Cutting the squish band is not the easiest to do I agree. I made up a jig for the lathe but it took a while to setup, certainly longer than it took to do the cut. The CNC has proved to make the process quite easy, I can typically adjust the program and run it in about 30min including setup. Maybe even faster actually. The full cut only takes 5 mins or so.

That is quite a lot of squish area you have in that cylinder, as you would expect to get the compression you describe. Are you able to shed any light on which is the better way to go?

 

[wcorey]

Cutting the squish band is not the easiest to do I agree. I made up a jig for the lathe but it took a while to setup, certainly longer than it took to do the cut. The CNC has proved to make the process quite easy, I can typically adjust the program and run it in about 30min including setup. Maybe even faster actually. The full cut only takes 5 mins or so.

The majority of my professional career somehow or another involved machining but I managed to avoid cnc the whole time, lol.
Would love to have the capability but I just don't have the head for cad and programing, tried many times, just no go. I have vague hopes of chat/gpt solving some of that for me, playing with it a bit but...

Takes me maybe 10 min to cut a band with an adjustable mandrel setup. Just clamp it in a vise, drop the cylinder on it and there's a screw adjustment accessible through the ex port that moves those black nylon channels out to the bore size. I have a set of a few nesting channels that get me from I think 36mm through 60mm. Set the depth stop from the base with feeler gauges, then apply even pressure while tuning it 'round and 'round. Sometimes not as neat as machine turning due to a bit of chatter but some sandpaper on top of an old piston or base turning mandrel easily cleans that up. @Treemonkey gets credit for putting me on to the whole mandrel cutting thing (and many other useful tidbits).



Just for shiits and giggles (and because someone suggested it couldn't be done) I made an adaptor to hand cut bases on skirted cylinders.
Not that I'd ever want to actually do it that way...

 

[jacob j.]

The majority of my professional career somehow or another involved machining but I managed to avoid cnc the whole time, lol.
Would love to have the capability but I just don't have the head for cad and programing, tried many times, just no go. I have vague hopes of chat/gpt solving some of that for me, playing with it a bit but...

Takes me maybe 10 min to cut a band with an adjustable mandrel setup. Just clamp it in a vise, drop the cylinder on it and there's a screw adjustment accessible through the ex port that moves those black nylon channels out to the bore size. I have a set of a few nesting channels that get me from I think 36mm through 60mm. Set the depth stop from the base with feeler gauges, then apply even pressure while tuning it 'round and 'round. Sometimes not as neat as machine turning due to a bit of chatter but some sandpaper on top of an old piston or base turning mandrel easily cleans that up. @Treemonkey gets credit for putting me on to the whole mandrel cutting thing (and many other useful tidbits).

View attachment 394002

Just for shiits and giggles (and because someone suggested it couldn't be done) I made an adaptor to hand cut bases on skirted cylinders.
Not that I'd ever want to actually do it that way...

View attachment 394004

That's awesome stuff Bill.

 

[Darryll]

For now I have moved away from angled squish bands. I do believe there is some potential there to be realized if I start making my own pistons. I set the head up in this thread pursuing a different line of thought with a square band and the result was encouraging. However I think to get the best out of that I will need to make my own piston also. All roads seem to be leading me in that direction.

Some very interesting observations noted here. It seems no one has any thoughts on the 500 style head so I might just design one up and make it at some point in the future. For now, unless someone has a better idea I'll do a 60% area band with a 2mm radius at 10:1 and I think I'll add 1 degree to the squish band. I'll decide that when I design it.

I brought the tooling I need for the piston some time ago and I have now found some 1mm pins to locate the ring. As for the material here in NZ its hopeless. All I can find 6061 which I have read others have used successfully but I'd like something better than that.

If you all have other suggestions I'd be happy to discuss them.

 

[wcorey]

That is quite a lot of squish area you have in that cylinder, as you would expect to get the compression you describe. Are you able to shed any light on which is the better way to go?

I don't port enough of the same models to have a handle on particulars such as this, usually just stick to tried and true with maybe a bit of experimentation on transfer angles. The only model I ever did many multiples of is the husqvarna 350 'project' but that's another story.
In the case of this 394 there's just so many variables over so much time I really can't quantify the various outcomes other than it ran exceptionally strong until the tuning issues, then I somewhat neutered it in fixing that.

There are some older models (jreds and partners come to mind) with very wide squish and deep/tiny diameter chambers that run and mod quite well. Then there's others with narrow bands and wide/shallow chambers that also run strong. So going by examples that I can see, I never put much effort into chamber proportions other than to increase comp. Obviously it's a thing, I just don't know where to go with it, seems mostly the domain of serious race stuff rather than work saws.

 

[Darryll]

The majority of my professional career somehow or another involved machining but I managed to avoid cnc the whole time, lol.
Would love to have the capability but I just don't have the head for cad and programing, tried many times, just no go. I have vague hopes of chat/gpt solving some of that for me, playing with it a bit but...

