GY6 Big Bore Kits and "P" Head Discussion By budvoss (Read 14,021 times)

[budvoss]

what vendor would you all recommend for a big bore kit, 180 cc and up for GY6
thanks

[sprocket]

Depends.

Has your current engine got a 'P' in the engine number?

Like LN 'P' 57QMJ

If it does there is only one 180cc BB for it.

If not, you can use any kits.

At 180cc you will have to have the case machined as well.

[jake aka:ventoscoot]

I really like my ceramic 160 kit from scrappydog they carry a 180 kit as well.

[budvoss]

thanks for the info. I do have a p in the #

[budvoss]

could you please tell me what the difference is with the P motor
thanks

[sprocket]

The P engines have a taller head, (73 mm) a different cam lobe, larger valves and bigger ports. They produce a couple of HP more than the 'standard' GY6...

You can use a 'standard' BB on the P engine, but you can't use the standard cam chain. As I recall the P engines can chain is one link longer. I think the chain sprocket on the crank shaft is the same, but I'm not certain...

I think that is the only change you need to make...but for 180cc you need to enlarge the hole in the case...

The reason I mentioned this is many guys have put standard BBs on their P engines and have a lot of trouble even getting them to run...until they replace the cam chain...

Just trying to inform and save you headaches...

[bige]

there are a couple of different "P" motors, one has a 3 inch stud spacing and the other is 3.5 If I rememeber right

[monstergy6]

I have a nice 63mm kit with ceramic coated piston, the stud spacing is 3" diagonal.

I also have pre bored out cases for the big bore kits, in long or short case

[budvoss]

monstergy6 thanks for the reply. i need to call your shop and get something going on a kit. can you address the cam chain issue for P engine that sprocket mentioned
thanks

[monstergy6]

Most of the motors we work on are the 1P57QMJ, I have not found those to have the taller cylinder. My personal scooter has a 1P57QMJ and it takes the stock everything, I have a 63mm kit on mine.

Now the motors that have a two bolt valve cover, some of these have a different cylinder, if you have a two bolt valve cover, then measure the cylinder bolts diagonally, if they are 3" then the stock sizes apply, if it is 3.25, then that is a different cylinder and harder to find.

I have also found the scooters made by QJ (Quing Jang), Keeway, I think the UM Matrix, and there are a few others, also have the different motor with a different bolt pattern and taller cylinder. About 18 months ago I worked on this Keeway with the weird motor, It looks like that motor number is KW158QMJ it had the odd bolt pattern for the cylinder, it also had a longer timing
chain, the bore was 57.5mm, the crank was also different.

It gets really confusing, but again I have not found the 1P57QMJ to be a problem, If you want to do the 63mm (180) kit we have the cases that are already bored out.

I hope this helps, all the input from everybody is helpful when it comes to this issue.

[sprocket]

Any jug will fit a 'P' type engine...the heads are where the difference is.

However, this can be fixed as I mentioned before by, simply changing the cam chain length

The two bolt valve covers are a competitive designed engine from what my research says...perhaps designed by Yamaha

I'm currently working on a book about the GY6 history...

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[budvoss]

so if i use my head with a 180cc jug i should be ok ?
thanks

[sprocket]

No, you can't use your head with 180cc jug. The only time you can use the 'P' head is if you simply swap the jug for another stock sized jug, like if you have piston cylinder score etc...

So to make this simple... put on any BB you want. Just understand that you can't use the stock cam chain. You must switch it out for a standard length chain...

BUT the lower cam sprocket on the crank may also have to be changed...I simply don't know...

[monstergy6]

Maybe I am missing something here but on my P157QMJ & all the other ones we work on and build the regular head works just fine, my cam chain is the stock size. Now agreed when you use a BB kit you want to go to a BB head as well otherwise the combustion chamber is too small.

By the way what is the make and name of this scoot in question?

[budvoss]

monster, its a home built trike i use to get around town . The motor was bought off ebay new a couple of months ago.
the # off motor is BD1P39MB and other # on motor is 50030667.For all your trouble i will be buying the parts from you
thanks