BGF T80 total teardown

maintenancenazi · Oct 28, 2011

Hey biknut, I do have a quick question. This was my fault, and I should have done the research before I attempted this. i was going to reinstall the cylinder jug today, and without knowing at the time that the piston has those little notch pins for the rings. Well long story short, I ended up breaking the top ring!

So after I read on here the correct procedure to do this, I had no problem installing the piston with the remaining ring into the cylinder. So now my question is, since these rings seem fragile, will I be ok installing the new one without some ring expanding pliers? I have installed rings before, and have never had the need to have this tool. I just installed them carefully by hand. So do you think I can do that on THIS engine without problems?

biknut · Oct 28, 2011

maintenancenazi said:
Hey biknut, I do have a quick question. This was my fault, and I should have done the research before I attempted this. i was going to reinstall the cylinder jug today, and without knowing at the time that the piston has those little notch pins for the rings. Well long story short, I ended up breaking the top ring!

So after I read on here the correct procedure to do this, I had no problem installing the piston with the remaining ring into the cylinder. So now my question is, since these rings seem fragile, will I be ok installing the new without some ring expanding pliers? I have installed rings before, and have never had the need to have this tool. I just installed them carefully by hand. So do you think I can do that on THIS engine without problems?

You shouldn't have any trouble doing it by hand on these little engines. Make sure the top of the ring is facing up. It it breaks it's not necessarily your fault. It might just be a crappy ring. The quality is all over the place.

maintenancenazi · Oct 28, 2011

Thanks for your advice and reply, that's what I was thinking. Just wanted the 411 from someone who's " been there and done that ". Hopefully this little snag will appease the trouble Gods!!

Peace, James

mapbike · Oct 28, 2011

Hey great info. biknut

none of my BGF engines had nearly that rough of a combustion chamber, but on the other hand none of mine have a head with a squish ring like yours does either, the only lemon I've had so far is the engine with the lower free floating rod pin bearings, looks like my problem was that not enough pins were installed causing a serious tolerance issue, that started all my troubles which lead me to replace the crank/rod assembly, which made for one of my strongest running engines for about 450 miles, then a major air leak caused me to swap that engine out, crank shifted in case and caused mag rotor to eat into case and seal behind the rotor, I think I have a main bearing seized on the crankshaft which probably spun in the case until it caused my problems.

I just hope that you have a great engine when you're done upgrading that "T80 " engine, I'd sure replace that upper bearing as well and then with all the good bearings and seals you should have a great engine, I have a "PK80" from BGF sitting NEW in the box I plan to do the same way you are when I have the time.

Again best wishes and I look forward to following this one to it's end.

Peace, map

(^)

biknut said:
Off with it's head. Humm, this combustion chamber looks pretty rough machined. Besides that, it doesn't look very high performance.

This is what I mean about the performance. The T80 head on the left has no squish band. The head on the right is from a X80A, also from The Rico Group. Notice the narrow squish band circling the outer edge of the combustion chamber. A squish band improves performance by allowing the piston dome to come in close contact to the squish band. Gas molecules close to the cylinder wall are squished to the center of the combustion chamber causing a more complete burn. This also reduces piston ping and allows higher compression.

Tohri · Oct 28, 2011

I just did what you're doing a couple of months ago. Points of interest:

Case match that sucker, the cases I had were way off.

Throw out that head, it warps easily and can't dissipate heat like it should. https://www.treatland.tv/puch-moped-70cc-hi-compression-head-p/puch-hi-comp-head-70cc-maxi.htm This is what you want.

Replace every single load bearing stud on that thing. No, seriously, they're dead soft. Cylinder studs stripped, intake and exhaust studs stripped, rear motor mount stud sheared off clean at the cases. I had to specially grind a drill, chisel out the stub, retap for 5/16" threaded rod and make new studs. Be careful, the motor mount studs could be fine thread at the cases (Which is appalling) and means the only option is getting m8 grade 5 bolts off mcmaster Carr.

The porting on these motors is abysmal. The intake manifold needs major attention, the exhaust port on mine was rough and slagged, the transfers were a horrorshow... Thankfully I'm neatly equipped with a flex shaft dremel.

You will need a Dellorto carb for decent performance. I reccomend the 16.16 SHA. You'll be glad you did.

If you feel up to the task, Boost ports and piston transfer holes can be cut just behind the transfer ports.

I haven't been able to get the balance on my crankshaft right, it still vibrates way worse than my 55cc. Your mileage may vary.

Lap all mating surfaces as flat as possible. The surface where the cylinder meets the cases was so horribly skewed I had to rough it in with a file before lapping (!).

