Questions about Squish

[MrGiggles]

I put a Huztl kit on my 50 Special a couple weeks ago. It runs good enough but after a squish check it was less than ideal at around .060. From what I've read 20-30 is good.

I bought a Caber ring for it and will disassemble it to get a more accurate check and address the issue. I have access to a lathe and cutting the cylinder base is no big deal since I have the old cylinder to practice on.

My Questions:

1. Would bringing the squish into the desired spec give any discernible increase in power? Will the juice be worth the squeeze?
2. Will cutting the cylinder affect port timing enough to hurt anything? I don't like the idea of taking the grinder to my new cylinder.
3. Upon reassembly, do I need to prepare the cylinder in any way to seat the new ring?

This is not a race saw but for firewood only. Mostly limbing and smaller work but if it runs good enough it's a lot lighter than my 066 and could see harder use.

 

[mdavlee]

There should be a good difference if you can get it down to under .030". You should notice a good increase in compression with that also. The base is the only part needing to be cut to lower squish. Did you check it over the wrist pin towards the intake or exhaust?

 

[MrGiggles]

There should be a good difference if you can get it down to under .030". You should notice a good increase in compression with that also. The base is the only part needing to be cut to lower squish. Did you check it over the wrist pin towards the intake or exhaust?

I could not get a very good test. The only solder I had was .06 and it did not noticeably smash it. I could tell it made contact but it didn't make much of an impact.

 

[steve316]

squish

the squish ban is on a taper or angle. So your squish will be tighter closer to edge of the piston. .060 is a lot ; you may want to check it again before cutting the base. Just to be sure. Steve

 

[50blues]

Make sure you use at least 2 pieces of solder and put them over the pin. Not sideways to the pin. The piston can move around a little and give incorrect readings. .040 is a long ways to go.

 

[tolman_paul]

Figure if you cut .025" off the cylinder to tighten up the squish you'll loose about 4 deg of transfer and exhaust and gain about 4 deg of intake duration. A quick way to check out whether you like the performance without a permanent mod is to pull th base gasket and reassemble with a light coat of rtv. Should take .020" off the squish

 

[blsnelling]

.060" is WAY excessive. AFAIC, that's a manufacturing defect. I'd be shooting for .018"-.020". Regardless of how it affects port timing, this needs corrected.

 

[heyduke]

I've just been thru this with a pair of makita 6401's and 84cc p/c's. i'm not going to admit that i learned this by hard experience, but, sounds like you may have left the old gasket stuck to the case or cylinder base AND used a new one as well. the math is right. if the gaskets were .020" thick and you removed both, you'd lose .040" and be right in the .020" squish clearance range you're looking for. also be aware, again i'm not going to admit that I screwed up, but, when substituting compound for paper gaskets you should use a solvent based gasket compound like permatex grey for motorcycles, watercraft and small engines, which resists gasoline.

 

[barneyrb]

60 thou with or without a base gasket?

 

[mdavlee]

That series of saws does have high squish. The 50 I have around is around .050" on the squish. I'm sure it would benefit to have the squish tightened down to around .020"

 

[mopar1rules]

You could try it without the base gasket like stated above but dont use rtv as sealant. Instead use threebond 1194.

also be aware of any possible free porting that may occur if lowering the cylinder by .040"

 

[MrGiggles]

.060" is WAY excessive. AFAIC, that's a manufacturing defect. I'd be shooting for .018"-.020". Regardless of how it affects port timing, this needs corrected.

I'm not the only one. A couple other folks have had excessive squish. It's to be expected with cheap chinese kits.

I've just been thru this with a pair of makita 6401's and 84cc p/c's. i'm not going to admit that i learned this by hard experience, but, sounds like you may have left the old gasket stuck to the case or cylinder base AND used a new one as well. the math is right. if the gaskets were .020" thick and you removed both, you'd lose .040" and be right in the .020" squish clearance range you're looking for. also be aware, again i'm not going to admit that I screwed up, but, when substituting compound for paper gaskets you should use a solvent based gasket compound like permatex grey for motorcycles, watercraft and small engines, which resists gasoline.

