Will 253FMM engine take 170 pistons?

[lingeringstin...]

I can't find much information at the moment but the SMC 170 piston looks very like the 125 pistons used in the SMC250 twin engine, which is a Chinese 253FMM (whatever that means).

What I want to know is if there's enough liner in the barrels of a 253FMM to bore them out to take a pair of SMC 170 pistons. I can't find the dimentions of the pistons anywhere at the moment.

I know the Chinese 253FMM engine comes in 250 and 300 quadbike versions (among others) but are they different barrels with different amounts of liner or is one just factory bored out more?

A pair of 170 pistons would in theory make it a 340cc give or take a bit, which would be interesting if there's enough liner to do it.

[Rogerborg]

[Teflon-Mike]

Before Sowie made plan redundant and bought the bludi-guzzi, notion had been to build up a 200-Super-Benly lump, loosely based on the older 'sloper' CB200, which was a screamer, with the same 41mm stroke of the 125 twins, but a 55mm bore.... Oh-Kay, so use 125 crank, CD200 barels, 309 cam and away we go..... y-e-a-h....
Not so pertinent to you, but the barrels are stepped at the sleeve, and crank cases have to be matched to the sleeve.
Pertinent to you, the standard over-bore piston's on the 53mm motors run out 'early' compared to the 43mm motors that have standard over sizes at 1/4mm intervals that will take them all the way to I think it's 144cc @47mm bore dia. Ie; the liner, which I think is aprox 8mm wall thickness is down to half the wall thickness.
On the 233's, standard over-bores run out at I think 242cc @ 54mm Bore Dia... the liner thickness seems thinner from the start, but I think it's particularly thin at the crank-step.
Measuring up, I reckoned that there was enough meat in the sleeve to 'just' about eek a 55.5mm bore into one, to get 198cc from a 125's 41mm stroke crank, but, it would be pretty thin, and there may be some holes in the liner at the crank-step! @56 bore, it would certainly risk break-through at the step, and @57, you would be pretty close to running on the alloy of the barrel.
53x53 gives you 233.8cc
54x53 gives you 242.7cc
55x53 gives you 251.8cc

300cc on a 53mm stroke would take a 60mm bore

That begs a few of notions; first s that 300c Benly derived Chnky twins, use barels with an even bigger liner, and modded crank-case to accept them; and/or they use a longer throw crank, and/or they aren't actually 300cc, which given tendancy to 'round-up', which when 143cc engines are toutinely badges as 150's or 233's badges 250's, s more than likely.

I am going to hazard a guess that the SMC170 slugs are 'big bore CG' pistons; I cant find specs, but coding of one I found sort of implies its a 63mm diameter, which would be about right to get a tad under 170cc on a CG stroke... would give 330 on a 53mm stroke twin.. IF you still have a cylinder liner and aren't running rings direct on the ally and 'porting' between holes between fins or ventng nto the cyl-stud galleries! A-N-D, the piston will ft your con-rods....

This was big issue I discovered on the 125 motors that use, OTMH an 11mm Dia gudgeon; the 233's most often seem to use a 12mm gudgeon, I think, the CG, ISTR a 13mm.. begging a host of perms and coms tryng to match pistons to rods to craks, which are an intricate 5 bearing pressed up affair... How handy are you with the V-Blocks and coppa mallet?

As they say, anything is do-able.... whether its worth it, THAT s the big question!

I have found the Benly based motors to pretty uch be a nervouse breakdown in progress; and from comments in other posts, the Chky derivatives ramp that enormousely, offering 'suggestion' of so many mix and match bits for cheaps.... B-U-T.... so often mix, but don't 'quite' match.... and to what end?

All are restricted in the cam; and all but the CB125 motors are 360 timed... they shale like an old Brit-Twin, and whilst they do have a pretty well supported 3 main crank, I suspect that the 14Krpm 125's were 180 for that very reason, and the big-bore motors, if they might be encouraged to rev to get the power, will probably shake themselves to bits pretty quick.

