SYM Owners Forum

General => General Discussion => Topic started by: Thoppa on March 25, 2021, 06:49:25 PM

Title: Changing cylinders - anyone done it?
Post by: Thoppa on March 25, 2021, 06:49:25 PM
Hi everyone,

Has anyone tried swapping cylinders/pistons from the different SYM engine sizes? How about using different piston rings? I might have to 'cos of a parts availability issue so if anyone has looked into this, or done it themselves, please let me know. As far as I can see, swapping the 249cc and 263cc parts is straightforward, albeit time-consuming, but the 278cc might need major modifications.

So why do I ask? My 12 year old Citycom is just shy of 50,000 miles and is using oil (300ml extra needed between changes), has low compression, 10kg/cm2, and makes a knock noise when the engine is cold. It all points to worn piston rings/cylinder.

At the moment, it's looking like I'm going to have to fit a new piston, cylinder, etc from the 249cc engine, not the current 263cc, because that's all that's available at reasonable prices. It's no more expensive to convert than it is to repair, and the conversion would include a new cylinder too, so it might last longer/seal better than a repair. I don't think the 278cc cylinder will fit the engine block, although I'm in the process of checking for sure.

My problem is that I cannot get new piston rings locally - part no 13010-LEA-000 (the numbers denote a piston ring set, the letters denote the model) - the dealer doesn't stock them and won't order them. I can only get a complete cylinder and top end build and the dealer wants to charge me big bucks for this. Ebay is a similar story with 500% overpriced parts; however, I can buy a piston, cylinder, rings and gaskets for both the 249cc and 278cc engines, and for US$150 for either set - by far the best value. I've also reached out to family in Taiwan to see if they can get the 263cc parts but since these models are export, it's unlikely.

All the 249/263/278 engines have the same stroke but 2mm bigger bores - 71/73/75. The Citycom is 263cc - 73mm bore x 63mm stroke, and the cylinder/engine block gasket is the exact same part as the 249cc engine - both HMA. This means the smaller cylinder and its piston and rings should be fine. The piston will probably be lighter, which will reduce the load on the rod and big end. The issue will be whether the head from the 263 works efficiently with the 249 piston. I'm guessing they used the same design and sleeved the cylinder an extra 2mm for the smaller piston, with only a slightly different head shape. For example, the inlet pipe assembly, the camshaft, the valves... all the exact same parts for both. I've not been able to check exhaust, except for the gasket, which is the same. However, the head gasket is different - LEA for the 263cc and HMA for the 249cc. I will need to disassemble and measure to see which gasket would work best for the smaller diameter piston, assuming everything else mates up, as I think it will. Probably the bigger one, the LEA, is better to create a seal.

The 278cc is 75mm bore, same stroke - it has a different engine block/cylinder gasket part code (RFA not HMA) so the 278 cylinder might not fit the 263/249 engine block. The 278cc engines appear to be all RFA code parts for all gaskets and either L3A (GTS300i) or L9L (Z300) for piston rings/cylinder - 2 codes 'cos the latest 278cc cylinder is ceramic not steel lined.

Parts numbers are here:

The other option is to look into rings from a machine with a 73mm bore (Honda GX240 for example).

Here's some pics of the 2 troublesome parts that are specific to the 263cc engine - the rings and the head gasket. The other parts I need are the same (HMA) as the 249cc engine.



I can get this gasket for US$35 locally - the price in Taiwan is around US$10. The piston rings are on Ebay and they are about 5x the Taiwan price.



Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on March 30, 2021, 04:33:57 AM
I've also reached out to family in Taiwan to see if they can get the 263cc parts but since these models are export, it's unlikely.

My sister-in-law is a bloody legend - she has no clue about scooters but still she called around and eventually found a SYM dealer in Taipei who had them in stock. They became concerned that she was planning to do the repairs herself but she reassured them it was for an "experienced" mechanic. Actually, I've only ever once changed piston rings and it was on a real easy two stroke about 30 years ago! Anyway, she bought them today and is sending them on immediately - I doubt I'll get them in time to do the work over the Easter holiday but it isn't an emergency repair, yet. So I think I'll be fine to delay the repair until June when I have the time.

And the cost? Camcover gasket, US$5, head gasket and piston rings, US$20, cylinder/block gasket US$3. I'm not sure what shipping will be I'm sure the total cost is less than the cost of the piston rings alone on Ebay.

You might think the price doesn't matter that much but I'm planning to buy the Z+ 300 so this repair is just for "fun" - to see what state the cylinder/rings are in after so many years, and who knows, maybe get her near to full power again? Wish me luck!
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on April 01, 2021, 11:19:56 PM
The parts arrived already!

