pics wanted for cutting the head

hannesd said:
yes i realize,but if it's too hard to shape the bowl on a manual lathe,i'd go the lazy way and just measure the squish now and make sure not to shave the head too much so compression would go skyhigh.
i could get it done by a pro, you'll say, yes true, but therefor i'd have to find one in the area, wich i have no clue to.
plus most of all i'm a do it all yourself kinda guy, so id be more satisfied if i could achieve improvement if i did it myself.
some of you will understand my vision, others will say, don't bother trying :rolleyes:

thanks for all the input so far ,

ciao Hannes.
Click to expand...
Hey don't get me wrong , I'm in the same boat as you ,I have a lathe and I'd like to be able to do my own head too :D , it was when you asked which head Harry had skimmed 1mm off , making me think you thought it was as simple as that.
Up until a month ago I'd never really used a lathe apart from the small go at school , I've been reading up though and getting some practice in .
There is a device that I would think would work on the bowl of the head , but would take a bit of experimenting , something like this

http://www.youtube.com/watch?v=YSlS05unHkM

Chris
 
Chattering = high pitched whine,could not think of the right term at the time, but then F5 you would know that? as you were not taught by a muppet?

F5 said: You what? How does that work? (I'm self taught, read badly, as my tutor was a muppet)
 
As I said, I was taught by a muppet = Me!

I am all too familiar with chatter as I have a '40s Sheldon with an old Armstrong toolpost & chatter is my nemesis.

I still struggle to understand how the rubber damps the vibrations, but heck you could tell me to dip it in molasses & I'd probably try it.

So you are adding a tiny amount of mass around a solid object. :confused:
 
(F5) said:
So you are adding a tiny amount of mass around a solid object. :confused:
Click to expand...

It's not "mass," it's the damping effect of the rubber band. The rubber absorbs the vibrations, much as rubber engine mounts. It's not that they stop the vibrations, they simply quell the harmonics which provide feedback as noise and movement. As an experiment, get an empty wine glass and tap the side. Now put a rubber band around it and repeat. Vibration oscillations damped by the rubber. Similar concept with a bowl (or cylinder head) as the bowl transmits the harmonic frequencies as allowed.
 
Exactly what Mr Cook said, its an old trick used when machining auotomotive flywheels, brake disc/drums too stop them resonating:)
 
i'm guessing about 24cc?
could anyone confirm this?

thinking about this number, it can't be right since the i'd have a CR of about
8.3/1...:confused:
 
Hmm, I looked this up last night but didn't write it down to bring to my PC today.

I used one base gasket & skimmed the head to 0.9mm squish. This raised the comm too high so took some out of it.

From memory I think I ended up with 15.3cc for a 14:1 ratio. This needed decent fuel though. This measurement is with a bit of grease on the bore, piston to top & wiped off, bolted down & fork oil poured in till it come up 2 threads from the bottom. (engine propped up level ie front wheel tied up 2') Use a burette.

I did before & after comp gauge tests during this process out of interest & found they were totally misleading. Waste of time.

The effect was well worth it though.
 
(F5) said:
Hmm, I looked this up last night but didn't write it down to bring to my PC today.

I used one base gasket & skimmed the head to 0.9mm squish. This raised the comm too high so took some out of it.

From memory I think I ended up with 15.3cc for a 14:1 ratio. This needed decent fuel though. This measurement is with a bit of grease on the bore, piston to top & wiped off, bolted down & fork oil poured in till it come up 2 threads from the bottom. (engine propped up level ie front wheel tied up 2') Use a burette.

I did before & after comp gauge tests during this process out of interest & found they were totally misleading. Waste of time.

The effect was well worth it though.
Click to expand...

ok, thanks for that allready.
so why was a compression gauge test before and after misleading?
this measurement was for a 200cc ,right?
 
Have a read of this.. http://www.sacoriver.net/~red/uccr.html

Its probably best if you work with uncorrected compression ratios as its easier.

