Piston hitting head I think?? Engine knocking badly and keeps cracking the head.

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johnsoma

johnsoma

New Member
Jun 14, 2014
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Philadelphia
Hi guys,
I need your help. I just built a 80cc gasbike standard 2 stroke and it ran fine for all of one minute until it started making a loud knocking metal on metal noise from the top end. I took it home and saw a crack in the head. I unbolted the head and saw a clear crack down the middle. I thought I might have just over cranked the bolts creating too high of compression so I ordered another. The next head came today and I put it on being sure to not over tighten. Within a few minutes of riding the knocking began again and looking closely I have split another head. I can only imagine that the piston is traveling too far and hitting the head and splitting it. Is this normal? Perhaps it was built with too long of a rod or too long of a piston. Is there a measurement I can do to check this. I know how to take off the head and cylinder wall but do not know how to take apart the bottom end to check anything.

I have a video uploaded to youtube for you guys to listen to and offer advice. Here is the link. In the video I mention that I over cranked the bolts, but I really think it has to be a piston that travels too far and hits the head.

http://youtu.be/5WG9DnoG7_o

http://youtu.be/5WG9DnoG7_o

Any help would be awesome!
Thanks,
Mike
 
mapbike

mapbike

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Mar 14, 2010
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Did ypu buy the engine already assembled or did you assemble it from different parts bough seperately?

This may help us know what train of thought we should go on.

And no the piston hitting the head isn't normal..
Map
 
copmagnet80cc

copmagnet80cc

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Jan 7, 2014
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What could help is a thicker gasket to put more distance between the piston and the head...I would replace the head and then try a thicker gasket..Hope it helps!
 
johnsoma

johnsoma

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Jun 14, 2014
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Philadelphia
I bought a complete kit with the engine already assembled.

If I try a thicker gasket with a new head I run the risk of wasting another ten dollars on a new head with only changing a thickness of head raise of around 1mm.

Did you guys watch the video? Listen to that noise. what does it sound like? Have you ever heard anything like that before?
 
MotorBicycleRacing

MotorBicycleRacing

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Jul 28, 2010
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SoCal Baby!!!
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johnsoma said:
I bought a complete kit with the engine already assembled.

If I try a thicker gasket with a new head I run the risk of wasting another ten dollars on a new head with only changing a thickness of head raise of around 1mm.

Did you guys watch the video? Listen to that noise. what does it sound like? Have you ever heard anything like that before?
Click to expand...
Where did you buy the kit from?
what does the seller have to say about your damage?

Use your head.
Do not start the motor until you know it clears.

Pics would help.

Take one of the broken half heads and assemble with 2 head gaskets.

Turn the motor over using the nut on the magneto.

Check for clearance.
 
Last edited:
crassius

crassius

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Sep 30, 2012
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hard to tell from sound, but if piston hits head, then there will be a mark on the piston
 
Greg58

Greg58

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May 1, 2011
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I hate to say this but one of the rod bearings may be the problem, some of the stock needle bearings are not that great.
 
johnsoma

johnsoma

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Jun 14, 2014
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Philadelphia
So I tried two gaskets and it still makes the metal knocking noise. See video...
https://www.youtube.com/watch?v=QKWM5W5CE5A&feature=youtu.be

I took pics of both heads and gaskets. For the pictures I put little arrows pointing out the cracks in the heads. The gasket when on feels like it is hanging in the cylinder a little, it is not smooth transition up the wall of the cylinder and past the gasket. Could that be it. Pics of that as well. This engine has been running for less than 15 minutes on both heads and has cracked both of them. All of the pictures and videos together can be seen on my Dropbox at the following link.
https://www.dropbox.com/sh/byd4nuoircso55s/AADZMnSqdVACPP7ZGXDpBHlOa

Greg you mentioned a blown rod bearing? would that make this sound but also allow it to run as mine does? How hard is that to determine if true? and fix?

I am thinking about just buying a whole new engine. Thoughts?

Thanks for the help,
Mike
 
johnsoma

johnsoma

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Jun 14, 2014
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Philadelphia
Also when I remove the head and just crank the motor over using the pedal with clutch engaged it will spin over very smoothly and make no noise at all which makes me think that all the bearings are fine.

I bought it on ebay from rose326a.

I am trying to contact her now. However I bought it from her more than a year ago so I doubt she will care.
 
Greg58

Greg58

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May 1, 2011
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I have a engine from that vendor, it a 66cc 38mm stroke engine with the wrist pin 15/16 up from the bottom of the piston. The one I have is one of the smoothest running yet, I have also lapped the head down about 1mm or 38 thns. Piston clearance still is not a problem, that's why I would pull the cylinder and see if a bearing is bad.
 
johnsoma

johnsoma

New Member
Jun 14, 2014
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Philadelphia
What does that mean to lap the head?

Have you seen the video? What does it sound like to you?

Also why do you think it doesn't make any noise when I hand crank it over using the pedals with the head on and off? It only makes the noise when running? SHouldnt it make the noise all the time? Also if it was a bearing wouldn't it make a noise all the time? Im new to this and just throwing ideas out.

This six. Arggghh
 
maniac57

maniac57

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Oct 8, 2011
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Is there slop in the rod bearings? (does the piston move up&down with the rod held motionless?)
If it ran fine before, something has obviously changed.
 
Greg58

Greg58

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A bearing with play in it will allow the piston to hit the head, you can't spin the engine fast enough by hand. Removing the cylinder requires no tools if you have the head off other than removing the exhaust and carb. The cylinder will lift off just be careful or you'll break the base gasket. The piston has pins in the ring groove that you will have to line up to put the cylinder back on . the cylinder is tapered so all you need to do is squeeze the rings with you fingers to start them in the cylinder. Put a little oil on the rings before installing the cylinder.
 
Greg58

Greg58

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John, near the top of the page you will see the Google custom search box. This search engine will display threads about almost everything possible on these engines. The first few will be adds but you'll see the links. These are threads on the forum that are full of help.
 
johnsoma

johnsoma

New Member
Jun 14, 2014
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Philadelphia
Once I get the piston off how do i know if the rod bearing is bad or not?

I cooked the engine after applying the engine paint in the oven at 500degrees for half hour. Could that have done something to the engine?
 
Greg58

Greg58

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May 1, 2011
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Just get the piston to tdc and pull up and down on it it shouldn't move, also hold the rod and see if the piston will move up or down or if it will rock side to side. But look for hit marks on the head and piston first.
 
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