I bought the gasket for the 278 as well as the 263 to compare and they are 100% identical except for the bore on the 278 is 1mm wider all the way round - so 2mm in diameter more. If I ever need to, it's certainly possible to fit the 263 head onto either the 249 or 278 cylinder/piston. For the 249, it would all bolt on and there'd be a little more squish. The 278 would require some machine shop work to fit but it's doable.
278 vs 263 head gaskets
All parts needed for piston ring replacement
Piston ring set for 263
Title: Re: Changing cylinders - anyone done it?
Post by: Tromper on April 02, 2021, 01:00:59 AM
Thanks for posting all of this.
It's interesting 'nuff and cool that you're doing it.
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on April 02, 2021, 04:22:45 AM
Thanks - a bit of a mechanical adventure :-)

I got the body work and some bits off in the late afternoon. Under the skin, she was very dirty - huge amounts of dirt on the frame etc - I brushed some off before this photo. I am still wondering about the best way to get the engine out of the frame without damaging anything.

Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on April 02, 2021, 09:12:59 PM
I followed the manual and the only part that caught me out was draining coolant. I drained it from the drain plug but then when other hoses got disconnected, more came out from the drain plug and elsewhere every time a hose was removed. Even knowing this beforehand wouldn't have made sure I collected all the coolant because it comes out of everywhere else, not just the drain plug. Apart from that, quite easy. I decided to use a couple of scissor jacks and a trolley jack to support the engine  and then lift the frame and front end up and forward to take it away from the engine unit. I have fitted the new rings by following the manual and copying what was installed, and how, and that was straightforward. So now I'm at the stage of scraping off old gasket before the slow process of cleaning and rebuilding.
Draining the coolant using an old orange juice bottle
Piston cleaned up with a knife.
new rings - the piston measurements are all within spec so it just needed a de-coke. It shows that oil has passed the oil ring as expected, but perhaps more than expected because it has burnt on as a crust, and the same is less true of the second ring. The cylinder head also has a lot of this kind of crust. I'm surprised I didn't notice any white smoke out of the exhaust. It made me wonder if something else besides rings might be the issue, or if the problem had not go that bad yet.
The residual gasket between the cyilnder and engine block is a pain to remove - soaked in solvent and scraped.
The cylinder bore looks very smooth and polished - there's some honing lines criss-crossing the bore and no signs of damage or unusual wear of any kind. I couldn't measure distortion but the bore measures within spec so it should be fine, assuming my vernier calipers and measurement method are up to the job - the tolerances are very small.
And the gasket has all gone after an hour of work on it...
De-coked the head

I took a break for a long lunch and then started again in the afternoon. I got the head etc reassembled and the engine back in the frame by the end of the day. Tomorrow I try to reconnect everything...
Title: Can prayer overcome mechanical problems?
Post by: Thoppa on April 03, 2021, 07:35:44 PM
Happy Easter everyone! I have a miracle to report.

I reconnected everything, refilled the coolant, etc etc. and started her up. But no, she turned over but wouldn't start. So then I start praying extra hard.... and she started on some throttle with a puff of smoke, but she wouldn't idle and was shaking and vibrating a lot. A scary amount to be honest. I couldn't close the throttle or she would die. I ran her for a little bit with the throttle open enough to keep her running, and looked to see if anything else was wrong, but didn't see any issues. So I figured I messed up and that was that.

I started to think about whether it was worth it to begin the repair again, this time with a new cylinder, etc. I disconnected the coolant bleed hose to the thermostat to bleed air bubbles from the coolant, although I don't know why. I guessed she was a dead un.

Figuring I had nothing to lose, while praying for a miracle, I started her up again and tried to set the throttle adjusting screw so she would run on a small throttle without me needing to hold it open. I was adjusting this for a while and finally ended up turning it closed... and she was idling and not too roughly.

What happened? I have no idea. So I'm claiming an Easter miracle - thank God my scooter still works!
Title: Re: Changing cylinders - anyone done it?
Post by: Tromper on April 04, 2021, 01:55:32 AM
Smile, take full credit, & party 'til dawn to celebrate.

Congrats, & well done.
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on April 04, 2021, 08:42:30 PM
Well, it seems new rings have not made a difference.

I tried running her in for a day - very light loads, no static rpm for long, and no high rpm, and then today I did some more and a compression test. It is actually a bit lower than before. Although the cooling system is keeping temperatures in check, the engine is kicking out lots of heat, and I think the oil must be close to burning. One thing I dislike about these SYM is the lack of oil pressure warning/gauge etc.