Fel-pro 1/8" thick gasket material actually makes good motor mount shims. I run a BMX chain, but to do so you need to shave down the caveman-rough drive sprocket. I used a couple dremel cutting wheels stacked up as a grindstone and set to work. Very smooth motion.

Those clutch pads are really sketchy, you're probably better off cutting some from Semi-metallic clutch/brake friction material off mcmaster-carr, or buy some from one of the vendors.

I'm going to be picking up one of these motors as an experiment this winter and port the crap outta it, reed valve it, drop a highcomp head on, texture the crankcases thoroughly, make another attempt at crank balancing, and try and make a street tuned monster motor.

mapbike · Oct 28, 2011

Best wishes Tohri......

Peace, map

biknut · Oct 30, 2011

Tohri said:
I just did what you're doing a couple of months ago. Points of interest:

Case match that sucker, the cases I had were way off.

Throw out that head, it warps easily and can't dissipate heat like it should. https://www.treatland.tv/puch-moped-70cc-hi-compression-head-p/puch-hi-comp-head-70cc-maxi.htm This is what you want.

Replace every single load bearing stud on that thing. No, seriously, they're dead soft. Cylinder studs stripped, intake and exhaust studs stripped, rear motor mount stud sheared off clean at the cases. I had to specially grind a drill, chisel out the stub, retap for 5/16" threaded rod and make new studs. Be careful, the motor mount studs could be fine thread at the cases (Which is appalling) and means the only option is getting m8 grade 5 bolts off mcmaster Carr.

The porting on these motors is abysmal. The intake manifold needs major attention, the exhaust port on mine was rough and slagged, the transfers were a horrorshow... Thankfully I'm neatly equipped with a flex shaft dremel.

You will need a Dellorto carb for decent performance. I reccomend the 16.16 SHA. You'll be glad you did.

If you feel up to the task, Boost ports and piston transfer holes can be cut just behind the transfer ports.

I haven't been able to get the balance on my crankshaft right, it still vibrates way worse than my 55cc. Your mileage may vary.

Lap all mating surfaces as flat as possible. The surface where the cylinder meets the cases was so horribly skewed I had to rough it in with a file before lapping (!).

Fel-pro 1/8" thick gasket material actually makes good motor mount shims. I run a BMX chain, but to do so you need to shave down the caveman-rough drive sprocket. I used a couple dremel cutting wheels stacked up as a grindstone and set to work. Very smooth motion.

Those clutch pads are really sketchy, you're probably better off cutting some from Semi-metallic clutch/brake friction material off mcmaster-carr, or buy some from one of the vendors.

I'm going to be picking up one of these motors as an experiment this winter and port the crap outta it, reed valve it, drop a highcomp head on, texture the crankcases thoroughly, make another attempt at crank balancing, and try and make a street tuned monster motor.

Thanks for all the tips. Soon I'll be starting a new thread about the build, so don't go away.

Before I get started I have to find a crankshaft I like. Not having much luck so far.

rustycase · Oct 30, 2011

I'm very interested in balancing the flywheels and cutting a window in the piston skirt for improved intake... if someone gets around to that.
tnx
rc

richirich · Oct 30, 2011

Hey Tohri, That puch 70cc high comp head that you linked to, will it just bolt on to the 66/80cc motors? If you know , how would you think it would compare to the Manic Mechanic high comp head?

Goat Herder · Oct 30, 2011

biknut said:
Thanks for all the tips. Soon I'll be starting a new thread about the build, so don't go away.

Before I get started I have to find a crankshaft I like. Not having much luck so far.

Won't be long until the time ''labor'' and ''money'' into this diddy is = to the price of two Morini's

My crank is smooth... lea-surly cruising at 5 miles an hour and buttery smooth at red line With my S6S and S6T man I love them!

Ya knew... I was gonna tease ya eventually lol

Its still a ton fun after all! This is a great hobby! I have two new left over Happy Times to clean up so I am allways taking notes. I already sourced the upgrade parts etc. I have two decent cranks to static balance. Just keep putting them on the back burner... They will be guest machines. As long as my daily rider is cherry there is no '''necessity'' for invention as of yet. Then I think I am done with'em. Gonna clean them up right and proper then wash my hands throughly..

CroMagnum · Oct 30, 2011

biknut said:
This is what I mean about the performance. The T80 head on the left has no squish band.

Are we looking at the same picture? From what I see, the head on the left has a HUGE squish band. In fact it looks much bigger than the nominal 50% squish band recommended by Gordon Jennings.

I would consider the area I marked here in red to be the squish band:

Tohri · Oct 30, 2011

Richi: As far as I know, Yes. I bolted a stock Puch head up to my brother's BGF66 a month ago, that one only required a dremeled out combustion chamber because the piston was 10mm bigger than the chamber (!). But you DO need to drill out the stud holes for the M8's. Puch use M6 studs.

