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 Rank: Pro   Groups: Registered
Joined: 23/07/2013 Posts: 120 Points: 336 Location: Brisbane Australia
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Finally put my motor together all went well I thought? My piston is jamming at the bottom of the stroke; can look through the exhaust port and see it at the bottom so I known it's not going tight at the top of the stroke as explained in the other topic on piston jamming. I can't recall reading anything on this problem? Have doubled checked the assembly but I must be missing something, anyone had a similar problem?  Chasecar
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Rank: Pro Groups: Joined: 24/08/2009 Posts: 48,827 Points: -13,348
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Info about the piston sticking at the top is in this topic.
http://forum.model-space...spx?g=posts&t=12278
However, as your's is sticking at the bottom stroke, I would disassemble and re-assemble as it sounds like something is not properly seated.
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 Rank: Vice-Master   Groups: Registered
Joined: 05/04/2013 Posts: 540 Points: 1,630 Location: England
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You can loosen it off a bit with by heating it up with a hair dryer. I often do that when breaking in a new engine even more so when its cold. Finished 3D Printer, RB7, Hummer, Skyrider drone & Combat tank collection http://www.model-space.com/gb/
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Rank: Pro  Groups: Joined: 11/04/2013 Posts: 119 Points: 347
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Hi Chasecar,
Something is amiss with it, so be gentle as u can. Do u notice the motor being tight at the botom only when u pull the pull-starter or is it the same when you turn the engine over by the flywheel?
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 Rank: Semi-Pro Level 1 Groups: Registered
Joined: 15/01/2014 Posts: 67 Points: 197 Location: Essex
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Id start by taking the engine out of the car (if its in)and removing the head. When its off take a look (best you can) through the exhaust port to see if you can see any warping in the cylinder liner.
Failing that take off the pull start and see if you can rotate by hand the shaft it attaches too. If not, take the shaft (and the engine back plate) off and ensure the piston is correctly fixed onto the cam. I noticed during assembly that if its not quite right it will feel stiff as there's almost 0 tolerance between the two pieces.
Failing that take out the cylinder liner and inspect it properly.
Essentially, dismantle the engine piece by piece, inspect each part for damage and verify the operation of the entire assembly at each stage by hand. Eventually you will come across a part that's either damaged or not fitted correctly, or possibly both.
I'm going on the assumption here that you definitely do mean BDC and not TDC as the posts that Tomick has linked too explain the TDC 'Jamming' thing.
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Rank: Semi-Pro Level 1 Groups: Registered
Joined: 16/06/2014 Posts: 65 Points: 188 Location: Austin, USA
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I bet you have the piston rod turned the wrong way. The groove should be towards the inside and not visible when you remove the pull starter plate. When I was assembling mine it did this and that was the problem. Hope that helps.
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Rank: Pro Groups: Registered
Joined: 23/07/2013 Posts: 120 Points: 336 Location: Brisbane Australia
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Thank you all for your helpful replies. Yep definitely sticking at BCD. Aquanin has hit the problem spot on, I put the piston in facing the wrong way. I think in trying to be so careful during assembly I got the front and rear of the crankcase mixed up  When the piston is in the wrong way the bottom of the skirt jams on the crankshaft counter weight, only just but enough to stop it. So glad I didn't force it would have done damage to the piston for sure. I'll update my diary build shortly with a couple of photos of what not to do. Chasecar
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Rank: Semi-Pro Level 1 Groups: Registered
Joined: 15/01/2014 Posts: 67 Points: 197 Location: Essex
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Glad you've found the issue 
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Rank: Semi-Pro Level 1 Groups: Registered
Joined: 16/06/2014 Posts: 65 Points: 188 Location: Austin, USA
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Missed this. Glad it helped!
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