Anyway, for sure it is not repaired and still has the feeling it could clunk out mid journey. I noticed in my pics a small longitudinal mark on the cylinder wall - it is visible in the top left of cylinder bore in the the pic above. It is more of a scuff - my fingers didn't feel any scratches. But whatever, I had previously thought the cylinder was without any such issue, but now it seems it is not so it is damaged to some degree. I'm pretty sure my attempt to use new rings was not enough and I will need to go with a new piston, rings, and cylinder repair if I try again. Now I've done it once, I know how to strip and rebuild so it should be somewhat more straightforward next time. I may even catch all the coolant next time :-)

I measured the outside barrel of the cylinder and it was 78.89mm and the hole in the engine block was 79.36mm. So I'll see if I can find someone selling a 278cc cylinder to measure it and work out if it will fit.

I have no time left for this now so the Citycom will be off the road until the summer. If the Z+ 300 becomes available soon, I'll buy that and perhaps not bother repairing the CC.

EDIT - I continued to try to break in the new rings but the heat turned out to be too much. It has eventually caused the camshaft to fail and the end of the shaft has sheared off. As some mechanic websites tell me, too much heat can cause the head to swell in the middle, and this shape change is enough to cause the shaft to bind so it doesn't want to turn, and then it breaks. So the engine now needs a new camshaft and bearings on top of the other parts, and the head would need to be stripped and rebuilt.

On the plus side, while stripping the engine a little to check out what had gone wrong (still in the frame), I did manage to catch all the coolant. The trick is simple - first drain the coolant from the drain screw and the remove the highest hose that connects to the thermostat - a second surge of coolant from the drain plug gives up all the coolant in the system.

Anyway, I might not bother with the repairs - too much time and money and I'm not confident it would work either. So, my Citycom is probably finished, just shy of 80,000kms. I reckon I could have got to 85,000kms and by then she'd have been such a heavy smoker that there would have been a "natural" end. Rest In Pieces Citycom. :-)

ANOTHER EDIT: I've been thinking about what the manual doesn't say about refitting the parts - it doesn't mention careful alignment of the crank position. So although I thought I had aligned the timing, I now wonder whether I aligned the camshaft with the crankshaft exactly right? I didn't check the timing mark on the crank, trusting instead that a piston that looks like it is at top dead centre IS at top dead centre.

But if the piston was not, and preferably at the compression not overlap tdc, I might have changed the timing by a few degrees when I fitted the camshaft gear drive sprocket onto the end of the camshaft. It's certainly possible this is what I did wrong. I know for sure I did everything else right - it was all correct on disassembly. And this timing error would explain why the engine would run but was running too hot.

So maybe it is worth trying a second time, and this time making sure everything is exactly right and not making assumptions. A new camshaft is available and US$45. I could try next month.... :-)

Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on May 02, 2021, 02:04:35 AM
Smile, take full credit, & party 'til dawn to celebrate.

Congrats, & well done.

Hey Tromper, so now I'm celebrating - seems to have worked second time, with a new piston pin and new camshaft (from the 250) and lots of prayer and cursing and lost skin.

The difference this time was using the crankshaft timing mark to get as close to TDC as possible so I was sure the timing was as close to correct as could get it. I hope dead on. I also disassembled the camshaft chain tensioner by removing the spring before removing the tensioner from the cylinder. Then I could reset the ratchet on the plunger so when it was reinstalled, the plunger wasn't applying any tension to the chain. Then after that, fitting the sping and hearing the ratchet click through to apply the tension.

And so far so good - she idles sweetly from the get go, actually smoother than I can even remember. I ran her to get her hot enough for the fan to turn on ... and no issues.  I'll leave her overnight and see if there any leaks in the morning :-)
Title: Re: Changing cylinders - anyone done it?
Post by: Syd on May 02, 2021, 08:48:13 PM
Well done, Thoppa. "Smile, take full credit, & party 'til dawn to celebrate."
Title: Re: Changing cylinders - anyone done it?
Post by: Tromper on May 02, 2021, 10:01:20 PM
Excellent.  You were probably below the critical loss of skin & cursing levels the first time.
Now smile and enjoy the ride.
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on May 03, 2021, 01:11:37 AM
Thanks Syd and Tromper - I am feeling super good - the adventure seems to have a successful end... and no sign of oil or other leaks so far either! Cursing and skin loss are key!

