Cylinder mod for piston cooling

[StepVino]

Hi, next weekend I hope to be taking my cylinder
off (Taiwan cast iron 47mm) for new rings and base
gasket due to a seize. Base gasket was leaking, jetting
was for Texas summer, and weather was very cold. I'll
be taking care of all that, but for extra protection I was
thinking of doing a cylinder mod to help cool the top of
the piston.

Mod would consist of cutting a channel in the liner, under
exhaust port, from the skirt up to the second ring (which I
don't use). Also lowering the boost port to just under the
second ring land. At BDC, if there's still pressure in the case,
some mixture should be forced up through the new channel,
flow around piston through the second ring land, and exit into
boost port.

Any thoughts?

 

[DRXRACING]

What your cutting is basically referred to as a finger port. Generally I cut from the bottom of the transfer down the liner .
Look at where your hot spots are on the piston, 4 corners? You can always start at the transfer closest to the exhaust port and worm the chanell down the sleeve to end up underneath the exhaust port. This will provide lubrication and cooling.
Put the piston in the cylinder ocassionally and visually measure the depth of the finger port.

 

[StepVino]

Thanks Dx, but I can't quite picture it. From under the tranfer,
closest to exhaust, go down then up again to under exhaust port,
or sideways to under exhaust port? Also, how deep should the
finger ports be? 1-2 mm into barrel?

 

[Wieger]

The piston is cooled enough. Think when the piston closes the transferports. And importantly, the piston is hollow. The cool fresh mixture will come around the smallend and will cool the top of the piston. No need to adjust the cilinder for significantly less cooling. The only thing you will get is seizure.

 

[DRXRACING]

Thanks Dx, but I can't quite picture it. From under the tranfer,
closest to exhaust, go down then up again to under exhaust port,
or sideways to under exhaust port? Also, how deep should the
finger ports be? 1-2 mm into barrel?

Draw a straight line with a black marker from the bottom of the transfer port WINDOW
all the way down until you run out of cylinder wall to draw on. This would be a pattern for you to follow to make a straight groove cut into the cylinder wall.
Now imagine you put a dog leg in that straight line. The dog leg would move the port over a few mm at the base/bottom of the cylinder.

Air fuel mixture will flow thru this groove from the bottom of the cylinder to the transfer port as the piston reach's bottom dead center.
The air fuel mixture contains oil and gas which is lubrication and coolant.
So in effect you can start the groove(Finger port) at some point beneath the exhaust port, at the base of the liner and end up at the bottom , middle of your transfer port window.
Depth can usually be 3 mm or so. You need to be aware how thick the liner is especially at cylinder stud area.
No need to lower the boost port opposite of the exhaust port.

 

[DRXRACING]

Also where did the piston seize. If you have a reed valve motor the inlet side of the piston can be cut to supply additional coolant to the top of the piston and top bearing.
If you have a good expansion chamber the flow will transfer because the exhaust will scavenge fresh fuel up thru the transfers.

 

[StepVino]

That was another possible plan, a small hole at the top/side
of the piston, opposite the boost port. The engine is Min. Hor.

I don't know where the seize was yet. The parts won't be in
till next week, and it still runs, so I didn't take it apart.

Here's the best I could draw from your description. I probably have
the dog leg wrong because I wasn't sure exactly what that was:
Maybe I can do both a vertical finger port under the transfer, and
one under the exhaust port like my first drawing, except with exit
at the transfer port not boost port.

Hi Wieger, I understand what you are saying about there being
enough cooling, especially if I add a small window at the top of the
piston, by the boost port. I don't see how the extra passages I'd like
to make would hurt though, or if I understand correctly, you say they
would reduce cooling and make a seizure more likely?

 

[Wieger]

More cooling would reduce seizure of course. But in the picture at the first post it's not clear how wide the 'cool'-port is. If this pretty big the piston would expande there more (although the cooling), and will cause seizure.

