Lifan 140cc 1P55FMJ Cam Sprocket?!

Mini Dirt Bikes & Pit Bikes Forum

Help Support Mini Dirt Bikes & Pit Bikes Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Thumpstar Pro

Well-Known Member
Joined
Jul 14, 2006
Messages
88
Reaction score
0
Hey guys,

I bought a second hand lifan 140cc engine which didnt come with a cam sprocket cover. The cam sprocket itself looks really small? The one on my genuine thumpstar pro is alot bigger, yet they both have the large sprocket cover? Check these pics and tell me what ya think? Have they put on a different cam sprocket or something?
By the way the engine is not running yet, i am waiting to pull the topend down to check the bore and head.

This has got me stumped, any help appreciated.

This is the Lifan 140cc
IMG_0053.jpg


This is the Thumpstar 125
IMG_0054.jpg


Just remember the cam sprocket cover is the same size?! What the?
Is this the genuine lifan sprocket? Please help ASAp!
 
Are you sure? The engines are both N1234. I would have thought as the 140 has the same style head the sprocket would be the same size?
 
well if its already on the engine , and the engine is suppose to run? kick it over a few times if it doesnt seize up after a couple then your sweet lol

it looks the same as the 140cc engines i have seen

The teeth on the Crank? will most probably be different , and will make up the difference in the Cam gearing/timing.
 
Last edited:
i noticed this the other day when i stole one of my mates heads... the ducar it came off had a small sprocket, while the loncin i wanted to put it on had the big sprocket. shame the head itself has a different stud pattern... but yeah, theres two size sprockets out there.
 
Yes it's OK dude ... Lifan 140's run a 28 tooth sprocket on the cam and a 14 tooth sprocket on the crank ... All 4 stroke cams are driven one revolution to every two crank revolutions (2 to 1 ratio) ... no matter WHAT brand of engine (car , bike , tractor , mower etc etc etc) ... They've used 28 to 14 to maintain the proper cam chain tension with the longer cylinder that Lifan 140's and 150's have ... ie the cam and crank axis centrelines have been stretched further apart ... so they've reduced the diameter of each sprocket to avoid cam timing changes and be able to use the stock tooth pitch chain ... IF they tried to use a longer stock pitch cam chain with 32/16 sprockets ... they'd have had to add two links since you can't just add one ... two links are 13 mm long c to c ... so halving that the cylinder would have to be 6.5 mm taller than a 120/125/127/138's cylinder to maintain the stock tension and timing mark positions ... Since the 140's cylinder is only about 1 mm longer , altering the sprocket diameters was the logical way out without encountering problems ...

All other Lifans and Jialings run a 32 tooth cam sprocket and a 16 tooth crank sprocket to maintain the 2 to 1 ratio and proper chain tension with their shorter cylinder deck height ...

It's extremely important to remember all this info especially if you fit a race cam with only two sprocket bolt holes into a Lifan 140 ... You HAVE to buy a 28 toothed 2 bolt sprocket to suit the cam ... I've seen an ebay listing in the US where they're selling a "race" cam for Lifan 140's with a 32 tooth sprocket ... Damann33 almost got sucked into destroying his new OGM Lifan 140 when he got sold a TBW300A cam and 32 toothed 2 bolt sprocket by an extremely well known US seller of pit bike race parts ...

Lifan engines that have 55.5 mm strokes have a head which accepts both the 74 mm long small bearing cam that also suits Loncin 120's AND the big bearing 70 mm long Honda type cams such as the Z28/Z40/DRATV Mega cam/TBW300A ... I'm not sure if the 140/150 heads are cast and machined to suit both cams like the 120/125/127/138's are since I haven't checked yet ...

Lifan 140/150's take the small bearing 74 mm long cam ...

As far as I know ... YX140's which have a 57 mm stroke use the 70 mm Honda type cam and a 32 tooth cam sprocket ...