Combustion chamber vol = I measured mine on a glass surface with a smear of grease around the lower edge. Used a syringe to fill until I reached the bottom of the spark plug hole. CC's noted.
Deck height volume = (use this calc with a Radius of 31.25mm, height is your squish height. IE how much the piston sits above the cyl. Your result will be cubic mm which is same as ccs.)
Piston Dome volume = You can either use a calculator or work out a way to cc the dome. I have volumes recorded in a thread for the 250 and 300 but unfortunately haven't been able to spec up a 200 piston.

Trapped volume = (Combustion chamber vol + Deck height volume) - Piston Dome volume

The other way of measuring trapped volume is to do it with the bike assembled. Pull the head off, put piston down and run a smear of grease around the cylinder. Cylce the pisotn back up to TDC and lock it in place. The grease will seal the rings without taking up too much volume. Put the head back on and then carefully use a syringe/accurate measuring device, and metholated spirits, and begin filling through the spark plug hole until you reach the bottom of the plug hole. Once at this point record how many cc's. Then pull the engine down. The metho will evapourate off. Accuracy is most important.

Now go back and calculate your Uncorrected Compression Ratio
(Cyl. Vol. + Trapped Vol.) / (Trapped Vol.) = UCCR:1
Cyl Vol for a 200 is 199,4 cc

Hope that helps..
 
Hey jake

Is there an optimum compression ratio we should aim for , if the squish is way out from the factory would the compession ratio also be off the mark too .
How do you translate the correct cr into the correct amount to machine from the head ?
Chris
 
Correct Chris.

A large squish gap increases trapped volume, which reduces the compression ratio. Thats why either removing base gaskets, or skimming the head will both increase the CR.

http://www.2strokeheads.com/tech2.htm - This is an interesting read regarding compression ratio and cranking compression. I'm not sure everyone here will agree with it though. Some say aim to get cranking around 190-200psi for uses with normal fuels.

I'd be more inclined to aim for an UCCR between 11:1 and 13:1 for use with 92 Octane fuels, or 14:1 to 15:1 for 100 Octane. You can go higher but then really start getting into race fuel territory.

When I had my 250 engine setup we worked with base gasket stacks, the S3 head and the high comp insert. I managed to get a config to suit my purposes by running a single 0.5mm gasket. It put the piston about 0.7mm above the exh port floor at BDC but that was ok as the S3 cylinder was race ported for top end. Basically the port timing allowed me to run similar to a stock 250 cyl while still having more top end. Timing for full top end would have sacrificed some off the bottom and mid. The gasket stack allowed for a squish height of 1.25mm as is without skimming the head, however the combustion chamber had to be machined out. We went from 17.5cc to 21cc from memory to give a final UCCR of 14:1 which I run on 98RON with no issues. The full thread is here

Basically GG set a large squish and low compression ratio so that no matter where they ship a bike in the world it should run on any fuels. No matter who pulls the stack of gaskets down with the stock head it shouldn't make contact with the piston.

I'm no mathematician but working from "Trapped volume = (Combustion chamber vol + Deck height volume) - Piston Dome volume" once you've decided on a gasket stack and removed material for the squish height your deck height volume becomes fixed, as is the piston dome volume. You just need to work out the chamber volume you require to give you the correct trapped volume for the CR.

In the case of a 200 aiming for approx 13.5:1 UCCR you'd want a trapped volume of 16cc.

16cc = (variable + known) - known. Even if you're no good at manipulating forumulas good old trial and error will get you pretty close!
 
hannesd said:
ok, thanks for that allready.
so why was a compression gauge test before and after misleading?
this measurement was for a 200cc ,right?
Click to expand...

Sorry been away.
Yes for the 200 I had an '02. I checked my notes & it said 15.2cc for 14.1:1 ratio. I was running on Av /91 RON mixed 50/50 which would be equiv of 98 RON I suspect. Europe uses RON from what I can see.

Compression gauges are simple to use & you get a number, but when you get into decent compression of a race 2 stroke I think they are total waste of time. between testing I raised then lowered my compression (measured by volume). The comp tester (german reasonable qlty) went up & then slightly further despite being the wrong way. It is flawed. Just a little residue oil thrown up by the flywheels can scupper the results.