But search for some pictures of old cilinders/pistons, the have already cool holes under de transferports. An some of them also have a sort of transfer0channel In the piston to replace the mixture.

 

[StepVino]

The channel in the first pic I was not sure how wide to make it.
Mixture would have to be fast enough to be pushed through the
ring land, and into boost port (now transfer port), before boost
closes. At 12k rpm it would have 1.8ms to do that , which is
not much time.

I was thinking the channel shape could be a 1-2mm recess (now 3mm)
triangle (whole inside of triangle is a recess), with the vertex
up and below exhaust port. Maybe the exit at vertex of triangle
would act as a nozzle, and mixture would flow very fast into ring
land.

Wasn't sure about the bottom width (mixture entry into triangle).
I wondered if there might be reasons not to remove too much contact
area, between piston and cylinder wall, down by the cylinder skirt.
I'll do a search here to try and find the pictures you mention.

 

[Big B NRG]

Another, easier thing you could do, to cool your piston, is rich up the MJ 1-2 sizes. So that it's running a little rich. I think that's an easier way for what you want. Letting it run alittle richer at WOT, doesn't affect the power that much (you won't notice it/only see difference on a dyno).

Sounds like an easier way to me

Also, how much mixture is going thru that port at 1,8ms, maybe you can widen it up..

 

[mopedadenyou]

i wouldn't do it ... with all my experiences the taiwan cylinders sieze becuase their normally out of round. due to bad casting i would take it to a machine shop and have the bore re-cut and an oversize piston.

 

[StepVino]

Another, easier thing you could do, to cool your piston, is rich up the MJ 1-2 sizes. So that it's running a little rich. I think that's an easier way for what you want. Letting it run alittle richer at WOT, doesn't affect the power that much (you won't notice it/only see difference on a dyno).

Sounds like an easier way to me

Also, how much mixture is going thru that port at 1,8ms, maybe you can widen it up..

Yes, richer would be better first step. I am running with a forced air
scoop, and 125 main jet (stock 14mm carb bored to 17mm), which
worked perfect in summer. I will try removing the scoop, keeping main
jet the same first. Hadn't thought of removing the scoop, it's been
there so long that I forgot about it.

I recalculated the time that a finger port to second ringland would be
open. 1.8ms is for transfer port duration, but ring land is only ~2mm tall,
which would make it's duration closer to 0.8ms.

As far as rejetting being easier, of course, but I like to make changes
to the cylinder. Have a new dremmel tool that I can't wait to try .

mopedadenyou, I've been looking at bore scopes on Ebay. Hope to get
one before summer, and a honing tool.

 

[Big B NRG]

Yes, richer would be better first step. I am running with a forced air
scoop, and 125 main jet (stock 14mm carb bored to 17mm), which
worked perfect in summer. I will try removing the scoop, keeping main
jet the same first. Hadn't thought of removing the scoop, it's been
there so long that I forgot about it.

I recalculated the time that a finger port to second ringland would be
open. 1.8ms is for transfer port duration, but ring land is only ~2mm tall,
which would make it's duration closer to 0.8ms.

As far as rejetting being easier, of course, but I like to make changes
to the cylinder. Have a new dremmel tool that I can't wait to try .

mopedadenyou, I've been looking at bore scopes on Ebay. Hope to get
one before summer, and a honing tool.

If you got a new dremel, make some power-increasement adjustments. Make it do more RPM (with more power offcourse, not the no-torque-rpm), the fan spins faster and sucks more air.

Leave the scoop off, it doesn't increase cooling, sometimes it even decreases cooling.

 

[StepVino]

Correction, it looks like I meant air scoop on the fan inlet, but
air scoop is on the caburettor inlet. Foam pod filter with half
plastic cylinder covering back half of filter, catching the air
that would otherwise flow right through, perpendicular to carb
inlet. This made 2mph difference with 50cc, but have not tried
removing it on 70cc to see the difference.