Worth noting that the cylinder head torque is a ridiculously low 7-10ft-lb, and that the studs clamp down through the rocker bosses that also retain the cam-bearings, which would make the potential to move the cylinder head studs to fit bigger liner/bore, more vexations, as well as beg query over how much stress/vibration thee motors could take before they start popping had-gaskets.

To wit. conclusion I drew was that the potential in any of them is pretty limited; and 20-25bhp is about the best you might realistically hope for from one; 200 Super-Benly for Snowie, when she was on 33bhp restrict, and after all the work and money sunk into her 'Pup', was as said ultimately abandoned when she bought the Bludi-Guzzi, and I built up a mongrel T/TDC 125 motor around 309 cam I'd procured for it, as a 125, to go into the project bike she and my daughter pulled to bits, threatening to 'do' for daughter's CBT.. I still have to find motivation to finish! If it makes more than 15bhp for a four-stroke 125, then I'll be happy with it; (actually I'd be 'happy' with it if it was just one peice not strewn around the sheds!) It would still be an achievement for a four-stroke 125!

From comment in other post, vis the legacy of the MZ.... hmmm.... I recall a fair few coming over in the early 90's after the wall came down; former soviets flogging them and trabants on to try buy Voltswaggens! The trend reversing in the late 90's early naughties as poles hunted them out and took them home, abandoning many in re-united germany along the way, for the Hipster-Geeknic's to play with!

I am sanguine that the Chinks that have pry-barred themselves into that market, but unlike the Soviet tackle that was built to be fixed with a Zenit Cammera for a hammer, seem to be built to break, so they can flog another one, and keep the Dollar Drain war going, have the nature to be shedologsted to great advantage in the same way....But still...

Short answer is that I think that anything much over 55mm bore on a benly motor, you are likely to run into issues, if not in the gudgeon diameter, or crown heights, remember its a semi-hemi head.... then liner thickness and the liner over-hang are likely to cause hassle, if you don't run into issue with the cylinder head stud spacing.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Commuter_Tim]

[Rogerborg]

[Snod Blatter]

If it helps (or maybe this just muddies the water!) there is a 320cc motor in China that looks an awful lot like a bored out 233cc copy. Here it is in the AJS Regal Raptor 350:

https://suprememotos.com/uploads/postfotos/ajs-dd-350-e-9b-stellar-regal-raptor-12-reg-full-service-history-4087-miles-11.JPG

Specs say bore x stroke is 62x53mm, which is interesting. I wonder if you may actually be able to buy the parts off the shelf..
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL

[Rogerborg]

They used that frugal detuned engine in the "AJS" SPT "350", with a 27 litre tank that tested close to a 500 mile range.

Brilliant concept, a near perfect commuter bike that destroyed the Inazuma 250 on value for money.

They sold four of them in the UK.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[wr6133]

Look on taobao. There used to be 300 bore kits (complete barrels/pistons and even head if you wanted one) for those motors I considered one for my old Jinlun.

[lingeringstin...]

I've seen a couple of pics of that AJS engine that's supposed to be somewhere over 300cc. I can't really tell if it's a close copy of the 253FMM or not. Maybe it uses a few of the same external bits but the innards are all different. I'm too new to all this Chinese engine stuff to know what's up yet.

I do know that having taken my engine mostly apart now it's not as cheaply made as I was led to believe. It seems rather OK really. It does have a couple of what I think might be trouble spots, such as the fact the gudgeon pin runs directly in the con rod without a bearing or a bush or anything. I don't think I've ever had an engine without some kind of bearing in the con rod but that's a minor niggle if I wanted to modify it to take a bush or something. Who knows, maybe it just works well enough.

I probably shouldn't be worrying about modifying my Chinese engine. It's probably not wise and is probably too much work for little gain, like trying to make an MZ go fast. It can be done but it blows up every time you do it. There's a happy medium between the power and reliability you can get out of something and personally I need the reliability.

So no use messing about with alcohol carbs and funny gasses on my Chinese motor then.