Yeah headsmess ... The Loncin has the stock CRF50 head bolt spacing and Lifan BVH's have a wider spacing on a few of the studs ... You CAN adapt a BVH to a Loncin by slotting some of the stud holes in the head ... a few yanks have done it to fit heads to Hondas ...
 
Last edited:
i was thinking of doing that to it, seeing as its got little more than scrap value to me at the moment... just, do i attempt it on my dodgy lil mill or go see a mate with the cnc? my only concern was the oil gallery and which stud holes i should adjust...spose all of them would make sense...then theres the dowels.... and damn, its definitely different to a ct90 stud pattern. though i lost those heads like i said in a thread once upon a time...

but i could shave it a bit while im at it...

now regarding these sprockets...where do you find this shit out? the change of sizes makes perfect sense when you explain it like that... some clever asian thought of that... makes a change from copying everything...but then maybe they copied that too? actually, they did if i remember my 1940's engineering manuals correctly... simple chain centre/pitch/diameter calculations...

and i hope damann did a dry turn over test to find out and not a kick her over and start it up test...:p
 
Last edited:
No he didn't get to put it into the engine ... he came on this site and found out first ... I coached him in emails to help him get things done the right way and ended up buying the TBW300A off him ... I don't think they fit 140 heads anyway since they come stock with the exact same type of cam that's in a Loncin 120 ...

I've also looked at what's involved in fitting a Lifan BVH to a Loncin 120 since I was going to do it myself at one point ... I just ended up porting the Loncins head instead ... Then bought Lifan BVH engines ... You can fit a 110 Loncin E22 head to a 120 to boost the compression since they have a smaller combustion chamber plus the 110 actually has bigger and better ports as stock ... The ports of the 120 head are smaller and rougher and need a lot more metal removed ... especially around the guides and where the port meets the valve seat insert ... They're tedious to get into at first ...

As to fitting a BVH ... The oil gallery is always thru the bottom RHS cylinder stud hole ... What you need to do is first put the Loncins top head plate onto the top of a BVH to see which holes in the BVH don't line up with the Loncin 120's stud pattern ... don't touch the holes which line up ... just neatly and carefully mill slot the holes in the head which don't line up with the Loncin holes ... If the holes with the dowels need slotting ... then you can fit a few extra dowel pins to positively align the head to the bore ... The main thing to ensure is that the cam sprocket lines up with the crank sprocket ... I haven't checked the cam bore centreline distances from the deck surfaces yet ...
 
i suppose the cam chain is the most important bit, hence the need for the dowels... more to keep things square than about the exact line up...theres always a little bit of end float...

handy pic i found:) oh im so proud of myself right now...
dratv_2052_199993857


can see straight away the bottom two are the same, thats the oil gallery and one dowel that will be fine...

i still prefer the idea of getting the centre of the combustion chamber in the centre of the barrel though
 
Last edited:
Yeah you got that pic off the DRATV site ? I've already posted them on here a few years ago ...
 
yep:) dratv alright. and took me a few hours to find it again last night...stupid history

now, decided to do this on the CNC, have to go bribe someone now...easiest way to find out offsets for the other holes... move one down 2mm, the other 2.5, but do you move them across 1mm, 2mm, that sorta crap... and damn thats a deep hole to scrape one side of...finding an end mill long enough... this is why i dont do machining for a job.

maybe ill just give up like usual and leave it alone....

now, you need to really have just a thread of your own...stickied... or have all your posts put in one neat pckage like a book, easy references... mods?
 
Yes you want the combustion chamber dead centred over the bore and that's where you'd work out what holes to shift by how much ... Once again , you'd use a Loncin head gasket as a template on a Lifan BVH to work that out ... With luck , the two bottom studs will be in the right position ... You could most likely drill the stud holes in the head bigger , then press in sleeves that are drilled offset in a lathe to suit the stud positions ...
 
cactus jack. I must ask you, what is your job???? because you know basically everything mini related????
 