Far safer to measure like a scientist would, rather than a Scientologist. (just made that up, & a bit proud of it)

With one gasket a skim to correct squish & some taken out of the bowl to reduce compression to a sensible amount (I'd use lower if you intend to use lesser fuel than I did). & you will have a nice bike.

Don't be afraid, we are talking easy measurements & easy cheap machining so that isn't a barrier.

I put a road tyre on it & dyno'd to find best ign timing & stock was best so you'll be safe there.
 
(F5) said:
Sorry been away.
Yes for the 200 I had an '02. I checked my notes & it said 15.2cc for 14.1:1 ratio. I was running on Av /91 RON mixed 50/50 which would be equiv of 98 RON I suspect. Europe uses RON from what I can see.

Compression gauges are simple to use & you get a number, but when you get into decent compression of a race 2 stroke I think they are total waste of time. between testing I raised then lowered my compression (measured by volume). The comp tester (german reasonable qlty) went up & then slightly further despite being the wrong way. It is flawed. Just a little residue oil thrown up by the flywheels can scupper the results.

Far safer to measure like a scientist would, rather than a Scientologist. (just made that up, & a bit proud of it)

With one gasket a skim to correct squish & some taken out of the bowl to reduce compression to a sensible amount (I'd use lower if you intend to use lesser fuel than I did). & you will have a nice bike.

Don't be afraid, we are talking easy measurements & easy cheap machining so that isn't a barrier.

I put a road tyre on it & dyno'd to find best ign timing & stock was best so you'll be safe there.
Click to expand...

Well said F5! Now if only I knew how to use a machine :) Hahaha!

I agree whole heartidly regarding cranking compression readings. A couple of cc difference in the adapter can even skew the reading way out. Then you get into port timing and duration and the effect that can have on cranking compression vs engine compression. They are however kind of usefull for comparison from fresh to worn engine (assuming all other variables similar)
 
(F5) said:
Sorry been away.
Yes for the 200 I had an '02. I checked my notes & it said 15.2cc for 14.1:1 ratio. I was running on Av /91 RON mixed 50/50 which would be equiv of 98 RON I suspect. Europe uses RON from what I can see.

Compression gauges are simple to use & you get a number, but when you get into decent compression of a race 2 stroke I think they are total waste of time. between testing I raised then lowered my compression (measured by volume). The comp tester (german reasonable qlty) went up & then slightly further despite being the wrong way. It is flawed. Just a little residue oil thrown up by the flywheels can scupper the results.

Far safer to measure like a scientist would, rather than a Scientologist. (just made that up, & a bit proud of it)

With one gasket a skim to correct squish & some taken out of the bowl to reduce compression to a sensible amount (I'd use lower if you intend to use lesser fuel than I did). & you will have a nice bike.

Don't be afraid, we are talking easy measurements & easy cheap machining so that isn't a barrier.

I put a road tyre on it & dyno'd to find best ign timing & stock was best so you'll be safe there.
Click to expand...

ok, great, i've got some numbers now :D
i think i'll go with a little lower compression, 14.1:1 seems kinda high.
98 ron (octane) is what we have here, aswell as 95.
with this 95 in mind, i'll keep comp somewhat lower to be able to fill 'er up with 95 too when 98 is not available.

so how can you discribe the difference in power/torque.
i'm hoping it'll make a very noticable difference but being realistic, it'll only be moderate improvement :rolleyes:

thanks allready, i'll definately post a review on mine when i get 'er done.

ciao, Hannes.
 
Jakobi said:
. . . They are however kind of usefull for comparison from fresh to worn engine (assuming all other variables similar)
Click to expand...

I've just bought myself a ring seal tester, which for a 2 stroke is a valuable tool as you lock the piston at TDC & introduce air.

With the reduction of squish (less waste & better piston cooling) + an increase in com it is only a good thing. Lowering the barrel suits trail riding & I didn't note any fall off of peak.

+ you learn a new skill. I bought a plastic burette online for sod all. I use thin fork oil & be prepared to measure a few time till you are confident. Front wheel needs to be plonked up/tied down so plug hole is horizontal. Flywheel likes to move with the magnets a bit so tape/wedge it in position.
 
A leak down tester?? I have considered buying one of these more than once. More so for the 4T than the 2T.
 
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