rs125 unknown Knocking issues

Hey all,

So not posted much on here but with the sun being out I finally got round to having another go a fixing my rs125, after about 4 years of storage.

Now briefly, I had the crank rebuilt by PJME some 4-5 years ago (bare in mind bike has done ZERO miles since this) as the conrod had movement in it. I also replaced the main bearings and ALL the crank case bearings and seals. Put the it all back together and DAMN, its knocking. It was a consistant knock that happened at the same point on every revolution of the crank.

After a bit of fumbling around and no success, I got fed up with it and busy with work so it ended up being dry stored, until yesterday...

So where I am at now is, I took all the engine apart as I thought id'e missed a washer off the gearbox which might cause one of the cogs to foul against something, but fortunately I cannot see any evidence of anything fouling on the cases or gears. So I assume this wasn't the case. All the bearings feel nice and smooth still too.

The only sign of damage I can see, is on the piston and barrel. What looks like typical scoring/scrapes. Here's some pictures of said piston and barrel.

What do you think, could this be the cause of the knocking? Bear in mind the piston and rings are relatively new with probably less the 1000 miles on them.

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_55_05_Pro.jpg

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_55_29_Pro_1.jpg

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_53_41_Pro_1.jpg

Hey all,

So not posted much on here but with the sun being out I finally got round to having another go a fixing my rs125, after about 4 years of storage.

Now briefly, I had the crank rebuilt by PJME some 4-5 years ago (bare in mind bike has done ZERO miles since this) as the conrod had movement in it. I also replaced the main bearings and ALL the crank case bearings and seals. Put the it all back together and DAMN, its knocking. It was a consistant knock that happened at the same point on every revolution of the crank.

After a bit of fumbling around and no success, I got fed up with it and busy with work so it ended up being dry stored, until yesterday...

So where I am at now is, I took all the engine apart as I thought id'e missed a washer off the gearbox which might cause one of the cogs to foul against something, but fortunately I cannot see any evidence of anything fouling on the cases or gears. So I assume this wasn't the case. All the bearings feel nice and smooth still too.

The only sign of damage I can see, is on the piston and barrel. What looks like typical scoring/scrapes. Here's some pictures of said piston and barrel.

What do you think, could this be the cause of the knocking? Bear in mind the piston and rings are relatively new with probably less the 1000 miles on them.

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_55_05_Pro.jpg

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_55_29_Pro_1.jpg

http://i174.photobucket.com/albums/w91/tomo9992/piston%20and%20barrel/WP_20180325_22_53_41_Pro_1.jpg

I use to race 1/5 scale models with 2 stroke engines and only time I have seen that sort of scoring was down to lack of lube or some sort of particals, how deep are the scoring?

Sent from my SM-G950F using Tapatalk

Hi Rider,

yes the scoring is on both sides of the piston, worse on the exhaust port side though. I cannot say what position the piston would be in as it only knocks with the engine is running, hand cranking the engine doesn't reproduce the knock, so I'm guessing somewhere just before TDC?

Would that kind of scoring be related to the knocking? The scores on the barrel cannot be felt, however you can feel the scores on the piston but only very very slightly, though by looking at the piston, the scores do look deeper than you can feel, if that makes sense.

I am looking at a top end rebuild because of the scoring, or can the barrel and/or piston be reused?

Thanks for you reply.

A scored piston wouldn't produce a knocking noise. Knocking noises are made by big and little end bearing wear or the piston crown hitting the head. Could be pinking knock caused by too much compression ratio and over advanced timing. Piston noise could be piston slap caused by piston being too loose in the bore.

A scored piston wouldn't produce a knocking noise. Knocking noises are made by big and little end bearing wear or the piston crown hitting the head. Could be pinking knock caused by too much compression ratio and over advanced timing. Piston noise could be piston slap caused by piston being too loose in the bore.

Okay thank you,

The big and small end bearing are basically brand new. The head doesn't look damaged in any way, does this rule out the piston hitting the head?

If pinking knock, i'll need to run a compression test, correct? and over advanced timing, is that the result of incorrect setting of the plastic gears/timing?

How loose would the piston have to be in the bore to cause slapping?

I wonder if the rebore was done to accurately match the new piston . Was it done by an experienced engineer in a reputable engine shop or was it done by someone who happened to have a boring machine ?

Sounds like problem may have been caused by recent engine work.

Just because parts are new doesn't mean they are correct or good quality.

You will need a workshop manual to confirm ignition timing correct.

I'd check the piston and bore diameters in several places and check manual for tolerances.

I wonder if the rebore was done to accurately match the new piston . Was it done by an experienced engineer in a reputable engine shop or was it done by someone who happened to have a boring machine ?

No rebore has been done, AFAIK it is a standard barrel with a new-ish piston.

Was think of just either replacing the barrel, along with piston etc, as I assume that the barrel on is still the one from factory. Or upgrading to the 140cc kit.

You need to find the reason for knocking first.

whats the best tool to measure the barrel and piston?

I've heard the haynes manual for the timing is incorrect, is this true? If so is there anything online this is 100% correct?

Micrometer and inside telescopic gauge are best, but a digital caliper and inside calipers are cheaper.

Timing usually involves turning crank until piston at top dead centre and looking at the rotating element mark under the ignition or generator cover. Haynes should be correct. Google it.