60 thou with or without a base gasket?

Yes I cleaned the old gasket off the crankcase. Used the new one that came with the kit.

That series of saws does have high squish. The 50 I have around is around .050" on the squish. I'm sure it would benefit to have the squish tightened down to around .020"

You could try it without the base gasket like stated above but dont use rtv as sealant. Instead use threebond 1194.

also be aware of any possible free porting that may occur if lowering the cylinder by .040"

That is what I'm worried about.

What sort of porting are we talking about here? I have a dremel already and am not above doing a little grinding, but it makes me a little nervous since I've never done it before.

 

[heyduke]

You could try it without the base gasket like stated above but dont use rtv as sealant. Instead use threebond 1194.

also be aware of any possible free porting that may occur if lowering the cylinder by .040"

i'd suggest you take it apart, reassemble it without a gasket or compound, measure the squish clearance. I'm guessing you'll see appx .035". if that's ok, do it again with gasket compound. check your compression. it will probably be around 130psi. go cut some wood.

 

[50blues]

when you reassemble it without the base gasket. turn the crank so the piston is all the way down and look in the intake port. If you see a gap between the top of the piston and the top of the port, where you can see into the cylinder, that is called freeporting. It will allow air from the cylinder to exit into the intake and can actually blow the wrong way threw the carb. not good.

 

[blsnelling]

I'm not the only one. A couple other folks have had excessive squish. It's to be expected with cheap chinese kits.

Yes, I know Like I said, I consider that a defect. There's no reason to be that sloppy in the manufacturing process.

 

[MrGiggles]

when you reassemble it without the base gasket. turn the crank so the piston is all the way down and look in the intake port. If you see a gap between the top of the piston and the top of the port, where you can see into the cyl
inder, that is called freeporting. It will allow air from the cylinder to exit into the intake and can actually blow the wrong way threw the carb. not good.

I will definitely try this.

Yes, I know Like I said, I consider that a defect. There's no reason to be that sloppy in the manufacturing process.

Thank you for the replies.

I'm not really surprised.. That's what you get with a 50.00 piston/cylinder kit. I would rather pay that and get the valuable modification experience on a saw that's not worth much. In the Huztl thread there were people having the same defects with more expensive kits.

Say I have enough room to cut the cylinder without causing free porting.. Would changing the port timing by that much affect my performance at all?

 

[tolman_paul]

As I mentioned previously, when you cut a piston port cylinder down you increase the duration of the piston port opening, but you decrease the duration of the transfer port and exhaust port opening. The effect will depend on the durations of the ports prior to you changing them.

The longer the piston port is open, the more time the crankcase has to fill with air and fuel at higher rpm, however the flip side is more air and fuel is lost in the crankcase at lower rpm. So this effectively makes the engine a higher rpm engine.

Decreasing the transfer port duration decreases the time the tranfers are open which increases the velocity of the new incoming fuel air charge. This can be a mixed blessing. It tends to make the engine produce power over a narrower powerband, but if the velocity is too high you loose some of the air fuel mixture out of the exhaust port and without a tuned pipe, this is a bad thing.

Decreasing the exhaust duration limits the amount of air the engine can pump through it so it essentially makes the engine a lower rpm engine which can be counter to increasing the piston port duration. However, it does raise your effective compression ratio because there is a longer period where the exhaust port is closed and hence a longer period for the hot ignited gasses to provide power to the crank.

If your engine before mods didn't quite have enough intake duration and had a tad too much transfer and exhaust, then these changes would help. If however you already enough or a bit too much intake duration and your transfers and exhaust were already on the small side the effect would be to loose your bottom end due to the increased intake duration, narrow up the powerband due to the decreased transfer period and decrease your top end by decreasing your exhaust duration. So to answer your question, you need to know where you are starting at before we can answer if where your going will help you or hinder you.

As far as using a moto tool for porting, there are fairly limited areas you can get a moto tool into a small two stroke cylinder to make improvements. You really need a good 90 degree head and a small dia straight grinder body to get to the ports.