It's in this old thread ...

http://www.miniriders.com.au/general-chat/8406-official-what-your-job-thread-7.html

I've also studied automotive engineering and all types of engine modification including supercharging ... my main hobby was drag racing cars so I needed to know the ins and outs of a cats backside about everything to do with race engines ... I gave up on that and I sold the car due to a lack of places to race in NSW ... So it's back to dirt bikes again ...
 
^^^^well that answers most of it... you find lathe work boring and like cylindrical grinding? wtf? you must have some patience as well...
cactus jack. I must ask you, what is your job???? because you know basically everything mini related????

i think we all want to know this but you have the balls to ask:p brave man.

not just minis but motors in general related...so far, drag races, mini bike races, xr75 races...i aint reading every damn post to say anymore! theres a lot of knowledge gained over 30 odd years stuck inside that head(and how old are you cactus? and what movie is that name from?)... and just be grateful some of it overflows into our youthful spongelike minds:)

my only issue with drilling and making offset sleeves is the rocker shafts will need cross drilling...accurately..., but its worth contemplating.... grrr, stop making me think!

now i gotta go post on that thread...
 
Last edited:
Yeah lathe work can get highly repetitious , monotonous and booooriiiing ... I liked doing one off jobbing work , that's why I went over to fitting , welding/fabrication etc ...

You have to be patient to be a toolmaker and certain types of welding require a lot of patience ... porting heads requires a steady hand and a lot of patience ...

Three of the discs in my neck are worn (cactus) from the type of work and overtime I put in over the years and my name is John ... hence the fictitious name of Cactus Jack ... I'm 50 but I'm one of those people who don't look their age ... I've got two teenage kids to teach riding etc so that's why I'm back into the mini bikes ... easy to transport , store , cheap to buy and repair etc ...

With the bushes for the head ... just put them into the holes where the dowels are ... then you can counter bore them to suit the stock Loncin dowels ... They don't need to be full length ... the rocker pins are bigger than the stud holes anyway ... But if you wanted to do a really good job , you could press hat bushes in above the pin bores ...
 
Last edited:
mmm, i hear you on the lathe work... but my experience of grinding was really monotonous and you dont see sweet f all happening either... at least lathes spit chips at ya:p

:p the " resident idiot" we had at my last job...opened the lathe door with the coolant going....LOL!!!!!!

and, lemme see, left a diamond tipped insert in a rock drill that was in the mill...machinist was onto the last side...and BANG!!!! then the idiot decided to get an oxy torch into the mill... that was his last day:)

the machinist amazed me...ran three machines simultaneously(two being manuals on auto feed), then even swept the floor when they were all going... and i never saw him stuff up(except that rock drll...complacency kills). had a name like a chinese engine as well...yizhong:)
 
The cam sprocket size does not matter at all, as long as its exactly twice the size as the sprocket on the crank.
 
Hey headsmess , I just checked with a Loncin 120 head gasket on a Lifan BVH for you and it looks like the bottom 2 stud holes , the combustion chamber and the outside bolt all line up spot on ... Only the top two holes need to be moved downwards and in towards the chamber ... The dowels are on the top RHS and lower LHS stud holes so the top RHS dowel would need to be replaced with a bigger diameter one ... OR a stepped dowel ... You can't move it to the bottom RHS stud because that's the oil gallery and they don't have a dowel there ... but then again you might just be able to get away with doing it ... I'd say the cam sprocket would more than likely be in the right place for proper alignment ... You could slot the holes by hand with a rats tail file coupled with a lot of elbow grease , time and patience ... LOL ... No need to bother with sleeves/bushes ... although one in the top RHS hole would solve the dowel problem ...
 
I just remembered WHY I gave up on the idea of the BVH head swap at the time ... Loncin 120's have a crappo dished piston and coupled with the huge combustion combustion chamber of a Lifan 120 BVH ... you'd lose more power than with a stock head due to the compression drop ... AKUNAR wasn't around at the time like they are today so I couldn't find a 52.4 or 54 mm high dome piston ... which is what YOU'D have to get ...
 

Latest posts

Back
Top