[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

good suggestion about the extra bolts, didn't think of that. but sadly the outside of the cylinder is only 55mm, with a bore of 39. that leaves only 8mm to place a bolt. even using only M4 would only leave 2mm of gasket, too little I thinck ?

am going to make a copper gasket of 1mm thick, so need to take 0.5mm of the head, and was thinking of leaving a ring of aluminium stick out that would be firmly pressed into the copper, but am a bit in doubt as to how much it needs to stick out ? 1/10th ?

JanBros a écrit:

I'm prepping a Honda Sky for moped racing, but after only +/-6 runs down the street, the (original 0.5mm aluminium) head gasket is already blown/burned (guess I did my job well).


head and cylinder are bolted to the cases with only 4 long M6 bolts. so I'm wodering if they aren't strong enough for the increased power, and that the bolts stretch too much ?
If so, there is not enough meat in the cases to put in M8's so I was thinking of turning down the threads of 10.8 alen M8's to M6, drill out the holes in the head and cylinder to 8mm and bolt them like that. those M8's shouldn't stretch so much ? Or is it not a good idea to use plain 10.8 steel bolts ?

and if it's not the bolts stretching, copper gaskets are stronger than aluminium ? does the thickness matter ? I've got 0.8mm and 1mm thick copper.
I'd prefer to ust O-ring the head, but I don't want to reshape the combustion chamber (because of losing the thickness of the head gasket, the sparkplug isn't straight in the center so it's difficult to get the head correct on my lath). Or should I use a copper gasket AND O-ring the head and cylinder ? using a thicker gasket, I only need to mill the head to the correct height.

4pcs on M6 studs are enough for this engine...in scooter races is no problem 30hp with original M6 studs...I think you have too small or bad shaped combustion chamber. more than 15:1 compression ratio is bad for gaskets. on my previous engine I have bronze ring before viton gasket for 16,5:1 compression ratio, but better results I have with 15:1

I run 4cc heads so that's about 13.75:1 compression ratio.
squish area = 50%, squish = 0.70 , combustion chamber = conical TZ type

what torque do you use for the head bolts ?

torque...by the hand ... each season I give new studs and nuts...

Jan Bros....

What did you use to make the cylinder base gasket ?

A piece of cardboard from a Honda gasket set ?


OK....

Remove that cylinder and lap the top.

Do the same for the head.

Clean both parts in HOT soapy water to remove all traces of lapping compound paste.

If the cylinder is Aluminum, use " aluminum " paint to seal.

If the cylinder is cast iron ( ? ) , use something like KW copper coat to seal.

I would make double-ended studs from alloy steel and use nuts and flat washers.

My arrangement would not blow any gaskets, nor leak.

Tried and proven from many years ago.


Now, what's this talk of a 30 HP " 50 " ???


BTW, you do realize that the clamping pressure between two pieces is greater than a simple cummulative summation of the head bolts' torque spec.

30hp on piaggio 50ccm engine with 94ccm big bore cylinder...still use the same M6 studs as 50ccm 6,5 hp factory cylinder...

Oh, that one again !

It must have a modified clutch to hold 30.

I have an old TZ125G.

Runs an OEM 34 Mic with power jet.

Based on other similar Yamahas ( earlier with reed valves ) I have built, tested, & raced, I would GUESS that the TZ-G * might * have 30.

But, I don't think a 100 cc moped will out run it.

i find only dyno run with 70ccm, but 26,6 hp:

Hello! I recently started ,my newly built scooter engine. It startet perfectly with few kicks . I let it run for couple of minutes and bleeded the cooling. It run for 15-20 minutes with pauses and the temperature never went higher than 50 degrees. Yesterday i started it again for a minute and turned it off because i wanted to see what the piston and zylinder look like. I was shocked when i saw that the zylinder had 4 vertical seizure marks in the length of the traveling piston ring. The rest of the zylinder is okay. I used 1:33 Premix with Motul. I revved the engine a couple of times when it was warm but not higher than 1/4 throtle. The engine is a piaggio with a stage 6 70 cc 47,6 bore with a 0,8 piston ring. I measured the piston ring gap clearance again and it was 0,15 mm. What do you guys think could caused this ?

ice t a écrit:

... I was shocked when i saw that the zylinder had 4 vertical seizure marks in the length of the traveling piston ring. The rest of the zylinder is okay. I used 1:33 Premix with Motul. I revved the engine a couple of times when it was warm but not higher than 1/4 throtle. The engine is a piaggio with a stage 6 70 cc 47,6 bore with a 0,8 piston ring. I measured the piston ring gap clearance again and it was 0,15 mm. What do you guys think could caused this ?

