gt5 new jug and piston

IbedaYank

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gt5 new jug and piston now the piston hits the head and locks up the motor.
The fix for this is? I dont want to start doubling/tripling up headgaskets because these motors ar low enough compression already. stack the bottom cylinder gasket till it no longer locks up?
 
usually when the piston hits the head, a low comp ratio is your last concern...

start measuring bits and find out what bit isnt the same as the original ;)
 
This is just my 1 cent:

You can ask a industrial gasket supplier and get some "asbestos" covered metallic gasket (similar to a automobile head gasket) to use under the jug. Doing this you can increase the piston deck measurement, mine engine I found it to be 0.7 m.

I will also raise the ports too, so you have to check the amount off raise w/ this gasket. Based on my inspections raise the jug can also increase the "overlap" time among transfer and exhaust ports, if I'm right it can make the engine runs stronger at high RPM.

Also the cylinder liner is conical, the more it raise worse the rings will seal (due to increased gap at the ring ends).

Felipe
 
usually when the piston hits the head, a low comp ratio is your last concern...

start measuring bits and find out what bit isnt the same as the original ;)

no really i would not have guessed
being that these are the only replacement parts available theirs not much choice is there? Now the question is how to get around it.
 
Felipe
jugs/cylinders match but the pistons are slightly off on the new piston.
The wristpin is just slightly lower causing there to be just enough increased stroke to lock the motor up
the joys of made in china quality control....
 
Felipe
jugs/cylinders match but the pistons are slightly off on the new piston.
The wristpin is just slightly lower causing there to be just enough increased stroke to lock the motor up
the joys of made in china quality control....

I got, maybe they changed the connection rod length, in this case you will have to shim up the jug. But take care and check to ports timing and position.

Felipe Cobu
 
any access to a lathe? trim the crown of the piston... but 4mm is heaps! down to the top ring land! that cant be right? if you turn it over without the head, how far does the piston stick out? just remove that much... :p might only need a trim... a bit of modelling clay helps to check clearances ;)


gah! its 4mm matey! :p these are metric motors and should be measured as such! gripe over... it might actually be 3.5...? :eek:

thats a lot. and well, shimming up the cylinder is probably the only answer. if turning the piston is out of the question. this can be tricky. it has to be 4mm( or 3.5?:p) ali plate cut precisely where needed and dead flat.

using anything "squishy" at that thickness is a no no. ali or steel plate. repeat after me. ali or steel plate. no matter how hard it is when you squeeze it, you are not a running engine! it is made from ali and steel for a reason!

you can do it if you have a fret saw file drill etc, combined with top notch metal fabrication skills,but its hard to not warp it. get it machined...made of cash? could get fancy with CAD and get it laser cut in various thicknesses...and materials! not terribly expensive and saves a lot of frustrations.

if the pistons 4mm higher, shimming the head alone will reduce your port timings to something ridiculous where it wont rev over 3000rpm or so... regardless of your comp ratio which will have increased anyway by having reduced the exhaust port timing...(if you measure comp ratios from the ex port closure)

you have to get the edge of the new piston to the same point in the new cylinder as where the old piston and cylinder were at TDC. stands to reason. um...iunno! id be getting a new one by now... :whistle: gawd im lazy...
 
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It's a tremendous error even for chinese stuff.

Probably your engine is the 70cc (40 stroke x 47 bore) and you get the jug of a 66cc (same bore x 38 stroke) and that why you piston is jumping out the liner.

Felipe Cobu
 
yeah im sure its a 66 cc motor

ordered the piston from www.gasbike.net
we will see what their tech line says later this afternoon
 

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