Thoughts on the "slant plug head"

[Pablo]

I thought I better start my own thread on the slant plug (high compression) head. By no means am I complete with this yet, so the writing will appear blogish.

I want to emphasize from the start, for $22 (delivered) this is not a plug and play bolt on, despite the web appearance of such. I think for the price it's a bargain. It looks like it has a nice squish band with a very compact combustion chamber. This must be the "advantage" vs. a regular hemispherical head - although interestingly I think a hemi head with a smaller chamber and squish band might really be the kat's meow. The slant head is smaller, and looks cool with plug laying at an angle. The visual appearance of the head is a little rough. Not complaining, but the mating surface did not look perfect.

Bolt on? Of course, that is exactly what I did. Took the old head off, threw a new gasket on, torqued the slant head on to 15 ft. lbs, screwed in a BP5HS. Bike fired right up, but I could really feel the clutch slip and the raised compression. Took off, but it seemed rough, clutch would slip when I changed gears - but there was a feeling of increased power and all the power, the power band, graphically looking at the power vs. rpm, seemed to come on at some much lower RPM and above that RPM there was very much a flattening and a drop off of power from the peak. The clutch slip was getting worse and very irritating.

Rode back to the shop. Messed with the clutch way too much. (separate thread)

I changed to a cooler plug without the extended tip (B6HS)

Retorqued the head. Rode for awhile. Felt better. Then I notice the head is leaking right in front (engine NEVER had a head gasket leak before). So I take it apart, aluminum paint a new gasket, reassemble and torque to 17 ft. lbs. Rides OK with the exception of clutch issues again. Only notice a slight throttle issue, so I move the e-clip up a notch. Makes a nice difference, seems to be dialed in now. Will check the plug soon.

So while messing with clutch again at the end of the day yesterday I look at the front of the head/gasket seal - it is goozing BIG TIME. So tonight I will pop the head again and check it for flatness as I should have done twice before!! Arggh.

[europorsche914]

I noticed when I first got my slant cyl head I did notice the head did not have a perfectly flat mating surface for the head gasket but neither did my factory one... any way 5 min of sanding I got a flat surface and with a new gasket and properly torqued head nuts, i dont have any head gasket leaks. As for the clutch I noticed once it is correctly adjusted, even without the clutch mod mentioned in the previous thread, when not abused I rode 600 miles before I beat it real bad one day which glazed the pads...

[bmc_az]

Has anyone tried turning down the stock head?

It's a 90 degree head should be able to make a adapter and chuck it up in a lathe and cut it down.
and reshape the chamber.

used to do it with B&S flat heads years ago.

bmc_az

[ThatPerson]

I installed mine today and I can't say it was a huge difference in power, but it did give a little more bottom end. The main thing it did for me was smooth out the motor. It just purrs from idle right up to redline. My old head gave the motor quite a bit of vibration and "chug" to it. (couldn't think of a better word for it.) I did have to use 2 head gaskets to get the clearance needed to run.

I just wish the shipping was quicker. It took almost 2 weeks to get here. I know it was slowed by the holiday, but still....

In conclusion, I would say it's worth the $22 that I paid. The motor seems happier and a little more peppier now.

[DIYMark]

Seriosuly, dont buy the head if you dont have the equiptment to remachine it lol.

My head was pretty much garbage, the bosses/holes where the bolts sit were counterbored in a drill press, not milled. The spark plug hole was counter bored too! FFS how stupid are these chinese.

So first things first. I re machined all chinese "machined" surfaces and that fixed that.

Now to the combustion chamaber. The squish band was terrible and would only work if the head was shimmed up. This then leaves a huge squish clearance than would make performance worse! and also raise fuel consumption! The squish band has to gapped to the minimal amount possible so a smaller percentage of the fuel/air mix is there and thus more is in the chamber where it burns.

Also the chinese squish bands blending radius was nothing! A 90 degree edge was it. this is not accetable as the thin blur on this edge would heat up rapidly and promote detontation.

So, I turned myself a whole new head on the lathe - I pretty much brought a casting... not a bolt on part. So now, after all the hard work I have got 65/35 squish band head gapped to 1.5mm (I'm closing this slowly, its just that under no load revs who knows was the piston max hieght with all the bearing play and con rod stretch) and wit ha blending radius of 1mm - oh yeah and all squish/combustion chamber surfaces polished; of course...