The skin loss was from pulling the cam chain to rotate the crank so I could get TDC - it's actually a lot of effort in places and then suddenly turns very easily and spins past TDC. Quite troublesome! There was also a lot of cursing while trying to get the piston into the cylinder without damaging the rings. Both of those jobs took a lot of time to get right.

Second time round I also realised there's no need to remove the bodywork - just loosen screws so panels can move and that gives enough room to access electrical cables and connectors. I also tied the handlebars into a centre position so I could lift the body away like a wheelbarrow and the front wheel wouldn't wobble. Lifting the "wheelbarrow" up and over the back wheel seemed like a lot of physical effort but manageable even though I'm out-of-shape and the wrong end of middle aged.

Another tricky bit was realigning the frame with the engine and then getting the long mounting bolt to pass through - it still had to be lightly hammered through even when aligned.

Anyway, now I'm watching for leaks and if none, then I'll run her in for a couple weeks ... I'm looking forward to giving her some beans and see if that 250 camshaft has made an obvious difference. It has the same size and same lift as the 263 but I guess more duration for top end power and less bottom end.

Beer time I think. :-)
Title: Re: Changing cylinders - anyone done it?
Post by: Syd on May 03, 2021, 01:21:13 PM
If you take the plug out, the engine turns much easier.
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on May 04, 2021, 03:54:29 AM
And it sounds awesome with no exhaust!

EDIT - I though you meant that as a joke but I realised I might not have explained what I was doing clearly - a lack of pics cos this site does not make pics easy. So, I was pulling on the cam chain to rotate the crank and piston to tdc with the head off. It's not possible to pull on the chain with the head fitted - insufficient access. When the crank marking is as close to aligned in its inspection window as possible, and that is not as easy as it sounds, I fitted the head down the mounting bolts while pulling the chain and guides through the slot in the side of the head. With the the head in place and bolted down, the cam chain can be worked around the cam chain sprocket (a little tricky due to the limited access) and then the sprocket is bolted onto the end of the camshaft - so the camshaft, the sprocket, and the crank are all at their tdc markings. When you torque the bolts that hold the sprocket to the end of the camshaft, the sprocket will turn and then so does everything else. That's when it makes sense to not have the spark plug fitted. The last thing is to add the camchain tensioner, and then the head cover, then the engine goes back in the frame and then reconnect cooling pipes, rear brake, throttle body, fuel, airbox, electricals, exhaust, rear suspension, etc.

It's mostly following the servive manual but, as I worked out from the first failure, also spending time thinking about what the manual isn't saying so stuff like the crank marking and camchain tensioner don't create problems.

Anyway, still no leaks and she's running well. I read up on breaking in piston rings and the "no idle and varying throttle and engine speed" doesn't need that long apparently. So I'm going to take her for a blast tomorrow :-)
Title: SYM, my Speedy Young Motor
Post by: Thoppa on May 05, 2021, 10:34:14 PM
How fast does she go mister? was a question I sometimes got asked on my Yamaha YZF many years ago, but it is not a question I expect to get asked on the SYM. But, just in case someone does ask, I need to be prepared right? :-)

So, I took out for a spin, got her warned up and then gave her WOT from a standing start onto {edited} for 2 miles, with no wind. So, I can say 130kmh on the GPS, that's 140kmh indicated, with maybe a little more with a new belt etc. She still has the K&N air filter but is running the standard exhaust - when she was fully tuned with a straight through exhaust she did 148kmh indicated, 135 on GPS. The calculation anomaly for the maths is likely tire diameter.

I also noticed that the "knock" noise the engine used to make when cold has gone, and the engine feels smooth or at least smoother, at all engine speeds too. I can't tell if the 250 camshaft has made any difference.

The next stage is to run her for month and see if she has stopped using oil, measure the compression pressure, and calculate fuel economy. 
Title: Re: Changing cylinders - anyone done it?
Post by: Thoppa on June 04, 2021, 08:12:10 PM
One month later... and still no leaks or problems of any kind. :-)

The ring replacement has restored engine power so the top speed is very similar to what it used to be as a low mileage machine. The engine is noticeably smoother and quieter. So far, she is not using much, if any, oil so I guess no top up will be needed between changes. The fuel economy is almost back to normal too - she was as low as 28km/l before the new rings, and now is a little over 30km/l so far, instead of the long-term average of 32km/l. I've been giving her more WOT than I used to - perhaps the camshaft has affected the bottom end power so I'm more inclined to give her lots of gas. Not sure.

Anyway, the ring replacement is a success - it's done a good job of renovating the worn engine. I wonder for how long...