79’ Yamaha QT50 - Really stuck piston

Picked up a 79’ QT50 over the weekend. It has an extremely stuck piston. Removed the cylinder head and the piston is about 3/4 down in the cylinder. Stood it upright and let it soak with Marvel Mystery Oil overnight Saturday. Sunday tried beating on the piston with hammer end and no movement. Repeated that process everyday since and no movement.

Tuesday I hit it with map gas for a good 8 minutes then beat on it with the hammer end, still no movement. Let it soak with the Marvel Mystery Oil again overnight.

I ordered a metal hole saw to use as a last resort and just drill the top of the piston off past the rings.

What other tricks can I try to get this thing unstuck?

Re: 79’ Yamaha QT50 - Really stuck piston

I had a bike like that just recently. What I ended up doing was removing the studs and twisting the cylinder to break it loose. Granted in my case the whole motor was junk so that piston was either coming out, or the conrod was getting cut.

Re: 79’ Yamaha QT50 - Really stuck piston

You're trying everything I can think of. Maybe PB Blaster soak.

Just be patient, don't want to mess up the rod on down.

Re: 79’ Yamaha QT50 - Really stuck piston

> Mitch Mills wrote:

> -------------------------------------------------------

> I had a bike like that just recently. What I ended up doing was removing

> the studs and twisting the cylinder to break it loose. Granted in my

> case the whole motor was junk so that piston was either coming out, or

> the conrod was getting cut.

Thanks yeah I considered doing this but I don’t think it would turn at all given how stuck it is.

Also tried a wheel puller attached to the cylinder studs, and turned that but all it did was put pressure on the studs so I removed it, not wanting to break off the studs and have to drill and retap.

Re: 79’ Yamaha QT50 - Really stuck piston

MMO is pretty thick stuff compared to real penetrating fluid .

Heck , a can of Coca-cola would likely be better . At least that eats rust .

Re: 79’ Yamaha QT50 - Really stuck piston

> Peter v. wrote:

> -------------------------------------------------------

> You're trying everything I can think of. Maybe PB Blaster soak.

>

> Just be patient, don't want to mess up the rod on down.

Thanks yeah I did try pb blaster for a few hours on Sunday, including down the back of the cylinder and through the exhaust port.

Re: 79’ Yamaha QT50 - Really stuck piston

> P D wrote:

> -------------------------------------------------------

> MMO is pretty thick stuff compared to real penetrating fluid .

>

> Heck , a can of Coca-cola would likely be better . At least that eats

> rust .

May try some kind of rust remover like loctite’s just for curiosity and see if that works any better. I’m assuming its the rust on the rings causing it to not come out.

Re: 79’ Yamaha QT50 - Really stuck piston

> S C wrote:

> -------------------------------------------------------

> May try some kind of rust remover like loctite’s just for curiosity and

> see if that works any better. I’m assuming its the rust on the rings

> causing it to not come out.

Rust is the usual cause , but , not always .

Re: 79’ Yamaha QT50 - Really stuck piston

Extreme solution- since the cylinder and piston are cheap to replace- put a block of wood on the fins- bottom side of cylinder between studs after taking exhaust off- break the cylinder with a few hits of hammer from opposite the wood with eye protection on. I've done it with no damage to the conn rod on badly seized engines where you're gonna replace the cylinder and piston anyway.

Re: 79’ Yamaha QT50 - Really stuck piston

> Bob Naske wrote:

> -------------------------------------------------------

> Extreme solution- since the cylinder and piston are cheap to replace-

> put a block of wood on the fins- bottom side of cylinder between studs

> after taking exhaust off- break the cylinder with a few hits of hammer

> from opposite the wood with eye protection on. I've done it with no

> damage to the conn rod on badly seized engines where you're gonna

> replace the cylinder and piston anyway.

Is the cylinder easy to just break apart? I know the fins are.

Re: 79’ Yamaha QT50 - Really stuck piston

The cylinder is iron. It's not going to break apart like an aluminum head.

Re: 79’ Yamaha QT50 - Really stuck piston

daan SJAARLOOS /

If you are doing a total rebuild with new crank you can just take out the studs and split the cases. Throw away the crankshaft with the cilinder still attached.

Re: 79’ Yamaha QT50 - Really stuck piston

@Peter - Yeah cylinder head is iron, not sure thats going to just break all that easily. Fins chip off but thats about it.

@daan - trying to avoid splitting the case and just put a new top end kit on. Otherwise yeah, would be easier that way for sure.

