Hey Teflon-Mike- What do you think?

[lingeringstin...]

Oh it gets even more complicated already. I just can't stop my mind from trying to modify things unnecessarily.

Just this morning I was randomly looking for some new gudgeon pins and clips and noticed that in the several years since I last spannered on a personal four stroke a lot has happened. I found loads of pitbike pistons in all manner of sizes. This automatically got my mind working.

Secondhand barrels for these China clone engines are cheap enough so I'm just going to HAVE to get one to experiment with. Tell me if these ideas are bollocks or what:

Firstly, I know a bloke with a machine shop and he's acceptable to working on my experiments as long as I pay him and his prices are good.

Now, to my mind a four stroke liner on a small engine like this does bugger all but give the pistons something to slide up and down in. I don't see why the liner really has to be stepped or complicated in the casting, I figure a decent heating job and interference fit should make a straight sided liner stay put in the barrel ally.

The first question I have is how big of a hole can I make to fit the biggest diameter liner?

There are plenty of pitbike pistons in all manner of sizes. Some use 13mm gudgeon pins so a nice bronze bush could be used in the con rod (I actually like that idea anyway). Many others use a 15mm gudgeon pin so that just fits. Lots of sizes out there. For instance one of the pistons I temporarily looked at this morning says:




This piston will fit any machine that uses a CG175cc Honda replica engine.

Dimensions
Upper piston diameter: 61.4 mm (2.42 inches)
Lower piston diameter: 62 mm (2.4 inches)
Centre of pin hole to crown: 24 mm (0.92 inches)
Piston height: 51.3 mm (2 inches)
Crown height: 49.8 mm (1.96 inches)
Gudgeon pin hole: 15 mm (0.57 inches)
Pin length: 50.25 mm (2 inches)
Pin diameter: 15 mm (0.57 inches)
Ring thickness: 1.5 mm (0.061 inches)
Exhaust valve indent length: 22.1 mm (0.87 inches)




So... a quick measure of a 253FMM piston shows that the above piston is only about 3mm taller between the gudgeon pin centre and the top. I'm pretty sure that's within limits for experimenting. I've been shaving bits off pistons for decades and it ALMOST never causes any trouble. The above piston (if the photo was correct) showed a basically flat topped piston that could easily be shaved around the top edge to create a domed top piston that would fit up into the 253FMM head.

I think given the amount of material in the top of the piston the valve indents can be ground out a bit more if need be without structurally compromising the piston crown to any great extent. And there you go, high compression pistons for not really a lot of faffing about.

As far as I know the bottom of the pistons can be shortened if need be without any complications and there MAY even be scope for adding a very small bit of spacer (make a really thick head gasket) between the head and the barrel if that would help. The cam chain would probably allow it. Might get as much as a mil if that were needed, but I doubt it would be necessary. Could probably also use different thicknesses of base gasket to help space things.

The bottom end looks pretty robust to me so I think it would take it. The clutch looks like there's room for improvement with better or more springs but it'll probably be OK as is. The gearbox seems to look pretty well made too.

Basically I think there's plenty of scope for getting more poop to the pop out of this engine design. The Honda design wasn't very stressed out to begin with and the availability of cheap nasty Chinese parts appeals to me. It might be like my MZ engines and blow up all over the place if you push it but if I could get another 5hp or so out of it without destroying it that would be fun.

[Teflon-Mike]

I don't have a motor open on the bench to measure the distance between the studs; that would be the limiting factor to how big a liner you might squeeze between them.

Bottom of the liner is only slightly stepped o the scrap cylinder I can lay my hands on; maybe 0.5mm or so, just enough to be 'matched' to the crank-case recesses.

Shriniking a bigger liner in, shouldn't be too big a problem; but getting those liner skirts into the crank-case likely will. How much you need step the liner or open up the crank-case aperture, is down to preference; but pay heed to the oil-ways, and how much land you have there to ream out.

Pistons? 24mm gudgeon to crown, and you have aprox 6mm deck-height? Err.... two thoughts on that one....

First; how much metal is there in the crown of Chinky CG copy pistons to reshape? All well and good having the height to re-profile or pocket for valves... but if pocketing breaks through the underside of the crown, you wont have much compression!

I would be more inclined to build up the crown of a dished stock-piston for more C/R with lumiweld! Then see how much I culd grund out the skirt with a Dremel.

Second: Raising the barels on a spacer/using thicker head gasket... and 'should still be able to get the cam in'.... Lol!

a) I wouldn't put bets on that one, I'm afraid! Effn pita to thread the cam uder the chain, and if you CAN on a stock height barel, its usually a good sign that the cam-chain is past it! They are usually pretty darn tight on a new chain... though that is on the original roller-chains. Does yours use the TD-J derived Hi-Vo?

b) IF you got the cam 'in' and as said, its a squeeze on a stock height barrel; extra height in the barrel would beg extra length in the cam-chain; it may be there in the 'slack' but that's on the back of the chain run; with however many extra mm on the front, that will try and advance the cam-timing when you come to clock it in; or would likely leave it retarded if you slipped it a tooth to compensate.... Begging slotting the cam-chain sprocket mount to offer some timing adjustment, and posibly some fort of pegging & pinning to make sure it stays timed.

5bhp.... it's at best a 20bhp motor.... increasing capacity to what did I work it out to for a 61mm bore? 330cc? That added capacity wont actually give you much extra power unless you can fill it with charge; and limiting factor there is the valve sizes and cam timing. Extra C/R is good, but in the soft cam, I suspect you may see some low-down gains, but up top, they are likely becoming negligible, as the motor is just running out of breath...

As the weak-link; even on a 125 motor; upping the C/R and even more greatly over-boring, putting an bigger pad-lock on the chain, and leaving the baler twine where it doesn't quite go round the gate-posts!

