Head/Piston clearance RM125M

[Graeme M]

Rebuilding the RM125M engine with a fresh piston and need to know minimum clearance between piston crown and head. I am using a CR125 piston and it looks to have about a 1mm higher crown on it. I've done some measurement using plasticene and it looks like at the closest I have just over 2mm right out at the edge of the crown.

Anyone got some advice on that?

[Freakshow]

Id say If you can stick some soldering iron down the spark hole and get a squish between the head and the piston you got enough clearance. Id say with 2mm youll be fine.

[Lozza]

Plasticine is a hit and miss way of measuring squish clearence. As Freaky said some solder and verniers is a more accurate way. Even the there are 2 schools, the cut 2 lengths, place a cross formation on the piston,tension head,turn over TDC, remove head and measure.The other is form the solder into a L shape and poke down plug hole IN LINE WITH THE GUDGEON PIN, turn over TDC and measure.As for the actual correct clearence ..............2mm is WAY to much, and it should diverge slightly.  

[Graeme M]

Thanks for that, I'll try the solder thing tomorrow. You're right of course, plasticene is pretty iffy at best. Never thought of solder...

[cyclegod]

This just got relisted, can't believe it passed in at this price

http://cgi.ebay.com/ebaymotors/Suzuki-RM125-nos-cylinder-set-1975_W0QQcmdZViewItemQQcategoryZ35581QQihZ009QQitemZ190186645507QQrdZ1QQsspagenameZWDVW

[Graeme M]

Yeah, that one was tempting but I just don't have that much available at this time of year. Sad but true. And my slack brother didn't win Lotto last night either!

Might keep an eye on it though...

[Doc]

Graeme, I know of 2 people who run the CR125M pistons in TM125 race bikes and have done so for many years with no ill effects and no modifying of piston crown. (not RM125M I realize but very very similar if not the same) I have a TS125 kit piston here and the crown height on that is 'over' 1 mm higher than any stock RM, TM or TS125 piston so the CR piston shouldn't have an issue 

[Graeme M]

I reckon you're right Chris. I am running a CR piston in the TM and it is fine. Just wasn't too sure about the RM as I thought the head and barrel are different on them. My collection of CR, TM, and TS piston kits all have different skirt lengths and crown heights, but a bit of trimming and any of them seem to work OK...

[Doc]

Yeah Graeme, you're right. The head and barrel are quite different on the M and S but they still run the same bore/stroke/rod etc.. I'm figuring unless the RM already runs super hi comp stock or has an abnormal squish chamber (which they don't so far as I've seen) I can't see there being much diff. I've was always told to inspect piston to head clearance whenever I have anything bored out past 1mm over as the piston can sometimes contact the outer edge of the squish area on hi-comp engines (especially fitted with aftermarket heads) in extreme overbore situations. I've checked and re-checked with every rebore I've had done to any 'stock' motor since but to date none have needed any further mods. Like you though, I'd be double checking this tolerance also for my own peace of mind  let me know if it goes bang! 

[TM BILL]

This just got relisted, can't believe it passed in at this price

http://cgi.ebay.com/ebaymotors/Suzuki-RM125-nos-cylinder-set-1975_W0QQcmdZViewItemQQcategoryZ35581QQihZ009QQitemZ190186645507QQrdZ1QQsspagenameZWDVW

Previously he had it listed at $299.00 buy now , Bit pricey as they ain't that rare. $99.00 start price is way more reasonable 

PS thanks CG for listing all these Egay goodies  i have had a few good scores from your leads , owe you a beer when i see you Bill 

[Hoony]

interesting stuff guys, enlighten me to why the CR piston is the go ?

is it simply they are larger than the TM 4th oversize or are the TM ones Unobtanium now?

[Doc]

it's just another piston option hoony. Honda pistons go up over and beyond TM pistons and in 0.25 increments. TS pistons also work. No advantage to any of them it's just another option when looking for parts.

[Lozza]

Just wondering if the head has a squish band or quiessent(ie no squish band)?Have you cheack the volume?

[Graeme M]

Yep, does have a squish band, and no, haven't checked the volume, but happy enough with how it looks so will reassemble with *some* confidence.

But another question. These CR pistons (and they aren't OEM I don't think) have two rings. One has a slightly tapered cross section, the other is rectangular. Which goes where? I am assuming the tapered one on top and the other on the bottom?

[Lozza]

The keystone ring will fit into both ring glands the flat ring will only fit into one ring gland. You will have no forseeable(?) problems with a 2mm squish gap, not if you count the torque your missing out on. 

[Graeme M]

It's an old school 125, who needs torque?

[Freakshow]

What model CR pistons fit ? and does this carry over to being able to be used in a TM250 ? i got a barrel here on 1mm that needs loving.

[Graeme M]

I don't think it works for the 250, it just so happens that the CR and TM125s share the same bore/stroke so the pistons are interchangeable with a little careful trimming. I am no expert on this, but I think the 74-77 CR pistons fit the 73-75 TM/RM125.

My collection of pistons include TM, TS and CR, all of which differ slightly in length of skirt, height of crown, and cutaway. I am sure there are slight differences in power as a result, but given that these things don't make much power anyway I don't think it matters much...