Big bore kits talk

Will'smotobikes19

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A while back I purchased a 48mm big bore cylinder kit to install on my 2 stroke engine from MZpartsmiami. However I ran into problems with piston height at tdc meaning I needed a spacer. The piston that came with the kit was already scratched up when I received it and it seems to be pretty soft aluminum. I was wondering if a 48mm Stihl piston would work or an aftermarket? They are flat top and I think the wrist pin is the same size. Would it be better to use a 38mm stroke motor or a 40mm? I am thinking about tearing the pk80 motor down replacing all bearings with Nachi and maybe seals. I have a mikuni VM18 and an OZ reed with a 23mm intake I was planning to use on it as well.
 
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Vikingimike01

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I think you should take pictures, and ask for a refund.. Hopefully you get it, keep the cylinder, throw the piston out, and get a diff. piston.
 

FNTPuck

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How far off are we talking? With the stroker crank + 48mm piston kits they state that you will need to use 3 base gaskets and possibly a second top gasket as well. If you are mixing and matching, you need to make sure you have the proper piston for your rod length. Stock crank shouldn't have ran into that issues unless you are using a different style piston with a lower wrist pin that pushes it higher up. If its the wrong style piston, you will probably need to get the proper rod or just get the complete stroker crank+rod setup.

If its not a huge difference and wrong piston, you can just throw 3 bottom gaskets on it then mic it. If its still too close, a thicker HG should take up the last bit.
 

gary55

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When you start swapping pistons around you can run to issues as you are with deck heights as well as skirt clearance issues to the crank. If you are currently clearing the crank than you should work on the deck height issue.
 

Will'smotobikes19

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It’s a pk80 type motor 40mm stroke. The thing with using that many gaskets is its more likely to have an air leak or have gas and oil leak out. The stock engine has 2 base gaskets. I have the stroker crank but worry it is untrue since I tapped it with a hammer trying to seat the bearings the wrong way.
 

Will'smotobikes19

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How far off are we talking? With the stroker crank + 48mm piston kits they state that you will need to use 3 base gaskets and possibly a second top gasket as well. If you are mixing and matching, you need to make sure you have the proper piston for your rod length. Stock crank shouldn't have ran into that issues unless you are using a different style piston with a lower wrist pin that pushes it higher up. If its the wrong style piston, you will probably need to get the proper rod or just get the complete stroker crank+rod setup.

If its not a huge difference and wrong piston, you can just throw 3 bottom gaskets on it then mic it. If its still too close, a thicker HG should take up the last bit.
I wonder if a extra thick copper gasket would work for the top.
 

gary55

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It’s a pk80 type motor 40mm stroke. The thing with using that many gaskets is its more likely to have an air leak or have gas and oil leak out. The stock engine has 2 base gaskets. I have the stroker crank but worry it is untrue since I tapped it with a hammer trying to seat the bearings the wrong way.
If you don't want to stack gaskets you could always make a min. needed spacer for the base and use a gasket above and below it.
 
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Will'smotobikes19

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I want the port timing to be as good as possible not sure how to do that but the exhaust is widened some and the intake is too. I really want this setup to work because I spent over $100 on the cylinder alone.
 

gary55

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I want the port timing to be as good as possible not sure how to do that but the exhaust is widened some and the intake is too. I really want this setup to work because I spent over $100 on the cylinder alone.
Since I've never worked with the set up you are attempting I would start with achieving adequate piston to head clearance, assemble and run the engine. Then I would have a base line to modify from, or who knows it may just run fine right out of the gate eliminating all the pre disposed doubts.
 

Street Ryderz

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This is partly why I ended up not approving of these over bored sleeves. This and the issue of heads not fitting the bore make this type of build very hard for the less experianced to get right!Gary is right in suggesting a spacer plate with the thickness to allow a gasket above and below it that totals the height required if using the 40mm stroke,However the timming of the ports is really meant for the 38mm stroke and high pinned piston Imo.The stroker crank requires the three base gaskets and two head gaskets as the product info states being a short rod and the big end pin being 1.5 mm farther out from center,this geometry actualy yeilds a 41mm stroke and does not gain the 7cc's they claim but rather 5 cc's.
 
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