Using stock gaskets on a BIG BORE KIT, can it be done?

DuroTools
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DuroTools
Edited Date/Time 12/4/2015 10:52pm
I am putting a +3mm big bore kit on my RMZ250. I was too cheap to buy a $60 big bore gasket kit when the only difference from stock gasket set is that the top gasket is 1.5mm less on each side of the circle diameter. I have read that many others have used a drum sander and just widened it that little bit. There is plenty of room to do this and 1.5mm is so small that I'm sure it will work fine but I noticed that the piston doesn't even come to the very top of the cylinder so is it OK if the gasket hangs over slightly? Right above that there is going to be solid metal once it is put back together so it will be stepped down (cylinder then gasket then head). I don't see any issues doing that. Has anyone done this or is there something I'm missing? maybe like once the engine spins faster the piston will go higher and peek above the cylinder. I doubt this because that would be dangerously close to the head but just wondering?
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jsmx97
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jsmx97 11/13/2015 2:47pm
11/13/2015 2:47pm
Its crucial to have the proper bore size head gasket.
DuroTools
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DuroTools 11/13/2015 4:02pm
11/13/2015 4:02pm
Thanks for the input but if the piston doesn't go through the hole in the gasket or even near it than why does it matter?
colintrax
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colintrax 11/13/2015 4:09pm
11/13/2015 4:09pm
It can mess with the flame front, and cause some weird turbulence. It'll most likely melt away anyways, and then you'll have bits of gasket in the combustion chamber.
I've never pulled the head on a rmz 250 but I assume it's copper?
You can put the gasket over the cylinder and use an exacto knife to carve it out. Then use an Emory cloth to clean up the edge of the gasket.
However IMO, buy the proper gasket. You have the money for a big bore kit, then uou should have the cash for the appropriate gasket
Paw Paw 271
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Paw Paw 271 11/13/2015 4:49pm
11/13/2015 4:49pm
If you use the stock gasket and it hangs over into the cylinder you could cause some preignition issues when the gasket gets hot.

Paw Paw

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DuroTools
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DuroTools 11/13/2015 9:08pm
11/13/2015 9:08pm
Thanks Colintrax and PAWPAW. After your comments I started to increase the bore of the gasket with a dremel stone grinder bit, just rotating around the circle so it keeps a nice round shape. The gasket has a line right where the 1.5mm mark is so it's the perfect guide, it's coming out real good but the grinding stone is so small that it keeps slipping off the edge. I'm gonna go get a bigger bit tomorrow and finish the job. I first thought it may cause some air / flow issues like you guys are saying but didn't initially think there was enough hanging over to do that along with the fact that the top part is flush against the flat part of the head but after your comments I relooked at the gasket and realized it was a multilayer gasket so those thin layers could definitely get flapping and cause some issues. I never even thought of the gasket melting or breaking apart, you really think it gets that hot to melt steel or any gasket metal for that sake? That scares!
BTW I got the big bore kit used off ebay with the piston and rings for $90. I was looking for a stock one and noticed in a picture it said Cylinder Works on the side, sure enough the description said it might be a big bore so that confirmed what I already knew. It's the 269cc, it has plenty of life left. Perfect for me to test and see if I like it over stock. U Gotta love eBay!
Paw Paw 271
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Paw Paw 271 11/13/2015 9:33pm
11/13/2015 9:33pm
I am going to make a bet that this modified gasket leaks and or fails causing water damage to the engine........

Paw Paw
DuroTools
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DuroTools 11/13/2015 9:47pm
11/13/2015 9:47pm
If you saw how little I have to shave off you would think it is going to be fine. It will be perfectly identical to what I would be buying if I bought a big bore gasket. If I see any leaks I will change it out quick and let you know it is an epic fail but I don't see one reason it would.
colintrax
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colintrax 11/13/2015 10:03pm
11/13/2015 10:03pm Edited Date/Time 11/13/2015 10:03pm
It's a steel gasket?
Regardless make sure you clean up the edge when you're done
Kawasaki-rider 11/14/2015 9:03am
11/14/2015 9:03am
Out of curiosity..

How much money have you spent on gear/bike/hauler?
But the right gasket ffs.
DuroTools
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DuroTools 12/4/2015 10:52pm
12/4/2015 10:52pm
Just wanted to give an update in case anyone was curious. It took 2.5 hrs to bore the gasket out with a stone grinder bit on a rotozip. The ground metal rolled over along the edges and there was three layers so I had to split one of the rivets and polish each side of the gasket layer flat and smooth with some very fine like 1000 or 1500 grit sandpaper. So did that to 6 sides total and it was a total pain. It did end up working and sealing fine in the end. I did buy get a new big bore gasket anyways because I scarred the coating on the stock one by accident during install and when compared them it was identical to stock obviously except the center bore, even the raised part of the gasket is in the same area around the bore. So moral of the story is unless you are broke as hell or you have a crap load of free time than it is not worth it, especially if you have a multi-layer steel gasket!

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