Two Stroke ebay Engine, Cylinder and Piston issues, Wrist Pin Bearing problem.

Hi I’m Dave welcome back to AuMechanic Channel I showed you this little cheap engine and I bought off ebay the 49cc
engine so I’m going to show you how to measure up a couple of things port
mapping compression ratio so forth so on the reason we need to do that not just
for this engine but for any two-stroke engine if we’re going to do any
modifications or we’ve got to measure everything up so that we know which part
of the engine is deficient you know and what actually needs modification on it
because if you’re going to get you know your best bang for your buck if you’re
going to do some work to it you want to find out where the engine is most
deficient and then when you do that single modification you get the most
value out of your work yeah and then other modifications might need other
modifications but you know this each modification has different returns and
you know you usually start with the one that’s going to give you the most you
know improvement that’s the first thing second thing is there’s a couple things
that with this engine that are not quite white so if you’ve got one of those
small engines that might also interest you so we’ll do a couple of things to
reveal a few issues with this motor so the first thing that’s wrong with this
engine this is two main faults for this engine one is that the small end bearing
to the hangs each side of the con rod I’ll show you that, the other problem is
is that this piston just doesn’t go anywhere near the bottom of the port at
bottom dead center you see and that’s not too good push that down
yeah that’s bottom there a couple of things we want to measure is
the piston clearance from the top of the head because this cylinder head is not
removable so when I’m doing the measurements I need to know what that is
so I’m going to show you a quick and easy way to measure the piston clearance
from the top but what we’ve also got with this motor is is this piston
doesn’t go all the way down so what I’m going to do before I take it apart is
I’m actually going to use a marker and I’m just going to mark this you see a
piston there so the once I take it off I can actually meet
you measure what the heart is there so I’ve got a piece of solder here you can
use a thicker piece of solder we’re gonna actually use that to squish it
against the top of the cylinder now because we haven’t got a removable
cylinder and normally if you’re going to measure the clearance you want to lay it
all the way around the cylinder all the way around the piston because if you
just measure in one side the piston can kick over and give you a false reading
so you want to put or at least put two pieces one side and the other side or
all the way around the piston without it I’m not on top dead center just put it
all the way around the cylinder like that you know a ring of the solder so
that when you bring it up the top dead center in you and you squish the piece
of solder you can measure it a couple of places and make sure that you’ve got the
you know the average clearance of all the parts in the cylinder because if you
just measure it once in one spot then there’s a side the piston has got a
little bit of clearance in the bore now I’m just gonna I’ve got a light there I
don’t know if you can see it but I can see it in there and I’m just gonna take
that up I’ve pushed it against the cylinder wall then I’m gonna take that
up the top dead center and now I’m just gonna push that down on the piece to
push it on this side so the cock it over this side like that just make sure that
you know if if the piston is pushed over that side so we get the the absolute
minimum amount okay we don’t to read the largest about tolerance we want to
meet the minimum of our tolerance I just push that down and then I’ll take that
out of there and then what I’ll do is that’s you probably can’t see that but
that is squash down it’s got a little bit of squash bit on the edge and I will
just measure it the calipers and I get 0.6 of a millimeter
now that I spoke about before with this you see the small end bearing okay just
hangs outside so so if the piston I mean the piston’s held about center of the rod
you know when the cylinders on but even so the bearing can hang out a bit and
that means the roller isn’t isn’t full width the roller is sort of on one side so
it’s it’s you know the stress on that rod is off-center so that could be a
problem for the rod and also that you know this can you sort of float from one
side to the other it’s just generally pretty shonky and could have been easily
resolved by whoever built the engine the manufacturers by just putting a little
aluminium washer each side of that now you can see the line from the that I
marked so that’s the difference between the bottom of the port the bottom of the
exhaust port and the top of the piston at the top of the piston should come
down to the bottom of the exhaust port and but I don’t think that’s normal for
these engines so I’m just gonna measure measures accurately as I can bit of an
angle there but it’s three millimeters if that’s the 34 mil stroke then for the
piston to go all the way down the top of the piston to come down to the bottom of
the port we had three that would have to be off an engine that had a 37
millimeter stroke so I don’t know if there’s any of these engines that have a
44 mil bore by 37 millimeter stroke or 44 by 36 millimeter stroke if anyone’s
familiar with these motors yeah let us know if these actually come with a with
a 44 by 36 or 44 by 37 this one’s 44 by 34. The next thing we’re
going to do now that we’ve measured we’ve got three mil on there
we’ve measured the piston clearance from the top of the cylinder is we’re going
to move to the cylinder and we’re going to take some measurements off the cylinder
so I think that will do it for now and we’ll leave the rest until the next
video bye for now folks


  • Frankie Rutherford

    Finally a real tuner I can learn from. Thanks

  • Learn something new from your channel always. Thx


  • great video! I'm interested to see the port duration's on this. Those piston rings look thick .

  • 34+3+3=40 stroke. 3mm at top 3mm at bottom . Great content .
    Exhaust calculation very enjoyable and rare to see in such a clear way.

  • Nice to see your face!
    Does it really matter that much if the port does not open completely? If there is not enough duration or port area just make the port higher. If we are talking crap chinese engines like the motorized bike I race…

  • I opened up my mates cheap 2 stroke generator that was running poorly due to running the wrong ratio oil.

    The motor was all coked up and after cleaning it up I noticed the ports weren't uncovering like yours.

    I think with a 5mm spacer at the base of the cylinder and then 5mm machining off the top should sort it out (luckily the head is separate) but would that be worth doing on a generator?

    I might take it a part again to check the little end, good spot that one.

  • From the looks of this engine, I'll be reconsidering what engine I'll be using in my project minibike. Although I'd like to use a 4-stroke engine, I need something that I can lay flat, and would run reliably in any position. So an engine like this one, but with better quality would be ideal.

  • Again even a polini has those insane gaps between the piston and the rod
    So youd recommend making aluminium washers to put in between?

  • Can you please do a video about optimizing a cylinder head, such as optimal squish angle (relative to dome of piston), radius of squish angle (relative to the dome of the piston), squish thickness, optimal percentage of squish band for applications such as drag vs street. Also the corner of where the squish meets the combustion chamber; should it be radius-ed or sharp angle (what makes the most power). Thanks

  • I was a little nervous about getting an engine like that just because the head top end isn't removable and might be 38 cc it really 38 cc ? the reason why I got the bicycle engine is it's 60 cc but the ignition cdi is odd unreliable cheesaneeze. Only now figured the flywheel magnet got too weak on that and I was thinking ignition coils were coming in the mail dead on arrival … the race version of the one you got. the head bolts extend up to the top end with the head removable ? That would probably make things easier if and when you Deck the cylinder and correct that exaust port . And compression I don't think that the Piston wrist pin being loose is much a problem so long as the cylinder hold the Piston right center. My nephew had a tiny scooter with the same engine it was no way near as fast As a tomos A35! moped.

  • Do you have a email where one could contact you besides your YouTube channel

  • Same thing I noticed while measuring my engine's port after watching your previous videos my exaust port does not opens totally the 2 to 3 mm of piston stays above the lower end of the port, Does it decrease the power of the engine??

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