I don't know about the seizure marks, Ice T, but I do know that your ring gap clearance is dangerously small.
As a rule of thumb the minimum gap should be 0,5 % of the cylinder bore.

What would be the best way to file the piston ring gap ?

ice t a écrit:

What would be the best way to file the piston ring gap ?

As everyone is thinking :

Very carefully !

I'll use Wet & Dry sandpaper on a small FLAT file and gently ease off a few " thousandths " per end.

Is the piston damaged ?

Ring(s) still free in the groove ?

Where are the score marks in the bore ?

Above the exhaust port(s) ? Transfers ?

If yes, maybe the port chamfers were not " dressed " smoothly.

Perhaps there was debris in the transfer passages ???

Obviously, any particles / debris that dislodge can contaminate and cause troubles.

BTW, I always " smooth " the edges of any ring AS NEEDED.

Compared to larger motors ( especially 4T types ), smaller rings will go on easily...

BUT, if there are sharp INNER edges on a ring, a guy might scrape off aluminum from the piston.

You don't need any resulting particles in the piston ring groove.

Once the parts are good, proper assembly is all about cleanliness and using common sense with hand-to-eye coordination.

JanBros a écrit:

good suggestion about the extra bolts, didn't think of that. but sadly the outside of the cylinder is only 55mm, with a bore of 39. that leaves only 8mm  to place a bolt. even using only M4 would only leave 2mm of gasket, too little I thinck ?

am going to make a copper gasket of 1mm thick, so need to take 0.5mm of the head, and was thinking of leaving a ring of aluminium stick out that would be firmly pressed into the copper, but am a bit in doubt as to how much it needs to stick out ? 1/10th ?

does your cylinder have a iron liner or a one piece casting with plating ? years ago some of the ktm heads bolted directly to the cylinder with no gasket at all, at the cylinder bore. and just a very thin paper gasket around the outer circumference to seal the water. it worked rather well, just as well as gaskets or orings from my experience. i have used this design at 15:1 compression with methanol and had no problems ever. also i have used it with gasoline also. maybe its a possibility for you ? but i think it would only work if the cylinder and head are of similar material. iron liner it may not work

Hello ambike!
The piston is less marked than the zylinder it has only 2 polished line marks at the exhaust side coresponding to the zylinder. The rings is completly free and has small rubing marks corespondig to the vertical marks in the zylinder. The score marks are on the sides of the exhaust port (single), between the A and B port and on the midle of the B port on the other side. The marks go the whole traveling length of the piston ring (not the piston). The zylinder was ported by me and all the ports were carefuly chamfered and then the chamfer smoothed and polished. The whole zylinder and piston were washed with alcohol,break cleaner, compresed air and with soapy wather and a brush. The only reason i can think of is that the piston ring had a cold seizure because of the small end gap. What do you think ?

The assembly method sounds good.

Of course Frits is correct about the end gap !

Procedure :

Check ring gap in bore.

Install ring in piston groove.

Check ALL clearances & fit.

If the ring has a correct fit, I will then insert the piston & ring into the zylinder's bore.

Next, I will ( gently ) twist / rotate the piston to make certain we have clearance between the ring's cut out ( or relief ) and its corresponding locating pin.

Many aftermarket pistons and rings from the " early days " will have plenty of clearance. IOW, it is easy to see.

But, when all tolerances are tighter, and especially after a ring gap has been filed, the pin clearance must be checked / qualified.


Is your zylinder plated, or with a cast iron liner ?

If it's iron and was bored-out, do you have a small chamfer at the lower edge ? That does help.

And if it was bored-out, there is always the possibility that the bore is not perpendicular to the base, or that you have taper and out-of-roundness at the ports.

Such errors can & do cause the extra wear & tear.

I have micrometers and dial bore guages with " one - tenth " resolution. That is " English ", so .0001 INCH, not metric.


I want to add something here....I build all kinds of early type motors like Bultaco.

Per OEM, these models came with Mahle pistons and Dykes type CAST IRON rings.