Takes me maybe 10 min to cut a band with an adjustable mandrel setup. Just clamp it in a vise, drop the cylinder on it and there's a screw adjustment accessible through the ex port that moves those black nylon channels out to the bore size. I have a set of a few nesting channels that get me from I think 36mm through 60mm. Set the depth stop from the base with feeler gauges, then apply even pressure while tuning it 'round and 'round. Sometimes not as neat as machine turning due to a bit of chatter but some sandpaper on top of an old piston or base turning mandrel easily cleans that up. @Treemonkey gets credit for putting me on to the whole mandrel cutting thing (and many other useful tidbits).



Just for shiits and giggles (and because someone suggested it couldn't be done) I made an adaptor to hand cut bases on skirted cylinders.
Not that I'd ever want to actually do it that way...

Nice work.
I'm not a machinist, in fact I have my own earthworks company and service the rural sector. I always wanted to be a fitter turner and I brought my first lath 30y ago now and a mill not long after. I learnt Fusion 360 back around 2010 and over the years I have got most of it pretty sorted. I always wanted a CNC. I saw one that would suit me 3 years ago so I brought one not knowing a dam thing about it. The Mill turned out to be a complete dog. I was absolutely shafted! I had 9k of junk in my shed and know idea how to fix it. So I started reading and watching and slowly I worked it out. I knew I wanted a high speed spindle so I brought an 18,000 rpm spindle and got some new drivers and mother board and over about 3 months I got to cut my first piece of wood totally under the control of the computer. I have since thrown half of what I brought out and replaced it with better gear and now I have a very very good mill. Super reliable and ridiculously accurate. It's not Kern good but it possibly owes me 15k now and for that money its awesome.

I can say the whole process was the most challenging thing I have done in my life. Although I was pretty pissed in the beginning it has turned out to be a blessing. The mill I brought and the mill I have now are not even remotely the same machine. Its cuts steel no problem at all and for the likes of the head I spin a 3mm cutter at 14k and it removes material pretty quick. I have mostly cut steel on it and Aluminum has its own challenges but I'm starting to get some nice surface finishes.

 

[Ketchup]

500i chamber. Sorry I don’t have time for measurements tonight.


261v1 chamber:


Exhaust exit angle on the 261: 18degrees


Factory squish angle on 2511 piston: 5 degrees



@Darryll, Just eyeball comparison, but I think the 500i chamber is not as deep as your second drawing.

 

[Darryll]

I don't port enough of the same models to have a handle on particulars such as this, usually just stick to tried and true with maybe a bit of experimentation on transfer angles. The only model I ever did many multiples of is the husqvarna 350 'project' but that's another story.
In the case of this 394 there's just so many variables over so much time I really can't quantify the various outcomes other than it ran exceptionally strong until the tuning issues, then I somewhat neutered it in fixing that.

There are some older models (jreds and partners come to mind) with very wide squish and deep/tiny diameter chambers that run and mod quite well. Then there's others with narrow bands and wide/shallow chambers that also run strong. So going by examples that I can see, I never put much effort into chamber proportions other than to increase comp. Obviously it's a thing, I just don't know where to go with it, seems mostly the domain of serious race stuff rather than work saws.

Yes that is a very fair answer. Unless you have a dyno its difficult to quantify your changes definitively you are a bit in the dark. When testing saws there is more to it than how fast it cuts. How they feel and load up etc all counts. My thing lately is ported saw feel slow. Running a stock saw curers that pretty quick though.

 

[srcarr52]

Nice work.
I'm not a machinist, in fact I have my own earthworks company and service the rural sector. I always wanted to be a fitter turner and I brought my first lath 30y ago now and a mill not long after. I learnt Fusion 360 back around 2010 and over the years I have got most of it pretty sorted. I always wanted a CNC. I saw one that would suit me 3 years ago so I brought one not knowing a dam thing about it. The Mill turned out to be a complete dog. I was absolutely shafted! I had 9k of junk in my shed and know idea how to fix it. So I started reading and watching and slowly I worked it out. I knew I wanted a high speed spindle so I brought an 18,000 rpm spindle and got some new drivers and mother board and over about 3 months I got to cut my first piece of wood totally under the control of the computer. I have since thrown half of what I brought out and replaced it with better gear and now I have a very very good mill. Super reliable and ridiculously accurate. It's not Kern good but it possibly owes me 15k now and for that money its awesome.

I can say the whole process was the most challenging thing I have done in my life. Although I was pretty pissed in the beginning it has turned out to be a blessing. The mill I brought and the mill I have now are not even remotely the same machine. Its cuts steel no problem at all and for the likes of the head I spin a 3mm cutter at 14k and it removes material pretty quick. I have mostly cut steel on it and Aluminum has its own challenges but I'm starting to get some nice surface finishes.

I run some older industrial machines. With a little knowledge of analog controls and DOS skills I can keep them running well.



This was on the day I was moving in the Partner 1 cabinet mill. The CNC knee mill is from 1996 and has been a workhorse! It may only have a 4.2k spindle but it can take surprisingly deep cuts for an R8 spindle.

I still run the manual lathe the most (1340 Jet). I can setup and cut a squish band in 15min or less.

