Gx200 clone (S170F) block preparation advise?

I R Me Can Nix 2

I R Me Can Nix 2

Member
#1
#1
Hello Everyone,
I am in the process of building a new 70mm bore gx200 clone. For a mini bike application. It is stamped S170F.
So far....

*Undecided crankshaft*
Arc std. rod with bearing
Flat top 70mm piston
Black Mamba JR camshaft
27/25mm std. clone head (p&p mild)
Stainless single angle valves
Automotive keepers/retainers
26lbs valve springs
*Undetermined pushrod*
"Champion" style rocker
Aluminum recast flywheel 28*
Std carb, 140 etube, blah blah....

Doing block prep, and have 0 experience with gx style engines.
However, This is just for fun and my pure playtime, no rush. Piddeling/Fiddling and up to no good.

I noticed the oil return between the lifters was casted with large amounts of flaws and flashing. So I promptly hit it with a drill bit of a much larger size. Taking the diameter to the outside valley wall.

My question is...
Is there a benefit to port matching the head to the block in the area of the top end oil spray channel?

The channel is teardropped shaped in the block deck, but only a round hole in the head. Beneficial turbulence?

Any other tips to make this thing more reliable are welcome!
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#2
#2
I will get some pics up tomorrow. Not near the engine right now.

I also have a questionable flaw on the crankshaft that I would like some opinions on, later.

Thanks for reading!
 
ole4

ole4

#4
#4
Leave the oil drainback alone no need to touch it. What CC's are the head? Because you are flow limiting the motor with a stock sized carb compression is your friend. The black Mamba Jr is a good choice as it is on a 104 lobe center and closes the intake earlier building a bit more dynamic compression. Which champion rockers the forged ones (good) or the stamped ones (junk in my expierence)
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#5
#5
Not sure of the head cc, haven't attempted to measured it. 18cc or 22cc maybe...

Forged set of rockers. I hate stamped items. Ugly lookin' and they bury into a little spot in the back of my brain, until changed.
17105451031439160967764667517423.jpg

Here is a shot of the valley hole, cleaned up.
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#8
#8
Back on topic... Here is the port that I am talking about. 20240316_070433.jpg

Seems that would cut oil right out of the air, to me.
Anybody enlarge this port? What is the thinnest area to watch for if I redrill and chamfer it?
 
SAS289

SAS289

Well-Known Member
#9
#9
I guess you can open it up but I'm not sure how beneficial it would be. I've never seen it done or even talked about.

If you are going to port the head you would want to go with a slide carb. I can't recommend a carb but from what I see guys who are serious about power are using flat slides. Otherwise the vm22 Mikuni carbs are pretty popular.

If you want to get the most use out of the power you are building for you may want a Juggernaut driver for the torque converter.

You mentioned "fun and my pure playtime". This year I'm going to have some fun with a stock 196 and a cheap Chikuni carb kit.
 
ole4

ole4

#10
#10
I R Me Can Nix 2 said:
Back on topic... Here is the port that I am talking about. View attachment 312248

Seems that would cut oil right out of the air, to me.
Anybody enlarge this port? What is the thinnest area to watch for if I redrill and chamfer it?
Click to expand...
Do not open it up no need you have already opened the one by the lifters. The hole you have circled is where gasket blowout usually happens on high hp motors. That looks like a 22 cc head. Pic below is a 14cc head which would raise your compression.
 

Attachments

I R Me Can Nix 2

I R Me Can Nix 2

Member
#11
#11
ole4 said:
Do not open it up no need you have already opened the one by the lifters. The hole you have circled is where gasket blowout usually happens on high hp motors. That looks like a 22 cc head. Pic below is a 14cc head which would raise your compression.
Click to expand...
Thanks for the gasket blowout tip!

This is why I asked. I appreciate it, and will now consider different surface finishes of the block and head mating surface.

I actually haven't ported the head yet, so this 27/25mm head will be milled an undertimended amount and then pushrods measured.

I will note to myself to get the appropriate surface finish. Any recommendations with MLS coated gasket? Copper spray coating?
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#12
#12
The lifer valley hole and the one circled in the picture are the only two ways I see for oil to get vaporized into the top end.

If gasket blowout occurs here, that would be excessive crankcase pressure. No?

Or does it blow from cylinder to port?

If it occurs from port to atmosphere. Then enlarging it would be an excellent idea.

I assume it is not discussed because most of the information I have gathered originates from ruled class racing. These modifications will get you noticed and then targeted. At least, it would bring unwanted attention to other aspects of your engine.

I am not racing. I will be trail riding with the wife. Who is about 100+ pounds lighter than me, on the same type of bike.

I gotta catch up!
And worse... if it blows up... she gets to giggle.

I do like the PWK flatside carbs. I see no reason to not go flatslide. 26mm sounds good to me... Behind a 27mm valve. I already own one 26mm, so I have the jets already.
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#16
#16
I assumed the valve cover just contained a baffle. I have a hard time believing the fiber plate I can see is sealing air tight. One way checked.

An actual PCV valve... is one way checked. Plumbed to engine intake manifold vaccum, not into the air filter vicinity. Unless boosted. I see your quotation on PCV, I assume we feel the same way.

My automotive experience tells me...
It only runs into the air cleaner housing to be nice and not vent crankcase blow-by into the open atmosphere. Your air filter also becomes more efficient when lightly oiled. Sticks more dirt and repelling water.

I would love to have a true vaccum pump pcv system on this motor. Except it would be literally sucking the oil vapor for oiling the top end away. No good without an oil pump.
 
I R Me Can Nix 2

I R Me Can Nix 2

Member
#19
#19
I need a headgasket with approx .26 .28 thickness. For clone, 70mm or 70.5mm.

I am thinking about stacking a superseal.09, bare stainless, then another super seal?

I am having a hard time finding 70mm clone gaskets. Not predator.
 
You must log in or register to reply here.
Top