Dykes rings use tight tolerances, and today, these OLD, original parts might have a small amount of oxidation. I don't use JUNK, but I get lots of practice in cleaning and checking things 100 % !

On occasion, I find a NOS piston kit without the correct ring set. So, if a guy plans for trouble and checks EVERY area in advance, he'll come out ahead.


One more thing....When doing machine work with cast iron cylinders / liners ( yeah OLD stuff ), a guy has to take care of himself on the boring bar.

If a motor warmed-up completely, BEFORE it was fired, tighter tolerances on the OLD stuff would be OK.

But, that does not happen.

Now....what about crankshafts ?

Running True with good rod bearings ?

When the crank is " out of spec " it must be replaced or reworked !

Would it be possible to have a direct injected 2stroke engine breath directly from the airfilter, with no reed valve in between, thus relying only on the injected fuel and a mix of exhaustgas and fresh air to start it up and reach the powerband? Or would there be too much exhaustgas and too little oxygen?
I know once it reaches the powerband a reed valve isn't necessary anymore, but couldn't you completely leave it out with direct injection?

I 'am not a Frits, but I still am a Jan so I'll try and answer the question (not that I want to compare myself with JanThiel )

once in the powerband , you can do whithout the read valves only because the expansion pipe is doing the sucking in of fresh air. without the pipe doing the work, it just won't work as there is no suction and no pumping from the crankcase.

JanBros a écrit:

I 'am not a Frits, but I still am a Jan so I'll try and answer the question (not that I want to compare myself with JanThiel )

once in the powerband , you can do whithout the read valves only because the expansion pipe is doing the sucking in of fresh air. without the pipe doing the work, it just won't work as there is no suction and no pumping from the crankcase.

My experience is that when the reeds are a little bit damaged the  max HP goes down!
And starting the engine becomes VERY difficult!
So I think there is no way you can do without a reed valve!
And even leaking rotary valves can cause a power loss and bad starting....
Direct injection will cause a power loss.
Because the charge is no langer cooled by the evaporating fuel.

JanBros a écrit:

once in the powerband , you can do whithout the read valves only because the expansion pipe is doing the sucking in of fresh air. without the pipe doing the work, it just won't work as there is no suction and no pumping from the crankcase.

Correct. So you need some form of inlet timing control to be able to start the engine.

Jan Thiel a écrit:

My experience is that when the reeds are a little bit damaged the  max HP goes down..... And even leaking rotary valves can cause a power loss and bad starting.

I have the same experience. If the carburation is optimized for an inlet system that does not leak, then leakage will upset the mixture strength with a resulting loss of power, and starting will very difficult. But once the engine is running in the power band, the exhaust suction alone can take care of breathing and the lower flow resistance of a 24/7 open inlet in combination with a correct mixture may promote power.

'May' promote power?
To believe this gives more power I would have to see it with my own eyes!
And then I still might NOT believe it......
I always saw that even small leaks caused big problems.
And here we are talking about a very big leak!

A leak certainly has negative effects. But I do not want a leak; I want to try about 360° inlet timing. We'll see, Jan .

Thanks for all the very helpful replies!

I know that with a mixture pumping crankcase it would not work offcourse, because there is no (or very little) mixture in the cylinder then.

But if you let the transferports breath directly out of the open air, and then injecting fuel after all ports are closed, it would only need some fresh air/oxygen to be able to burn the freshly injected fuel.

So if that could be supplied by the transferports (I think) it would work. But how could you make that happen...

A roots blower would be possible but that would make it way more complicated...
I am not looking for power gains here (though it would be nice).

Frits Overmars a écrit:

A leak certainly has negative effects. But I do not want a leak; I want to try about 360° inlet timing. We'll see, Jan   .

what about a 4 cylinder with two reeds ans two direct inlets ???

or maybe, a twin carburator. one with reeds, the second without, but the second carburator should not have "ralenti" and should remain closed at low RPM ???

bentou a écrit:

....a twin carburator. one with reeds, the second without, but the second carburator should not have "ralenti" and should remain closed at low RPM ???

Something along those lines, but with a piston-controlled inlet port instead of reeds, has already been tried:

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Senne s a écrit:

A roots blower would be possible but that would make it way more complicated...
I am not looking for power gains here (though it would be nice).

so basicly you are trying to re-invent the 2-stroke diesel and have it run on fuel