top end rebuild help - RS125

**STEPHASAUR** · 20th February 2017, 07:19

The time has come for a top end rebuild to be done on the little RS

However I have a couple of queries.
Online It seems I can choose between 5 piston sizes, all .01mm difference
the number on the piston is "5027DB 201 99-54" but a quick google search returned nothing so im not quite sure what to do from here :/

will the size matter given the small difference between pistons?
aside from the copious amount of tutorials online, is there a specific way one would recommend to measure the bore?
given this is something i've never done before i would also be willing to take the bike into a workshop and just learn. so if there are any bike groups in auckland

All advice appreciated!

**Drew** · 20th February 2017, 07:25

Measure the current piston. If the current clearance os in spec, that's the piston size you want.

**STEPHASAUR** · 20th February 2017, 09:19

Originally Posted by Drew

Measure the current piston. If the current clearance os in spec, that's the piston size you want.

yes but venier calipers don't have that level of accuracy, right?
so i could be 0.1mm off and order the wrong size piston, which is why i'm stuck for ideas.

**Drew** · 20th February 2017, 11:24

Originally Posted by STEPHASAUR

yes but venier calipers don't have that level of accuracy, right?
so i could be 0.1mm off and order the wrong size piston, which is why i'm stuck for ideas.

Should be using a micrometer. Take it to a machine shop, they'll be able to measure it accurately.

Take the barrel too.

**Akzle** · 20th February 2017, 15:01

jahjah. T bars and Mics.

you could do it with one or the other (the cock, or the sock) with feeler gauges, then get the other thing measured and do maths,
but by the time you're elbow deep anyway...

could be a good time to put a 155 top on...

**tigertim20** · 20th February 2017, 16:13

take the bits into a machine shop, and ask them to measure it for you. If theyre not a bunch of grumpy bastards, theyll do it for ya for nothing, will only take 'em a couple minutes. You need to use a micrometer as above.

You might want to go back to the shop later, as you might need some machining done if you arent putting the same size stuff back in.

**OddDuck** · 20th February 2017, 21:31

Bits and pieces...

The Mitutoyo verniers I have in the garage are rated at 0.02mm, well inside what you need to do. Most manual verniers should be able to easily beat 0.1mm.

Smaller vernier calipers may have jaws too short to reach around the piston properly. Although a 125 piston is probably small enough that a standard 150mm vernier should be OK.

If using digital calipers,wipe the jaws first, zero, take your measurement, re-check your zero afterwards.

There will be a correct way and place to measure the piston diameter - see if you can find a workshop manual with a picture of this.

There is a tool known as a telescopic gauge, you put this into the bore, lock the gauge, then remove it and measure the gauge with an external vernier or micrometer. Here's a set: http://tradetools.co.nz/products/4830320
They require patience and skill to use but are far cheaper than dedicated bore gauges. Errors in angle / centering will affect the measuement.

If buying measuring gear, Insize is good, Mitutoyo is very good. You won't find either of these at the local hardware shop, go looking at dedicated engineering shops. The cheaper brands like Kingchrome... Nope. Their manual stuff is OK but the battery powered digitals suffer track errors and eat batteries.

The gear's expensive. I spent around a hundy on the 150mm manual Mitutoyo calipers... it seemed horribly expensive at the time, they're now one of my most important workshop tools and I couldn't imagine doing engine work without them.

Finally... the advice about skipping all this and finding a friendly engineering shop or engine reconditioner's is spot on, turn up with piston and barrel neatly wrapped and cleaned (this will go a long way) and they should sort you out in less than ten minutes.

**jellywrestler** · 20th February 2017, 21:35

take the bits to hampton downs nationals in two weekends time and ask the countrys leading 125cc experts what to do, it's likely someone there will even have a piston available too, but they won't sell it till the end of the weekend, of course, if you're trying to get it running to race before then then that's not a good move.
or Taupo this weekend but it's the well the other side of the bombays....

**Drew** · 21st February 2017, 05:38

Originally Posted by jellywrestler

take the bits to hampton downs nationals in two weekends time and ask the countrys leading 125cc experts what to do, it's likely someone there will even have a piston available too, but they won't sell it till the end of the weekend, of course, if you're trying to get it running to race before then then that's not a good move.
or Taupo this weekend but it's the well the other side of the bombays....

This thread is about an Aprilia RS125 road bike I assumed.

**Akzle** · 21st February 2017, 05:39

Originally Posted by OddDuck

Bits and pieces...

The Mitutoyo verniers I have in the garage are rated at 0.02mm, well inside what you need to do. Most manual verniers should be able to easily beat 0.1mm.

Smaller vernier calipers may have jaws too short to reach around the piston properly. Although a 125 piston is probably small enough that a standard 150mm vernier should be OK.

If using digital calipers,wipe the jaws first, zero, take your measurement, re-check your zero afterwards.

There will be a correct way and place to measure the piston diameter - see if you can find a workshop manual with a picture of this.

There is a tool known as a telescopic gauge, you put this into the bore, lock the gauge, then remove it and measure the gauge with an external vernier or micrometer. Here's a set: http://tradetools.co.nz/products/4830320
They require patience and skill to use but are far cheaper than dedicated bore gauges. Errors in angle / centering will affect the measuement.

If buying measuring gear, Insize is good, Mitutoyo is very good. You won't find either of these at the local hardware shop, go looking at dedicated engineering shops. The cheaper brands like Kingchrome... Nope. Their manual stuff is OK but the battery powered digitals suffer track errors and eat batteries.

The gear's expensive. I spent around a hundy on the 150mm manual Mitutoyo calipers... it seemed horribly expensive at the time, they're now one of my most important workshop tools and I couldn't imagine doing engine work without them.

Finally... the advice about skipping all this and finding a friendly engineering shop or engine reconditioner's is spot on, turn up with piston and barrel neatly wrapped and cleaned (this will go a long way) and they should sort you out in less than ten minutes.

unless you pay ShitLoads™ for digitals, scrap the idea all together. my mitutoyo (300) have .02 graduations, plus inch scale, plus fine adjust (rarely used as have mics for accuracy) so, can realistically measure down +/-.005 mm by eyeball. or inch conversion.

as with all tools, you need to get a feel for them, and n00b measuring round thing... no.