Piston clearance issues

[jmc25fl]

I know this sounds like a idiotic question but a few months back I purchased a grab bag motor from Dax. It was a full 80 CC engine minus the head. I had taken some clutch parts from it, but now I am building as new bike and I have purchased the parts to make the motor complete. I ordered a high compression slant head from dax making sure he knew that I had a motor with the longer stroke. The piston comes up past the top of the head almost 1/8th of a inch. When I put the head on the piston hits and the motor wont turn over. I also noticed that on the down stroke the intake port never clears the top of the piston. Now I am a novice at this but I would think the air/fuel mix would need to make it to the top of the piston so the spark plug can ignite it. Or is that what the transfer ports are for?
Anyway I am hoping someone can point me in the right direction so that I can finish this build.

Also I tried butting three head gaskets on but even with that there is not enough clearance.

Thank you,

James

 

[Greg58]

You have a type 'A' piston with a crank that needs a type 'B' . The piston doesn't clear the intake port, the fuel mixture is drawn down below the skirt. The mixture first travels down to lube the bearings then is drawn into the cylinder through the transfer ports.

http://www.pistonbikes.com/v/vspfiles/assets/images/piston2.jpg

 

[jmc25fl]

The strange thing is though is that this piston came with the engine I got from dax. It was a complete engine minus the head. I check a few other engines I have and they have the same type piston.

 

[Greg58]

Have you contacted Dax to see if they can help you out? Sometimes when buying grab bag or clearance items you find odd and ends that have been put together from several different orders of parts.

 

[jmc25fl]

I have a call into him now. right now I am making a spacer place for the base of the cylinder as to increase the clearance on top. I give it a 50/50 chance of working, but it gives me something to play with.

Thanks for the tips

James

 

[jmc25fl]

update I made a really fat base gasket which now gives me enough clearance, but not to much so I am sure my compression is going to be pretty high, but the piston moves freely. Any ideas on anything I might be missing.

Thanks

 

[Greg58]

Check the piston skirt in the intake port at tdc, you may need to trim the skirt because of the thick base gasket.

 

[MotorBicycleRacing]

Post some pics of your piston, piston at TDC in the cylinder, etc.

 

[jmc25fl]

Thanks for the tip. There is just a little bit hanging down, but before I trim it I want to see if it runs ok..

 

[jmc25fl]

Here are some pics with my ultra thick base gasket installed. Let me know if you think my compression is going to be sky high. I want it high and plan on running high obtain gas anyway.

 

[MotorBicycleRacing]

Do you have a pic of the piston showing the wrist pin hole height?

Your piston deck height looks good.
How thick is your head gasket?

Would like to see a pic through the exhaust port with the piston at BDC.

 

[jmc25fl]

Thanks for your input. I can't thank you enough. I tried to take enough pictures so that you would have a good idea of the setup of the engine.

The photo's include both ports with the piston all the way up, a pic of the inside of the cylinder with the piston at the bottom, and a few of the gaskets and such. The head gasket is just a stock thin gasket.

Let me know what you think.

James

 

[jmc25fl]

Here is the other pictures. I tried to get everything you asked for.

Thanks again for the help.

James

 

[jmc25fl]

BuMp Bump.. any ideas anyone??

 

[Greg58]

How does it run? If it runs OK you can break it in and check the compression. Somewhere around 125 psi is what most slant head engines have stock.

 

[jmc25fl]

125 sounds low. I have a BGF ENGINE and stock it has 160 lbs compression . the motor runs fast. What is normal with a stock head and also with a aftermarket head?

thanks

 

[MotorBicycleRacing]

That PK 80 low wrist pin hole piston is blocking a lot of the exhaust port.

Replacing with a GT 5 high wrist pin hole piston would be the way to go.

 

[jmc25fl]

in the short term will it run as is?

 

[Theon]

I think you would be best to check your port timing properly, but it looks to have the wrong piston if you need that much base gasket.

 

[MotorBicycleRacing]

Yeah, I just said that and gave him the correct piston to buy.

 

[jmc25fl]

I ordered the piston last night. Thanks far all your great input !!

J