I'll post the timing on the cylinder when I take it off next weekend.
See where rpm + power can be added.

 

[StepVino]

Also where did the piston seize. If you have a reed valve motor the inlet side of the piston can be cut to supply additional coolant to the top of the piston and top bearing.
If you have a good expansion chamber the flow will transfer because the exhaust will scavenge fresh fuel up thru the transfers.

Jut took it apart today. Apart from the horrible looking seize
marks, it's actually quite promising, I think. The big deal was
the ruined small end of the rod, it was tight and I'm guessing
out of round. Tried new pin and mall bearing, and it feels like
new again.

Here's the pics of cylinder and piston. Top of piston seems great?

This seems strange, cylinder head looks like the day I put it on. Slight
oil on bottom only.

Rest are the seizure marks.

Probably won't put it back together till I get a hone. Meantime
I can work on the channels I want to make. Can't stop wondering
about mopedanyou remark about the out of round...

 

[scootertrog]

I'm not an expert on this by any means, but judging by the uniform scoring all around the cylinder bore, that tells me that the cylinder was not getting enough lubrication, not a heat-related issue. Maybe someone more knowledgeable here could confirm or refute?
I also noticed the "green" colored oil residue. What brand of oil are you using? The last time I saw anything like that was with Yamahalube and Poulan... yes the chainsaw oil.

scootertrog

 

[Zumaman]

What ratio are you running, Step? 40:1?

 

[StepVino]

Scootertrog, I'm using the pump with Yamalube injector oil,
and adding about 1.5% to the gas using Yamalube premix oil.
You don't think regular seizing would have that many scratch
marks? At one time I did find the oil line had popped off, figured
my addition of oil to the gas had saved the engine, and it never
quit running till I finally seized it, long after the oil line had been
reconnected. Maybe most of those scratches came from running
it a while (don't know how long) with only the 1.5% oil in the gas.

Hi zumaman, ..yup pump plus 1.5 in the tank.

Here's a pick of one mod I did today. There were some practical
problems in figuring how to route and make the finger port, so I
put it off a bit. I went ahead and started on the other mod I wanted
to try, cutting out the bottom of the boost port. What do you think
about trying to make two extra ports, one on each side of the boost
port? Seems like it would be easy to do. If I try it, what timing
might be best for them, same as aux transfers, or same as boost?
As suggeted, timing will be as close as I can get to 132 main, 128
secondary 124 boost.

 

[scootertrog]

Stepvino,
Generally, a good sign of a lack of lubrication is that the cylinder/piston has been "stuck", in otherwords you will see scoring all around the cylinder, and not much burn. From the photos you show, I see just that. I really don't see any scorching or burning on the cylinder head, no metal deposits on the head or piston dome. The piston does not look bad, and not much blow-by (you did a good job breaking it in ).
I am not a fan of adding oil to the tank while running the pump. Bad luck doing this in the past has pretty much cured me of that practice. I either just run the pump as is or straight pre-mix.
Yamalube is fine for straight de-restricted 49cc scoots. I'd really recommend you use something else with an altered setup. When on the ragged edge, which you were, a good synthetic might have just soft-seized.

As far as the bore being out-of-round, that can be easily measured. Again, that would suggest the piston got stuck, and not a heat-related problem. What did your plug look like? That will tell you if you overheated.

scootertrog

scootertrog

 

[StepVino]

Here is the plug, doesn't look bad. BPR7HIX

I checked under the piston, and didn't see any thick black
oil deposits. Still, it did just lock up. Strange though that my
cylinder head temp wasn't reading high either, maybe ~300F,
and it was about 35-40F ambient, going 55mph or so.

Also, even though I tried before a very quick burst to 60mph,
this was the first time that I held wot for much longer. Maybe it
was just a basic problem of piston clearance at high rpm, coupled
with previous damage due to low oil.