I can't wait to see how this turns out.



How are you going to bore it to 63? When I measured mine I thought the biggest that coul be done would be 61.

 

63mm tall, I WISH I could work a 63mm bore but I don't think it's happening.



This is also assuming I can find appropriate pistons as I'll be running a 'long rod' 13mm wrist pin setup. I suppose I could only shave 15mm off and use a 'short rod' piston, that might make finding a piston easier. I'm also operating under the assumption that the center of the crank is the same distance from the base of the cylinder on this motor vs a standard Honda. If not I may not be able to shorten a Lifan cylinder enough without welding up new cam chain sealing bolt hole bosses.

 

sounds like it will be fun project for sure..... it will definitly rev to the moon and back!!!!

 

63mm tall, I WISH I could work a 63mm bore but I don't think it's happening.

Ok, now it makes more sense. Not sure how I missed that. It sure will rev, have you thought about what cam you will put in it?

 

seems like it would be easier to get your SDG 107 crank destroked about 8 mil. That way you could just use your lifan 120 clutch and tranny setup the easy way. Price getting it destroked vs. what it would cost to run that alternative clutch setup and figure in the time monkeying with the oil path and making the oil pump work right. Maybe you will luck out and the jialing offset pin would fit your crank??



With the piston sitting 4mm lower in the cylinder after destroking you could run a 63mm tall cylinder with a 6V piston and a thick head/base gasket combo if needed. I guess I would start with a lifan cylinder and see where it takes you. 16mm is a lot of meat. You might be able to scrape 8 off each end, or 10 off one and 6 off the other if one side is thicker. That would be much easier than reworking a honda cylinder for sure.



Sounds like a really cool project, but full of misery and most likely a loud pop at the end

 

I'm currently quite afraid of any import cranks... hence my preference to run a Honda crank for now.



Looking at my stock Lifan cylinder, I think I can take 4mm off the top and still have a viable top deck, and some alu for the cam chain galley bolt to go through. Rather than rely on that little alu to hold threads, I'll just open up that hole and put a nut behind it. I know a local guy that's good with welding alu, if he's game I'll have him drop a blob and cut fresh threads.



It looks like I can safely deck 10 to 12mm off the bottom deck, there again enough material left to provide a mating surface as well as deal with the cam chain galley bolt.



Now, if I can find someone I trust to rework my SDG crank, as well as give it a clean bill of health, I may go that route... although I still wanna ditch the oil sling.



As far as cam, the most aggressive unit I've got specs on is TB's 300A Race Head cam. My hokey motor sim says the TB cam with a 27mm intake valve will have me hitting peak usable intake flow at 12k RPM. A bit more lift or a larger valve will slow that down letting me drag out peak power till later. I've no idea how overlap enters the equation so, ummm... lots of overlap good? : )



If there is a cam with more lift... I'm all ears!

 

I belive overlap will help acheive higher rpm to a certain extent.

 

make sure you get a video of its first rev out so we can all laugh when that 62mm pston wants to show everybody how dinky that honda rod is.you will save yourself about four months of hassle if you drop an sdg 107 crank in it.i have several stock stroke and 45.5 destroked cranks if interested.

 

be carefull these modern pitbike pistons come apart at 14k believe me i have had these chinese engine builds up that high.

 

sounds fun, i love messin with engines. just my current situation doesn't allow it. but everyone loves kids right?

 

Sounds fun! Buzzzzzzz-Booom!

Too bad there's not bucks for a Tak stock stroke crank. An XR80 rod is only 3mm shorter and a lot stouter, fits the 50 pin with 80 bearing. The stock 50 rod may cause rear traction issues, when it ventilates the cases...At that short of stroke, a destroked China crank should live just fine though. Reworking a Honda type jug should be easy. I'd bore at the new locations, and sleeve the holes down to size, then resleeve. Or.. cut the needed amount out of the MIDDLE of a tall China pattern jug! Parting tool just in to the sleeve, chamfer the edges, press it together, have the buddy reweld, then resleeve it? Leave a half mm or so for decking true when the new sleeve goes in. Are you planning an offset bore?

 

Hack - What stroke were you running? With a 41.4mm stroke, I won't be hitting 4000'/sec piston speed till just after 15k RPM. That said, I'm hoping to find a forged unit for this application. I am not looking for huge compression as that'd work against a high rev build. That should make piston shopping slightly easier.



Dave - Are you using a replacement pin to destroke the cranks, or modifying the crank bells? And could you drop one to 41.5 (4mm pin)? No chance the SDG crank and my Pro Mini crank share the same size pin is there? Being able to retain a 14mm pin would make finding suitable pistons much easier.



High RPM in general - My old E22 headed 72cc motor used to float valves right at the 14k RPM mark, using a stock Lifan cam and HD springs. I'm going to be dealing with larger/heavier valves and a more aggressive cam profile, and shooting for similar if not higher revs. Short of custom rockers made out of obsenely expensive materials, hollow stem Ti valves, and jamming a Desmo setup into this head, what little tricks are there to massage more safe revs out of the top end?



I plan on taking some material off the top of the rockers to lighten them, as well as polishing them so oil doesn't cling. I'm going to see about fitting TRX450 intake springs. (Interesting find, Kibblewhite lists a spring set for the ATC110, max rated lift of 7.4mm... wonder how those would work?) I know I can get Ti tappet adjuster nuts and save a gram on each rocker there, are there Ti spring retainers readily available off the shelf as well?