Drilled on it more yesterday until the battery died in the drill, looks like Im past both rings which is good and close to the wrist pin. Starting to look like the entire piston will need to be drilled out. Still not moving.

Re: 79’ Yamaha QT50 - Really stuck piston

Pushrod Fifty /

Pb blaster soaked for a few days with occasional heat from a heat gun to the cylinder usually let's me knock them loose with a hammer and large diameter wood dowel against the piston top. Heating the piston will expand it making it harder to push out. If you can get it apart without destruction you may be able to rebore it.

Re: 79’ Yamaha QT50 - Really stuck piston

> S C wrote:

> -------------------------------------------------------

> @daan - trying to avoid splitting the case and just put a new top end

> kit on. Otherwise yeah, would be easier that way for sure.

Sounds like you're already way past the point of being easier and just wasting effort on a hope .

Re: 79’ Yamaha QT50 - Really stuck piston

daan SJAARLOOS /

Personally I wouldnt like reusing a crankshaft that you've been beating on with a hammer. Do you plan on kitting it?

If so just rebuild it with fresh seals, bearings and crank. You'll end up doing it any way later

Re: 79’ Yamaha QT50 - Really stuck piston

Yeah good point, eventually that stuff will need to be replaced. Could just do everything now while the motor is off. Going to put a 50cc cylinder kit on it to get it running since that one on there is toast now.

Re: 79’ Yamaha QT50 - Really stuck piston

Well I was able to get the piston out during lunch with a little more drilling and tapping with a screwdriver and hammer around the edges. The circlips were still intact and wrist pin came right out.

Piston after drilling with hole saw:

749A05D7-A328-459B-A4F7-D8EBAFC8123A.jpeg

Re: 79’ Yamaha QT50 - Really stuck piston

WOW! that was stuck.

as long as the rod, crank, and crank bearings are OK you can do crank seals with the engine intact.

60 cc kits are available too. Bolt right on.

Re: 79’ Yamaha QT50 - Really stuck piston

> Peter v. wrote:

> -------------------------------------------------------

> WOW! that was stuck.

>

> as long as the rod, crank, and crank bearings are OK you can do crank

> seals with the engine intact.

>

> 60 cc kits are available too. Bolt right on.

Yeah I’m glad it finally came out!

Used this drill bit from Amazon.

Sipery Carbide Tip Alloy Hole Saw 38mm/1.5"

Rod and crank look good. Waiting on a right side cover gasket, and will pick up some flange bolts to replace all those screws. Once I get those I’ll replace both left and right crank seal.

Re: 79’ Yamaha QT50 - Really stuck piston

Wow! That's a good trick I'll have to remember that one.

I have the 'zoom zoom parts.com' pw50 kit on my kids pw. Took a little clean up but it's got a couple hours on it and runs strong. Kit came with a bunch of extra junk too like studs and head for $40. I didn't use them but might come in handy for you.

Re: 79’ Yamaha QT50 - Really stuck piston

> Graham Motzing wrote:

> -------------------------------------------------------

> Wow! That's a good trick I'll have to remember that one.

>

> I have the 'zoom zoom parts.com' pw50 kit on my kids pw. Took a little

> clean up but it's got a couple hours on it and runs strong. Kit came

> with a bunch of extra junk too like studs and head for $40. I didn't

> use them but might come in handy for you.

Curious what brand cylinder kit it was they sent you?

Re: 79’ Yamaha QT50 - Really stuck piston

It's just generic china junk but it was totally decent as far as that sort of thing goes.

I feel like it's better than just whatever thing off eBay, there are real people that answer the phone in Michigan. And you don't have to give half to bezo if that's important to you.

https://zoomzoomparts.com/products/yamaha-pw-50-pw50-qt-50-qt50-cylinder-piston-ring-head-gasket-set-kit-brand-new

Re: 79’ Yamaha QT50 - Really stuck piston

> Graham Motzing wrote:

> -------------------------------------------------------

> It's just generic china junk but it was totally decent as far as that

> sort of thing goes.

>

> I feel like it's better than just whatever thing off eBay, there are

> real people that answer the phone in Michigan. And you don't have to

> give half to bezo if that's important to you.

>

> https://zoomzoomparts.com/products/yamaha-pw-50-pw50-qt-50-qt50-cylinder-piston-ring-head-gasket-set-kit-brand-new

Good to know, getting people to answer the phone is worth the 45 bucks alone. 👍🏻

Thanks for the link 👊🏻

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