cylinder's head modification for overbore

[Onathan]

hi! i have this little rm80 1986 that i picked up for my girlfriend as her first bike.. it need new piston and it looks like the cylinder needs to be bored. Std bore is 47.5mm and right now it is 49mm. So i've decided to go with a wiseco 49.5 piston kit. i have to say it is my first to end job. Some people say that the cylinder's head needs to be modified when overbore is made, as well as the head gasket, to prevent from a too high compression ratio. Is this true for any application, or in my case, should i bring the head with the cylinder to the machine shop? thks and sorry for my bad english 

[cnrcpla]

You do not need to do head work for an over bored cylinder. The bore of the cylinder doesn't have much to do with compression. Not to mention, unless the high octane fuel isn't available to you, you can always just up the octane of your premix. I have head work done for higher compression, and I have to run 110 octane minimum to avoid pre-det. You should run a minimum of 93 octane (available at most gas station, at least around where I live) for the rm80.

[Onathan]

thks for the reply!

and for the head gasket? does it need to be the exact same diameter of the cylinder ? since bore is 2mm bigger now, i figured that the gasket will be like 1mm all around inside the cylinder? do i need tu cut out the interior a little?

[cnrcpla]

That I'm not sure of. I have never had a gasket that overlapped the bore and hung over into the cylinder. I would think that's not good if yours does, but I honestly don't know. I use O rings in my top end, but when you go to the parts store, ask one of the guys at the counter. They should know. Good luck with it 

[Stusmoke]

First off, welcome to the site.

Buying a 49.5 mil piston for a 49mil bore isn't going to fit, plain and simple. 49.5 mil cannot and will not fit into 49 mil. You need to look for a piston that is about 48.96 mil if the bore is exactly 49 mil. Make sure it is still designed for your bikes make and model though.

[scotty dog]

hi! i have this little rm80 1986 that i picked up for my girlfriend as her first bike.. it need new piston and it looks like the cylinder needs to be bored. Std bore is 47.5mm and right now it is 49mm. So i've decided to go with a wiseco 49.5 piston kit. i have to say it is my first to end job. Some people say that the cylinder's head needs to be modified when overbore is made, as well as the head gasket, to prevent from a too high compression ratio. Is this true for any application, or in my case, should i bring the head with the cylinder to the machine shop? thks and sorry for my bad english 

i would say take the head with the cylinder when you get it bored out, people who do it all the time will tell you what needs to happen

[Jeram]

if your head is as per the stock bore and now your bored out a full 2mm bigger then your cylinder area has increased by 8%. thats a fair margin so should be altered to match the new bore.

While your in there you can get your head modified to the modern shape/squish/volumes also so you can get the most out of the old girl.

So a combination of your head being for a smaller bore bike and it also being a 30 year old design is reason enough for me to recommend that mail the head off to a reputable two stroke tuner to get it set up properly.

This same tuner can most likely also make you an oversized gasket also as a big overlap is definitely a no-no!
either that or buy some copper sheet and make your own head gasket like I do, or simply send your stock gasket to a laser cutter and tell them you want that replicated but with an Xmm diameter hole instead. Will set you back 50-100 bucks so get a few made while your there as all the cost is getting it set up!

[2STROKEREVOLUTION]

if your head is as per the stock bore and now your bored out a full 2mm bigger then your cylinder area has increased by 8%. thats a fair margin so should be altered to match the new bore.

While your in there you can get your head modified to the modern shape/squish/volumes also so you can get the most out of the old girl.

So a combination of your head being for a smaller bore bike and it also being a 30 year old design is reason enough for me to recommend that mail the head off to a reputable two stroke tuner to get it set up properly.

This same tuner can most likely also make you an oversized gasket also as a big overlap is definitely a no-no!
either that or buy some copper sheet and make your own head gasket like I do, or simply send your stock gasket to a laser cutter and tell them you want that replicated but with an Xmm diameter hole instead. Will set you back 50-100 bucks so get a few made while your there as all the cost is getting it set up!

Listen to this. Good advice.

You can use the head mod to tailor the power to bottom-mid to be more rideable. Not sure who you are having do the overbore, but some, like Eric Gorr, do full packages with bore, port and head mod. Great to get the power delivery you want with your rebuild.