The result? Awesome. Ive bumped up compressiong to 12:1 (uncorrected - corrected its about 7.3:1 - like most motocross 2 strokes) and it pulls like a tractor up the hills. This is all using a modified stock exhaust can as a muffler and a expansion chamber (thats tuned for all out speed 6000RPM +, so infact that chamber caused me to lose some torque on the hills but meh). Oh yea the bikes engine had been balanced/trued, port matched transfers/intake/exhaust, my own home made straight intake manafold and last but not least - dual boost ports (took me around 5 mins to do on the mill with a 12mm end mill, just cut the port than blend the roof angle to 45* using a engineers scraper).

Oh yeah - if your slant head didnt seem to do much when stock it could have been like the one I brought turned out to be, a stock compression head with a angle spark plug hole - some slant heads actually raise compression while some just pretty much reposition the spark plug.

[Pablo]

Popped the head off again. Not even close to being flat. I am surprised it ran at all! And I did feel higher compression - too weird. Took some elbow grease to get it flat.

Don't have time to put any more into it right now. My duties call. I'm a little behind....or a big a$$...take your pick.....let me rephrase that......

[Cabinfever1977]

im glad i didnt get a slant head.
i have all the power i want with my regular one.
my bike takes off like a rocket and zooms down the street at full power.
i could drag a couple people down the street behind my bike(i wouldnt do that thou).

[Creative Engineering]

Originally Posted by DIYMark

Seriosuly, dont buy the head if you dont have the equiptment to remachine it lol.

My head was pretty much garbage, the bosses/holes where the bolts sit were counterbored in a drill press, not milled. The spark plug hole was counter bored too! FFS how stupid are these chinese.

So first things first. I re machined all chinese "machined" surfaces and that fixed that.

Now to the combustion chamaber. The squish band was terrible and would only work if the head was shimmed up. This then leaves a huge squish clearance than would make performance worse! and also raise fuel consumption! The squish band has to gapped to the minimal amount possible so a smaller percentage of the fuel/air mix is there and thus more is in the chamber where it burns.

Also the chinese squish bands blending radius was nothing! A 90 degree edge was it. this is not accetable as the thin blur on this edge would heat up rapidly and promote detontation.

So, I turned myself a whole new head on the lathe - I pretty much brought a casting... not a bolt on part. So now, after all the hard work I have got 65/35 squish band head gapped to 1.5mm (I'm closing this slowly, its just that under no load revs who knows was the piston max hieght with all the bearing play and con rod stretch) and wit ha blending radius of 1mm - oh yeah and all squish/combustion chamber surfaces polished; of course...

The result? Awesome. Ive bumped up compressiong to 12:1 (uncorrected - corrected its about 7.3:1 - like most motocross 2 strokes) and it pulls like a tractor up the hills. This is all using a modified stock exhaust can as a muffler and a expansion chamber (thats tuned for all out speed 6000RPM +, so infact that chamber caused me to lose some torque on the hills but meh). Oh yea the bikes engine had been balanced/trued, port matched transfers/intake/exhaust, my own home made straight intake manafold and last but not least - dual boost ports (took me around 5 mins to do on the mill with a 12mm end mill, just cut the port than blend the roof angle to 45* using a engineers scraper).

Oh yeah - if your slant head didnt seem to do much when stock it could have been like the one I brought turned out to be, a stock compression head with a angle spark plug hole - some slant heads actually raise compression while some just pretty much reposition the spark plug.

Good post Mark!

Guys just as a matter of reference; the cylinder head should have an angled area that is 3 degress off angle to a line tangent to the piston dome from a point on the piston diameter and another point 10mm inward from the radius of the piston. It is critical that this is correct for any real benefit to occur. Ideally the piston should come within .030" of this area in the cylinder head, (know as squish band).

As Mark pointed out...It is a good idea to sneak up on the piston clearance.

The first graphic below is a cut-away of the engine split right down the center.

The second is a close-up of the ideal cylinder head geometry for a 70cc.

Jim

[DIYMark]

Spot on, thats it.

Also what happend to the intake Jim? At TDC its only 35% open?

[Creative Engineering]

Yeah I know,

I think a lot of this may have been intentional to keep the power output down.

Some of it is simply sloppy machine work that is typical of a mass produced product where cost is the only driving factor.

The Chinese are quite capable of making quality engines.

Jim