How was this Twin 049 made?

WingingIt74 Thu Mar 22, 2012 2:34 pm

More precisely the crank. I can see how the case was made. I assume it just has a longer crank pin to accommodate both pistons? Ideally you would want them something like 90º from each other?

It's sprouted my imagination on trying to figure out and maybe building one.

How was this Twin 049 made? Screen11

http://www.ebay.com/itm/260982078736?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_500wt_1156

ahrma_581 Thu Mar 22, 2012 2:49 pm

For best balance you would want the crankpins at 180, like a BMW motorcycle.

From the pistons/exhaust ports, it looks as if both rods are on a single extended crankpin. If true, I don't think it would provide any primary (crankcase) compression: one piston is going up while the other is going down, so no net change in crankcase volume.

Might ask the seller if it actually runs! Smile

Ivanhoe Thu Mar 22, 2012 2:50 pm

WingingIt74 wrote:More precisely the crank. I can see how the case was made. I assume it just has a longer crank pin to accommodate both pistons? Ideally you would want them something like 90º from each other?

It's sprouted my imagination on trying to figure out and maybe building one.

http://www.ebay.com/itm/260982078736?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_500wt_1156

No, you have to have a crankshaft machined with two opposed webs, go here... http://upload.wikimedia.org/wikipedia/commons/8/8a/Boxerengineanimation.gif

to see exactly how a twin cylinder works

WingingIt74 Thu Mar 22, 2012 3:20 pm

It's not the only one built like that. Which kinda makes me curious. This one is also on ebay right now, just not as pretty.

How was this Twin 049 made? Kgrhqf10

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130667422142&fromMakeTrack=true#ht_500wt_1108

nitroairplane Thu Mar 22, 2012 3:31 pm

WingingIt74 wrote:It's not the only one built like that. Which kinda makes me curious. This one is also on ebay right now, just not as pretty.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130667422142&fromMakeTrack=true#ht_500wt_1108

You got me wondering.
Stick the two cases together and then drill out the crankpin and put a longer steel rod in there?

WingingIt74 Thu Mar 22, 2012 3:35 pm

nitroairplane wrote:You got me wondering.
Stick the two cases together and then drill out the crankpin and put a longer steel rod in there?

Technically, that won't work as it's not pulling a vacuum in the case to pull air/fuel in. I would assume that's how the JB weld engine was done. I asked if they would remove the backplate to take a pic, they replied they do not want to in fear of breaking it (the JB weld one).

nitroairplane Thu Mar 22, 2012 3:48 pm

WingingIt74 wrote:
nitroairplane wrote:You got me wondering.
Stick the two cases together and then drill out the crankpin and put a longer steel rod in there?

Technically, that won't work as it's not pulling a vacuum in the case to pull air/fuel in. I would assume that's how the JB weld engine was done. I asked if they would remove the backplate to take a pic, they replied they do not want to in fear of breaking it (the JB weld one).

True!
And get back to the chat lol.

fit90 Thu Mar 22, 2012 3:58 pm

I have a G-mark .12 twin on the way. Hopefully I will get it some time next week. When I do I am quite certain I will not be able to control myself and will almost immediately have it disassembled. When I do I will snap some pics and post them.

Bob

WingingIt74 Thu Mar 22, 2012 3:59 pm

The JB weld one, I told them to open it as it won't break it, as I need to see the crank to bid on it. They told me not to worry about bidding... go figure.

nitroairplane Thu Mar 22, 2012 4:00 pm

WingingIt74 wrote:The JB weld one, I told them to open it as it won't break it, as I need to see the crank to bid on it. They told me not to worry about bidding... go figure.

Yeah I bet!
I wonder if enough of us ask would they do it or withdraw the sale?

**RknRusty** Thu Mar 22, 2012 4:02 pm

WingingIt74 wrote:Technically, that won't work as it's not pulling a vacuum in the case to pull air/fuel in. I would assume that's how the JB weld engine was done. I asked if they would remove the backplate to take a pic, they replied they do not want to in fear of breaking it (the JB weld one).

Right, the only way to pull a vacuum for the intake and also compress the charge to force it up the bypasses is to have both pistons at TDC and BDC simultaneously. there has to be two opposing crank pins and there doesn't appear to be any way to do that in that crankcase. In the first pic, at least the cylinders are offset, but looking at the pistons, they're countering each other's actions. Inside the case, one is sucking while the other is blowing. It would need a considerably wider venturi too.

nitroairplane Thu Mar 22, 2012 4:02 pm

fit90 wrote:I have a G-mark .12 twin on the way. Hopefully I will get it some time next week. When I do I am quite certain I will not be able to control myself and will almost immediately have it disassembled. When I do I will snap some pics and post them.

Bob

cool!

nitroairplane Thu Mar 22, 2012 4:03 pm

Do you recon it could be done as a "V" so then they will be at 90 degrees to each other.

**RknRusty** Thu Mar 22, 2012 4:17 pm

WingingIt74 wrote:
http://www.ebay.com/itm/260982078736?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_500wt_1156

I sent him this question:
In the picture, one piston looks to be up while the other is down. How can it pull an intake vacuum or compress the fuel/air charge in the crankcase with the pistons working against each other? Am I missing something? Any chance you could take off the tank and show a picture of the rotating assembly? And the venturi too.
Interesting engine. Thanks,
Rusty

That crankcase is a leak waiting to happen, even if it would run.

ahrma_581 Thu Mar 22, 2012 4:23 pm

nitroairplane wrote:Do you recon it could be done as a "V" so then they will be at 90 degrees to each other.