 

Not a deal breaker but your cam timing will be off if you shorten the cylinder unless you can shorten it the exact distance of a link or something.

 

if the cylinder gets shortened to 63mm, there shouldn't be a problem with the cam chain as you could just use a honda cam chain... assuming the timing sprockets are 14/28

 

Hrmmm... if an XR80 rod is 3mm shorter than a stock 50 rod, and the SDG crank shares the same pin size as a stock 50 crank...



An import 49.5mm crank expects the piston to be 1mm lower from the deck, and uses a 6mm taller cylinder. So, it runs the wrist pin 5mm further from the center of the crank, but the stroke only accounts for 4mm of that, so the rod should be 1mm longer than a stock 50 unit? (Working with 0 actual measurements, inferring based on known numbers.)



Now I've got some choices, stock 50 rod, +1mm via the SDG rod, or -3mm via the XR80 rod. Dr ATV has a heavy duty stock stroke crank crank that may be viable as well.



Lets assume I'm going to use a 'short rod' pin height piston, 19.5mm from wrist pin center to the deck.



It sounds like the consensus is to destroke the SDG crank. Retaining it's rod, destroked to 41.5 means I need a cylinder to be 65mm tall. Use a 50 rod and I'm at 64mm. Slap that XR80 rod on, 61mm.



Firepower, what is the actual length of that XR80 rod, center to center? Everything I've read says I want the rod to be in the 1.5 to 2 times the stroke length range.



Anyone know the rod length of a Lifan 55.5mm crank? If it's in the right range, I'd like to have the 14mm wrist pin.

 

Kurlon, pm me.

 

Hack - What stroke were you running? With a 41.4mm stroke, I won't be hitting 4000'/sec piston speed till just after 15k RPM. That said, I'm hoping to find a forged unit for this application. I am not looking for huge compression as that'd work against a high rev build. That should make piston shopping slightly easier.



Dave - Are you using a replacement pin to destroke the cranks, or modifying the crank bells? And could you drop one to 41.5 (4mm pin)? No chance the SDG crank and my Pro Mini crank share the same size pin is there? Being able to retain a 14mm pin would make finding suitable pistons much easier.



High RPM in general - My old E22 headed 72cc motor used to float valves right at the 14k RPM mark, using a stock Lifan cam and HD springs. I'm going to be dealing with larger/heavier valves and a more aggressive cam profile, and shooting for similar if not higher revs. Short of custom rockers made out of obsenely expensive materials, hollow stem Ti valves, and jamming a Desmo setup into this head, what little tricks are there to massage more safe revs out of the top end?



I plan on taking some material off the top of the rockers to lighten them, as well as polishing them so oil doesn't cling. I'm going to see about fitting TRX450 intake springs. (Interesting find, Kibblewhite lists a spring set for the ATC110, max rated lift of 7.4mm... wonder how those would work?) I know I can get Ti tappet adjuster nuts and save a gram on each rocker there, are there Ti spring retainers readily available off the shelf as well?

Kibblewhites atc110 springs do not fit in there. They are too tight to fit over the valve seals. On the old atc110's they didn't have seals, they had a small oring mechanism that went over it and was locked in place with the inner spring. Been there, tried that. Made me quite mad.

 

kurlon,im using offset pins.an 8mm destroke is no problem.the 50/70 cranks have a 1mm smaller diameter pin and the lifan/rcm 120 has a 1mm larger pin than the sdg107/typical 90-110 import motor.also the sdg rod is the same rod used in the 90-110 imports

 

"Firepower, what is the actual length of that XR80 rod, center to center?"

91mm with a 29mm BE vs the 50 28mm, but the crankpin's the same so the 80 bearing takes up the extra. The XR 100 rod has a 14mm SE, 97mm long, 31mm BE, 26mm pin.



The cylinder height vs cam timing is about one degree of retard per mm height reduction with a 14/28 and .250" chain. Kitaco makes offset sprockets.



I've got a bunch of NOS S&W valve springs, and will do some measuring.

 

Cool, much appreciate the info guys. Cyclerider pointed out a couple things to me so now I'm deep in research on rod ratios. It'll be a few days before I wrap my head around this one completely.



On the crank front, I think I'm going to go with an SDG crank as suggested. Depending on what rod ratio I aim for I may try to have the crank pin reloacted rather than use an offset pin so I can cut the bells down. I may need the clearance for the piston and cylinder skirts.



Plan is to split my motor tomorrow and go over it to verify that it was JUST the crank that let go, and what the cause was. I'll also start gathering measurements so I can plan any required machine work.

 

You're a math man so I figured that'd be the best way to get you lookin. Have fun.

 

for reference,the crank throws on the sdg crank are the same diameter as the 50

 

I have a 107 2 valve engine that is like the SDG but I had the crank turned down on the end to accept a nut to hld the drive gears in place...then the very end was turned down to fit a seal that was inserted into a allumunim nipple(about 2"-3" long) that is pressed on the nipple of the side cover...kinda like a extended nipple with a seal to go onto the crank.....



This elimanated the need of the oil slinger..and might be the way to go if using the honda crank.....

 

not to thread jack or anything, but since i have a old 6v system on my passport, since the rod is shorter, in return i get a higher rev in the long run right?

 

Er, that's not quite how it works. Looking at the stock rod length, the rod ratio is 2.27:1, a 1mm shorter rod isn't going to alter that ratio appreciably.