139qmb (w/47mm BBK) top end / compression issues

[peapoorscoot]

Alright, first let me begin by saying I've turned plenty of wrenches, although I make no claims of being a mechanic. That said, outside of the drive gear end of the CVT I have been all the way through the 139qmb at one point or another, I've even split cases and replaced a crank in the past. However I currently have an issue that has left me in need of some advice in regards to my options at hand.

My Peace Sports (or Pea Por, as the remaining decals read! Yes, it's a fairly true statement lol!) 50 has a fairly recently installed 47mm Glixal BBK. I even pried the extra cash out of my extremely tight pockets for the matching bbk head as well. Less than 50 (maybe even 25) miles on the new kit I blow the rubber coating on the head gasket out. At the time I had no high temp spray paint, so I carefully and completely stripped the coating, and reinstalled with a thin layer of Victor Reinz high temp, gas resistant, oil resistant, RTV. During disassembly I also damaged my base gasket, and without a spare in hand I decided to replace it with the same RTV. However the instructions gave no instructions in regaurds to temperature during application, and I don't believe the temperature exceeded the mid to upper 40's during the entirety of the application or curing time.

Now we're getting to the issue at hand (I'm WELL aware and OFTEN reminded that I'm long winded.)

I gave the RTV nearly 2 hours of cure time above what the instructions required. Started the bike, sounded GREAT, shut it down, got ready for a test ride (5-10 mins tops,) and then started the bike and headed to the Dollar General that's a bit less than a mile away. The bike ran great otw, no noticeable issues that I could detect at all. I grabbed a couple items I needed at the store and came out to start the bike and head home, only to discover that I once again had no compression. I believe the temp was the issue. However I am open to other thoughts.

So the situation now is this. I still have no new head gasket (even if I did, I'm not sure how much confidence I have in the typical gaskets coating.) However, I do now have some felpro gasket material for the base gasket AND some other new supplies: some Indian head shellac, permatex aviation gasket maker, and some High temperature spray paint.

My question is, given what I now have available, and considering the expected temps in Southern Kentucky this week are in the 30's - 40's. How would you all go about it this 2nd go round to insure there was no need in a third?

I'm not opposed to pulling the engine and bringing inside if it is absolutely necessary but I'm DEFINITELY not enthusiastic about the idea. Also, no I don't have a garage available unfortunately. If I don't pull the engine, I will have to do the work outside.

Thank you very much for not only any advice, but also for anyone who simply took the time to read this Novel. Your time is very much so appreciated. I'm going to go stare out my window at Satan's scooter while I await your replies, and try desperately not to act on my urge to push it in the river! (If you can't tell, this particular China bike has been a bit of a problem child!)

Thanks again!

[jackrides]

Could your head bolts or nuts be bottoming out and not actually squeezing the head? Are you using a torque wrench and a cross pattern to tighten a few pounds at a time? Could one of the mating surfaces not be absolutely flat?

[GrumpyUnk]

You should not be popping head gaskets... if all the parts are flat(cylinder head surface/cylinder surface).
You are blowing the gaskets... and have LESS cylinder pressure than 95% of the 47mm bbks installed. My bet, based on nothing, says most do not buy the '47mm head' as it costs money.
One other thing could be how tight the fasteners are torqued to. Should be around 15-17ft lb, I think... look it up. Given new bits, there should be no leftover gasket stuff of any sort when you did the first assembly, and you likely made sure the same thing on the 2nd... so the problem should not be caused by leftover gasket material or sealing goo.
Have you used a straightedge across the cylinder and head to see if there is distortion from the factory? I would not be surprised... Does it seem the head gasket is getting sandwiched between spots on the cylinder head and the cylinder where there will be some compression? The 47mm head should leave a flat(higher than the machined recess) that would match the OD of the cylinder. If they sent a 52... where they machine out a slight depression ... at the 52mm diameter... your 47mm gasket would not match, would be 'inside' the 52mm diameter, and maybe not getting compressed.
Take out a mm reading ruler and measure across the bottom of the head, noting where it is machined out. Do the same on the cylinder, noting where the diameter of the iron starts(dia) where the gasket would be compressed. Do the same on the head gasket. Do all match? Is the head machining further out(larger than 47mm?)? If so, you got the wrong head.
Just my choice, but I'd use the original head, and a head gasket designed for 47mm. I prefer fire-ring gasket with 'fiber' making up the remainder. I do not care for stamped steel gaskets, but it is just a personal thing.
I figure with the fiber, you get a bit of relief from the higher compression ratio when using the OEM 39mm head and the 47mm cylinder.
I use no sealer or RTV or anything except bare metal. I have not had a failure yet.
You can if desired use some gasket making material and cut out a head gasket to match. It may burn over time and let go, but you could run for a while with that. I would not run at all if it failed as leakage could damage the head surface with hot gases flying past... but for testing it should be fine(and it likely will not last). The gasket should be cut to match the diameter of the cylinder so it is 'covered' by the sandwich of cylinder & head, and only the edge is exposed to combustion. That, using the orignal head if you find your new is the wrong diameter, would let you know if the dang thing is going to work. This engine is not stressed highly as sold. Adding a larger bore does add to the peak pressure of compression and the pressure of the fuel burn and the G-forces of a heavier piston, but seem totally capable of handling all that... it should not have failed as yours did unless you didn't torque properly, or had something causing the head to not lay flat against the cylinder... or .. ?? Be sure to keep the oil flow passage clear of ANY sealant. You do not want to block the oil going across the interface from/to the passage surrounding the stud. That would stop head cooling and lubing of the cam bearings, lobes, arm pivots and followers, and the temp would rise.
tom