But.. it be your baby...
____________________
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?'

[P.]

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[lingeringstin...]

Yep, the cam chain is a real bugger to get out and there's very little slack to the chain, so that's all gotta stay normal.

Bigger liners would be easy though. It does require a bit of contemplation to decide whether the bigger pistons would give much more power in the end. I rather suspect that yes you COULD increase the cc's but on so small a motor to begin with even a 20% increase in power wouldn't be much to notice so leaving things as normal just means there's ample room for future rebores.

I've no idea whether upping the compression would do much to each little piston. On the MZ single piston it was a fine art. There was certainly some gain to be experienced in power up to a point and then if you went over that point it just made things worse so getting it JUST right was a bit of an art. But that was on one big two stroke piston going bang on every revolution of the crank and not two little four stroke ones. I suspect you probably wouldn't get a significant improvement trying it on the twin.

Being as both pistons go up and down together you could rejigger a cam to fire both pistons at once so it acted like a single but again I doubt there's be much improvement on engine power so it's too much work for what it's worth.

The valves and intake on the 254FMM engine are quite tiddly and it looks like way too much work to change any of that for probably very little gain.

I do think I could get more low end grunt out of this engine by adding cc's but I'd bet there's no easy way to add power at the high end due to the way it breathes. No idea how much it's supposed to rev to but these days I gravitate more toward grunt than high revs so some low end poop and higher gearing is probably what I'll go for. Looks to me like the easiest way to gain a little grunt is just bigger pistons and some carb trickery.

On the Honda Rebel forums people claim up to 90mph on those engines but I rather doubt it. I figure I can realistically get somewhere over 75 with a bit of luck and that's probably all I can hope for really.



Just thought I'd mention that YX140 pistons look very promising...

[Teflon-Mike]

I dropped a pretty stock 200 benly engine into a 125 Super-Dream chassis... I can report it did have a lot more low end grunt. The 125 motor made quoted 13 Learner-Legal bhp; Benly 200 on single carb aprox 15bhp. Gearing on the 200 motor was a tad short, as it has 4-speed box and should have had a smaller whee; sprocket to compensate for 17" wheel, where the D-D has 18's.

I have GPS clocked a couple of S-D's with stock motor'sin the mid 70's & I actually have a snail-trail from a 2-up trip with Snowie on the back, that shows 69.9 for about six miles, 'upright', with a 14T drive sprocket instead of 15, which probably helped a bit.

The 200 motor was no road-burner, and on slightly short gearing topped out about the same peed, 70 'ish' maybe a tad more.. it might have pulled a genuine 80 If I'd had more cogs to mess with to try and optimise gearing, but I only had stock 15T and the smaller 14T front.

So 90 isn't far off what I'd expect the Two-Fifty to pull in more favorable conditions, with a claimed extra 4bhp, and I'd certainly take an indicated 90 as completely reasonable! (S-D speedo is WAY off the scale at GPS 70!)

But, lets talk torque; swapping out the 125 for the 200; the difference i charecter was palpable, and the difference WAY out of proportion to the 2bhp or so difference they might have had in peak power.

The 125, made peak power at a tad under 12K, and had to be revved hard, and a little all or nothing on the throttle. You'd pin it through the cogs to get some speed up; then come a corner, ince at scrubbing any of it off; rolling off, even the slightest you would be effectively coasting through the bend, and had virtually no 'drive' to pull you round, and were simply hanging on waiting for it to wind up as you came out, hoping there was enough straight before the next one to built it back up again, or a series of corners would see you gradually grind down to a walking pace! Trying to make good progress down a twisty NSL road, and keep ahead of the golf club carriers was something of a test of nerve, faith in the tyres, and how much sole you had on your boot!

The 200 motor on the other hand.. felt like a slightly soft big single dirt bike motor, that wasn't quite so 'lumpy' on the throttle, but shook just about as much! Low down torque was just night and day compared to the 125... as in it felt like it had some! And I would have been short-shifting had it not been for the rather wide gaps of that 4-speed, as where the 125, would be a little limpid, then start to come alive (a lttle!) at about 6K revs and want to rev out to 12K, the soft-cam 200, would pull hard to about 7K, and then start to cry 'enough' as it ran out of puff, even though it was still pulling and made peak not that much lower i the revs, about 10K ISTR.

On the road, the bike handled completely differently; you'd not have to wind it up anywhere near so much, it was far faster to 60, and rolling off for a bend, there was just so much more 'drive' available on the throttle you could drive' it through on the throttle, and exit pretty much pinned wide open; it actually responded to throttle input and you could actually 'steer' it, rather than just leaning.

Been tooo long since I thrashed a 2T... so I would be guessing at a comparison, but would suspect a stock piston ported MZ to behave more like the 200 Benly than the 125, and thrive on that low-down 'grunt' of a double-banger..... that's certainly how the Cota motor behaves; makes sod all power, about 15bhp at most I suspect... at a lowly 6ooo revs.. but makes torque every-bludy where!

ONE things for sure though, my Cota's motor is a LOT crisper, more responsive and 'feels' ore grunty than the Benly motor by a long stretch... fact the thing wights probably 2/3 or less, and has incredibly short cogs probably amps the effect; BUT, I suspect you will likely find similar and that 'nature' of the four-stroke, the lack of throttle response, and the chuginess compared to a 2T will make it feel a lot less powerful, whatever the numbers may say! As sad, my Cota motor is a ruddy tractor; but the Benly motor doesn't even have the charisma of even that!

It's 'dfferent', and I have to say that I did like the 200 Benly motor's nature; it did feel more relaxed, and cornering on the throttle, I did feel more 'involved'... but it was no two-stroke! Even a humble static timed, piston ported one with a 6K peak power revs!
____________________
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?'