CroMagnum: Yes and No, the issue with the stock heads isn't Squish area, but that it's cast. You need a geometrically flat and Even squish band that matches the geometry of the piston, so you can set the headspace clearance on the piston. Otherwise all you're doing to wasting compressed fuel/air compared to a Hemi head. This is just another Cargo Cult design by the chinese: 'Maybe if we make it look like we know what we're doing hard enough it'll actually work!' And it may for some people, but a Real Squish band needs to be machined.

...Besides, those heads are flat as pancakes and warp under load mainly because of their lack of cooling fin area. Look at a Derbi Cylinder, or anything that Really generates horse in the 50-80cc 2 stroke world. Either it's Water cooled, or it's got almost 7:1 ratio of block area to fin area.

biknut · Oct 30, 2011

CroMagnum said:
Are we looking at the same picture? From what I see, the head on the left has a HUGE squish band. In fact it looks much bigger than the nominal 50% squish band recommended by Gordon Jennings.

I would consider the area I marked here in red to be the squish band:

Not exactly right. Only the X80A head has one. The T80 combustion chamber isn't machined to close enough tolerance to work as a squish band.

This is the squish band

biknut · Oct 30, 2011

This is the squish band on the MM gen II head

CroMagnum · Oct 31, 2011

I'd be really hard pressed to call what the either the X80A or MM gen II head has a squish band. It's pretty pitifully small.

OTOH, the T80 combustion chamber looks to me like a classic squish band. I'd be interested in knowing the clearance distance between that part of the head and the piston crown. It looks like it could be just a few mm. It might be rough cast, but it's the right shape for a squish band.

biknut · Oct 31, 2011

CroMagnum said:
I'd be really hard pressed to call what the either the X80A or MM gen II head has a squish band. It's pretty pitifully small.

OTOH, the T80 combustion chamber looks to me like a classic squish band. I'd be interested in knowing the clearance distance between that part of the head and the piston crown. It looks like it could be just a few mm. It might be rough cast, but it's the right shape for a squish band.

I'm pretty sure neither of these heads are taking advantage of any squish band.

In order for the effect to work squish clearance has to be set around 0.030".
That's what a lot of engine builders use. Less than 0.028" you run the risk of hitting the piston when the motor is cold. More than 0.040 and there's little effect especially when the motor warms up, because when the motor is hot the squish clearance increases.

I confess I've never properly checked the squish clearance on a china girl motor yet, but just eye balling with a caliper it looks like it's around 0.070" for the T80 head, and a little less for the X80 head.

The reason it's so big is because factories have to be able to use parts off the shelve to build motors. Since the parts exact dimensions vary a little bit from one part to another (especially on these motors), the factory has to make sure the squish clearance to so big that no combination of parts will result in the piston hitting the head. That's a lot more important to them than extracting the last bit of performance.

Of course when we're building our own motors the extra time and trouble to get it just right doesn't matter.

Harleys usually come from the factory @ about 0.060 or greater, and can grow to 0.070 after warm up.

For comparison, this is a 30 degree squish band of the Branch #5 heads on my Sportster. It's set @ 0.035

rustycase · Oct 31, 2011

Now we've gotten into technical fine points which will obviously cause a difference of interpretation...

IMO, depending upon the piston crown configuration, BOTH are different methods of establishing a squish band within the combustion chamber in order to promote a flame path for improved burn.

Fascinating stuff!
rc

Tohri · Oct 31, 2011

Rustypath: Both are squish bands. But a Home shopping network 30$ special replica katana, and a Hittori Hanzo sword are both technically Swords.

The difference is not in kind, but in degree. One does what it's supposed to, the other doesn't.

CroMagnum · Oct 31, 2011

biknut said:
The reason it's so big is because factories have to be able to use parts off the shelve to build motors. Since the parts exact dimensions vary a little bit from one part to another (especially on these motors), the factory has to make sure the squish clearance to so big that no combination of parts will result in the piston hitting the head. That's a lot more important to them than extracting the last bit of performance.

Actually that's a good point. Many years ago I was having a conversation at a motorcycle show about porting, performance, and why manufacturers make things the way they do with so much room for improvement. It turns out one of the factory reps was an engineer and he told me that the reason the ports were made that way on this particular engine was they had to trade off performance in order to get a low scrap rate during the casting process.

It would be nice to sell the absolute greatest motor in the world. But you've got to make 'em then sell 'em while competing with other manufacturers.

So biknut, what you're saying is on the T80 head it's not really a squish band, but just kind of squish-ish.

Tohri · Oct 31, 2011

The solution to all these problems costs about 30$.
puch moped 70cc hi compression head Nice and squishy.

BGF T80 total teardown

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