[peapoorscoot]

Jan 4, 2024 13:35:06 GMT -5 GrumpyUnk said:

You should not be popping head gaskets... if all the parts are flat(cylinder head surface/cylinder surface).
You are blowing the gaskets... and have LESS cylinder pressure than 95% of the 47mm bbks installed. My bet, based on nothing, says most do not buy the '47mm head' as it costs money.
One other thing could be how tight the fasteners are torqued to. Should be around 15-17ft lb, I think... look it up. Given new bits, there should be no leftover gasket stuff of any sort when you did the first assembly, and you likely made sure the same thing on the 2nd... so the problem should not be caused by leftover gasket material or sealing goo.
Have you used a straightedge across the cylinder and head to see if there is distortion from the factory? I would not be surprised... Does it seem the head gasket is getting sandwiched between spots on the cylinder head and the cylinder where there will be some compression? The 47mm head should leave a flat(higher than the machined recess) that would match the OD of the cylinder. If they sent a 52... where they machine out a slight depression ... at the 52mm diameter... your 47mm gasket would not match, would be 'inside' the 52mm diameter, and maybe not getting compressed.
Take out a mm reading ruler and measure across the bottom of the head, noting where it is machined out. Do the same on the cylinder, noting where the diameter of the iron starts(dia) where the gasket would be compressed. Do the same on the head gasket. Do all match? Is the head machining further out(larger than 47mm?)? If so, you got the wrong head.
Just my choice, but I'd use the original head, and a head gasket designed for 47mm. I prefer fire-ring gasket with 'fiber' making up the remainder. I do not care for stamped steel gaskets, but it is just a personal thing.
I figure with the fiber, you get a bit of relief from the higher compression ratio when using the OEM 39mm head and the 47mm cylinder.
I use no sealer or RTV or anything except bare metal. I have not had a failure yet.
You can if desired use some gasket making material and cut out a head gasket to match. It may burn over time and let go, but you could run for a while with that. I would not run at all if it failed as leakage could damage the head surface with hot gases flying past... but for testing it should be fine(and it likely will not last). The gasket should be cut to match the diameter of the cylinder so it is 'covered' by the sandwich of cylinder & head, and only the edge is exposed to combustion. That, using the orignal head if you find your new is the wrong diameter, would let you know if the dang thing is going to work. This engine is not stressed highly as sold. Adding a larger bore does add to the peak pressure of compression and the pressure of the fuel burn and the G-forces of a heavier piston, but seem totally capable of handling all that... it should not have failed as yours did unless you didn't torque properly, or had something causing the head to not lay flat against the cylinder... or .. ?? Be sure to keep the oil flow passage clear of ANY sealant. You do not want to block the oil going across the interface from/to the passage surrounding the stud. That would stop head cooling and lubing of the cam bearings, lobes, arm pivots and followers, and the temp would rise.
tom

In response to BOTH posted replies (thank you BOTH for your time and effort btw!)

Yes, I have been using a torque wrench, criss cross pattern. I typically tighten to 5lbs below spec, then start again and tighten them all up the additional 5lbs to spec. Then I warm the engine up, and check to ensure they're still in spec. afterwards. I am pulling the cylinder and head both today to do clean up and prep. I will at that time check the head diameter, as well as how true both surfaces are. I will of course promptly share my findings here. Thanks again!

Oh yeah, I also wanted to ask your opinions on the base gasket delete? Also, do you know of a online U.S. based source for the fire ring - fiber type gaskets? They are my preference too, but I can't find a U.S. based supplier of them online ANYWHERE! I look forward to your replies...

[GrumpyUnk]

I think I found them once on a US website, but it has been a while. All I can say is look. I did find them at aliexpress, and have ordered multiple from the supplier when I found them.
If you delete the base gasket, you are modifying the height of the cylinder by some number of thousandths as it will tend to be lower, closer to the engine base level. That will move the cylinder head down that much also. I doubt it will have big effect, but personally I would fear oozing gasket filling or clogging/restricting the cross block oil passage. Personally, I would smear a small dab of RTV onto the paper gasket to help conformal matching of the gasket to the block/cylinder surface. Not on the head gasket, just the base.
Did a search on big number using this: "139QMB 47mm fiber cylinder head gasket" and got some hits. I figure(possibly) if the gasket is not stamped steel, it is likely equipped with a steel fire ring, but don't bet the farm. Pictures help. One site to visit is gy6store.com as they seem to have them, but again, not sure.
tom