Could be done, and would probably start and run. But maybe not too well, as the leading cylinder would get the lion's share of the intake charge.

90 degree engines can be balanced to have perfect primary balance, altho there is some secondary vibration left. Ducati motorcycles take advantage of this.

Honda/Yamaha/Suzuki also used 90 degree two stroke twins for their GP 250cc bikes too. However, the cylinders didn't share a common crankcase volume; the cranks had a center main bearing and seal, with the cylinders' center lines offset.

nitroairplane Thu Mar 22, 2012 4:34 pm

ahrma_581 wrote:
nitroairplane wrote:Do you recon it could be done as a "V" so then they will be at 90 degrees to each other.

Could be done, and would probably start and run. But maybe not too well, as the leading cylinder would get the lion's share of the intake charge.

90 degree engines can be balanced to have perfect primary balance, altho there is some secondary vibration left. Ducati motorcycles take advantage of this.

Honda/Yamaha/Suzuki also used 90 degree two stroke twins for their GP 250cc bikes too. However, the cylinders didn't share a common crankcase volume; the cranks had a center main bearing and seal, with the cylinders' center lines offset.

Well then could you use a secondary cylinder with more boost flutes than the primary, say a primary surestart cylinder and secondary tee dee cylinder.

PV Pilot Thu Mar 22, 2012 5:56 pm

There is a teaser video on youtube of a Gmark .12 twin and a few sad second of a dissasembled one. No real close up's of the crank pin setup and tiny bits, just a drunken flyby with the camera. One single big reed valve under the carb to fuel the crankcase from what i can tell on my .12 twin.

**Cribbs74** Thu Mar 22, 2012 7:06 pm

WingingIt74 wrote:The JB weld one, I told them to open it as it won't break it, as I need to see the crank to bid on it. They told me not to worry about bidding... go figure.

Really? Might want to let them know you are a member of this forum and they just killed all potential bids with that last statement.

Think of it this way, if the builder chose to use JB weld to seal the crankcase then where else did he cut corners?

**RknRusty** Thu Mar 22, 2012 7:09 pm

nitroairplane wrote:
ahrma_581 wrote:
nitroairplane wrote:Do you recon it could be done as a "V" so then they will be at 90 degrees to each other.

Could be done, and would probably start and run. But maybe not too well, as the leading cylinder would get the lion's share of the intake charge.

90 degree engines can be balanced to have perfect primary balance, altho there is some secondary vibration left. Ducati motorcycles take advantage of this.

Honda/Yamaha/Suzuki also used 90 degree two stroke twins for their GP 250cc bikes too. However, the cylinders didn't share a common crankcase volume; the cranks had a center main bearing and seal, with the cylinders' center lines offset.
Well then could you use a secondary cylinder with more boost flutes than the primary, say a primary surestart cylinder and secondary tee dee cylinder.

A supercharger and computer controlled EFI would probably help. Laughing

nitroairplane Thu Mar 22, 2012 7:12 pm

RknRusty wrote:
nitroairplane wrote:
ahrma_581 wrote:
nitroairplane wrote:Do you recon it could be done as a "V" so then they will be at 90 degrees to each other.

Could be done, and would probablyhttp://modelenginenews.org/ad/marz.html start and run. But maybe not too well, as the leading cylinder would get the lion's share of the intake charge.

90 degree engines can be balanced to have perfect primary balance, altho there is some secondary vibration left. Ducati motorcycles take advantage of this.

Honda/Yamaha/Suzuki also used 90 degree two stroke twins for their GP 250cc bikes too. However, the cylinders didn't share a common crankcase volume; the cranks had a center main bearing and seal, with the cylinders' center lines offset.
Well then could you use a secondary cylinder with more boost flutes than the primary, say a primary surestart cylinder and secondary tee dee cylinder.
A supercharger and computer controlled EFI would probably help.

l
Then I might as well fly an electric Smile

fit90 Thu Mar 22, 2012 7:42 pm

FADEC, it's being used on GA reciprocating engines now. Why not just take it a step smaller? (FADEC-full authority digital engine control)

ahrma_581 Thu Mar 22, 2012 8:10 pm

PV Pilot wrote:There is a teaser video on youtube of a Gmark .12 twin and a few sad second of a dissasembled one. No real close up's of the crank pin setup and tiny bits, just a drunken flyby with the camera.

The google is your friend: http://modelenginenews.org/cardfile/gm12.html

Shows parts diagram and some photos of crank details. Could be duplicated with Cox cylinder/piston assy's, but I wouldn't want to build the bottom end with just a bastard file and a bench vise.

dckrsn Thu Mar 22, 2012 9:51 pm

......."but I wouldn't want to build the bottom end with just a bastard file and a bench vise."

Ha! Made my night. lol!

Bob

**RknRusty** Thu Mar 22, 2012 10:48 pm

Ahrma_581, welcome to the forums. That guy in your avie looks like Reverend Jim Ignatowski.

Big Al Fri Mar 23, 2012 4:17 am

WingingIt74 wrote:More precisely the crank. I can see how the case was made. I assume it just has a longer crank pin to accommodate both pistons? Ideally you would want them something like 90º from each other?

It's sprouted my imagination on trying to figure out and maybe building one.

http://www.ebay.com/itm/260982078736?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_500wt_1156

Several years ago that same engine (or one identical to it) was on eBay. At the time I also wondered if it could be a runner. I did a layout using stock Cox parts with thoughts of duplicating it and at first glance it looked like it might be possible by using a silver soldered crank assy. But alas, (see sketch) there was a serious fly in that ointment. Sad

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