 

[MrGiggles]

As I mentioned previously, when you cut a piston port cylinder down you increase the duration of the piston port opening, but you decrease the duration of the transfer port and exhaust port opening. The effect will depend on the durations of the ports prior to you changing them.

The longer the piston port is open, the more time the crankcase has to fill with air and fuel at higher rpm, however the flip side is more air and fuel is lost in the crankcase at lower rpm. So this effectively makes the engine a higher rpm engine.

Decreasing the transfer port duration decreases the time the tranfers are open which increases the velocity of the new incoming fuel air charge. This can be a mixed blessing. It tends to make the engine produce power over a narrower powerband, but if the velocity is too high you loose some of the air fuel mixture out of the exhaust port and without a tuned pipe, this is a bad thing.

Decreasing the exhaust duration limits the amount of air the engine can pump through it so it essentially makes the engine a lower rpm engine which can be counter to increasing the piston port duration. However, it does raise your effective compression ratio because there is a longer period where the exhaust port is closed and hence a longer period for the hot ignited gasses to provide power to the crank.

If your engine before mods didn't quite have enough intake duration and had a tad too much transfer and exhaust, then these changes would help. If however you already enough or a bit too much intake duration and your transfers and exhaust were already on the small side the effect would be to loose your bottom end due to the increased intake duration, narrow up the powerband due to the decreased transfer period and decrease your top end by decreasing your exhaust duration. So to answer your question, you need to know where you are starting at before we can answer if where your going will help you or hinder you.

As far as using a moto tool for porting, there are fairly limited areas you can get a moto tool into a small two stroke cylinder to make improvements. You really need a good 90 degree head and a small dia straight grinder body to get to the ports.

Thank you for the information. Is 15-20 thousandths a big change for port timing?

I took the saw apart. One of my favorite features of these saws is that I can turn it into a pile of parts in about 10 minutes flat.

Anyway, my squish readings were better this time. In 4 corners, without a base gasket, they were all .040 give or take a little. This is better but still excessive. Gasket was .025 so the numbers add up.

There is a good 1/8 inch before the ring will come into the intake port. Free porting isn't gonna be an issue.

Is there a rule for the relationship of squish and compression? Say 10 psi for every .010?

 

[tolman_paul]

I'd recommend doing a search for threads on degreeing a cylinder. It's important to be thinking in terms of port duration in terms of degrees of crankshaft rotation rather than changes in port height. Depending on your engines stroke and crankshaft/conrod geometry 15-20 thousandths will equate to roughly 4-6 degrees of intake duration. Whether or not that is a big difference depends on what your existing durations are. Lets look just at tranfer durations. You'll find that for a saw between 110 and 120 degrees is in general your ideal range. Some saws will tollerate a bit less or a bit more, but you should generally be shooting for that range for a non piped saw.

If your cylinder as is has say 116 or 118 duration, then loosing 6 deg will put you at 110-112 which wouldn't have a hugely adverse affect. But if you're only at 110 deg and drop the duration to 104 you'll probably see a much larger negative affect of that change in port duration.

As far as how much pressure you gain for a given cut of the cylinder, it depends on where you start. If you're cylinder is producing 130 psi, cutting 10 though off the cylinder will have a smaller affect than if you're starting at 160psi.

In short to see the greatest gains in performance you should simultaneously increase your compression ratio and idealize your port timing. Solely cutting the cylinder to bump up compression will adversly affect your port timings. How adversly they are affected depends on what they were before doing the work.

 

[heyduke]

when you reassemble it without the base gasket. turn the crank so the piston is all the way down and look in the intake port. If you see a gap between the top of the piston and the top of the port, where you can see into the cylinder, that is called freeporting. It will allow air from the cylinder to exit into the intake and can actually blow the wrong way threw the carb. not good.

maybe i'm confused, but if you lower the cylinder base by removing the gasket it isn't going to expose the top of the piston but just the opposite, the piston top will be further above the top of the intake port, inside the cylinder. Theoretically, couldn't you measure the gasket's thickness, throw it away, and then raise the top of the intake port by an amount equal to the gasket thickness without emotional stress. if you slipped and cut too much, then you could have a problem.