 

[Darryll]

I run some older industrial machines. With a little knowledge of analog controls and DOS skills I can keep them running well.



This was on the day I was moving in the Partner 1 cabinet mill. The CNC knee mill is from 1996 and has been a workhorse! It may only have a 4.2k spindle but it can take surprisingly deep cuts for an R8 spindle.

I still run the manual lathe the most (1340 Jet). I can setup and cut a squish band in 15min or less.

That Partner would be a good mill to have at home in the work shop. My mill will not do heavy cuts at all! I got it to do the type of things I do with saws and other interests I have and it has worked out well.

So much of what we do needs a jig. My jig on the lath was fair to average at best it possibly took me half an hour to dial it in lol. I find the CNC to be in reverse. The set up and cut is typically very quick especially with the probe etc but I still spend a fair bit of time in Fusion agonizing over combustion shapes etc.

This discussion with all of you has been great. It has caused me to think about different ideas and look deeper into the causes of detonation. I think you are onto something with that angled squish. I suspect a few of you are running possibly more compression than I am and having no trouble. I wonder if running super tight clearances like I do somehow caused the detonation? I was watching a guy on YT last night go through it all and he suggested it could and tapering as well but his reasoning was pockets of trapped gas. I doubt there are any pockets in a CNC squish or any machined squish for that matter. It may well have been that I was just over the limit on ratio. I have not had trouble before and it was pretty stout to start. I wish now I had done a compression test on it.

For a long time I was pretty convinced the a mildly hot compression ratio was better but the last few I built I have upped the ratio and they defiantly make more usable power. This cylinder head detonated under heavy load but if you backed off a bit it held the upper rev range and cut really well. When I cut back the ratio it was not as aggressive but run nicely.

 

[wcorey]

If anything tight clearance helps prevent detonation.

 

[Darryll]

500i chamber. Sorry I don’t have time for measurements tonight.

Interesting the 500 piston I have has a flat top.

Agreed the sketch I did was pretty piss poor actually. It was a correct ratio but I need to properly map it out and sketch it up. My interest in the 500 shape is simply that the saw goes well and someone was paid possibly 200-300k per year to come up with that design. Its a shame those guys don't come on this forum and give us a heads up. They probably come home on a Friday night and crack a beer reading though our *s-word for a bit of comic relief lol. That said we are making there very expensive R&D go better.

I have recently set up a jig and started probing all the ports and map them into Fusion. From there I can work out the entry angles for the transfers and I believe quite accurately calculate the actual port area. The few I have done have revealed some interesting info. They are not super symmetrical. In fact the Hyway exhaust port was a long way off. This gives me the ability to shuffle the ports around to get them spot on. I did all of this to the Hyway and it went really well and the numbers were very typical. Nothing special at all. Also with the top cut off I was able to view the transfers from the top and as nice as I think I port they did not look as good from above. So I cut everything perfectly symmetrical.

 

[Darryll]

If anything tight clearance helps prevent detonation.

Yes that has always been my understanding.

 

[wcorey]

The problem with assuming the engineers designing the saw know more than us is that we don’t know what was the goal of any particular aspect of the design. Could be for fuel efficiency, emissions, power, cooling, ease/cost of manufacture, weight, longevity, etc or even to compensate for another shortcoming.

 

[legdelimber]

Just wanted to say thanks to all of you folks who've been writing out this thread.

It's become about as close as I get to a lot of this stuff anymore.
Lost all of my access to any local shops over the years as times changed or owners passed on.

 

[Ketchup]

View attachment 394069
Interesting the 500 piston I have has a flat top.

Agreed the sketch I did was pretty piss poor actually. It was a correct ratio but I need to properly map it out and sketch it up. My interest in the 500 shape is simply that the saw goes well and someone was paid possibly 200-300k per year to come up with that design. Its a shame those guys don't come on this forum and give us a heads up. They probably come home on a Friday night and crack a beer reading though our *s-word for a bit of comic relief lol. That said we are making there very expensive R&D go better.

I have recently set up a jig and started probing all the ports and map them into Fusion. From there I can work out the entry angles for the transfers and I believe quite accurately calculate the actual port area. The few I have done have revealed some interesting info. They are not super symmetrical. In fact the Hyway exhaust port was a long way off. This gives me the ability to shuffle the ports around to get them spot on. I did all of this to the Hyway and it went really well and the numbers were very typical. Nothing special at all. Also with the top cut off I was able to view the transfers from the top and as nice as I think I port they did not look as good from above. So I cut everything perfectly symmetrical.

On the 500i, the most notable quality of the chamber is the squared transition from squish to dome. I assumed that was a product of manufacturing but maybe it was intentional. Does the squared transition created more turbulence? Does it distribute force to the piston better? Also, it will have an effect on chamber volume.

I think the R&D guys have clauses in their contracts that don’t allow them to discuss this stuff. I could see them occasionally reading to inform their own ideas though. They probably do laugh at us stumbling along.

Having a probe that would quickly and accurately measure area and angle sounds amazing. Jealous.