CR125R 2002 Cr125 piston clearance

[406flyer]

My nephew has a 125 that the top end was rebuilt couple years ago.  Cylinder was sent out through the honda dealer so I don't know who plated it but they did.   So it had a new cylinder and honda piston.  He's single tracked the bike probably 15-20 hours.   

We were replacing the crank seals last week and I decided the slip the top end off and measure.  Compression has always seemed week to me but it always started easy.  It has fouled spark plugs every 2-6 miles on every loop I've ever been on with him.  I rejetted it, replaced the little gasket on the Mikuni, JD jetted it, tried different oils, and nothing has fixed it.  That is why I was into the crank seal as it always looked oil fouled and not fuel fouled.  It looked fine.  Eric Gorr recommended a new coil so my brother in law bought a new honda coil.  We'll find out if either of these help.  

Back to my question.  I measured the cylinder bore with a bore guage.  It has no taper or out of round.  Looks perfect.  The piston was nicely polished.  

I could slide a .005" feeler gauge between the cylinder and piston.  My measurement verified .005" clearance also.     Factory manual show the #s

1. Cylinder 2.150-2.1253" with a service limit of 2.1265"     

2.  His cylinder measures 2.127" with a bore gauge.  Not calipers or telescoping gauges.  

3.  His cylinder looks new.  Since it has zero taper that means it was bored to this diameter when it was replated.  Is this close enough?  

4. Piston specs.  2.1233-2.1236" with service limit 2.1214"  His old piston measured 2.122" and the new one measured 2.123" so the new piston is only taking up .001 of the .005" clearance we had.  

We still have .004" clearance when the book shows .0014"-.002" with .003" service limit.  

Is this extra .002" clearance to much?   I don't know if the tolerances are as critical as they make them out to be.  

We did a compression check last night after we put it together and it was 140 wit 5 or 6 kicks until it quite moving.  Only thing I forgot was to open the throttle wide open.  For the old cars I work on this doesn't make any difference.  Might on the bike.  

Just curious what your thoughts are.  

[mlatour]

Re-check the compression with the throttle wide open, it will probably gain 10-15 psi but IMO that's still on the low side of ideal.

You cannot accurately check piston-to-cylinder clearance with a feeler gauge, a telescopic bore gauge and micrometer are needed.

 

Did the piston show signs of blow-by below the ring? (darkened)

If so, don't be too 'easy' on it during the next top end's break-in

link: http://www.mototuneusa.com/break_in_secrets.htm

 

Choice of premix oil and, moreso the float level might be contributors to the frequent fouling.

 

Edited March 5, 2021 by mlatour

[406flyer]

My clearance measurements were exactly what the feeler guage measurement told me.    .005" piston to cylinder clearance with the bore guage/mics or dropping a feeler guage.  

Piston looks great.  Polished look everywhere except the carbon on top which I consider normal.  

I put a rebuild kit into the carb and checked the float height early into this.  The needle no longer shut off the fuel supply.  

I don't have 125 experience except my 83' CR when I was 12 year old.  I do have a lot of carb tuning experience on 250's and 500's.  I dropped pilot jets until it no longer wanted to start and it would still foul a plug.  

I will try the compression again with it wide open.   I don't like being twice the recommended clearance on the piston.  

[SlowDinoDog]

0.005" piston clearance seems like a lot. My Wiseco 250 2T pistons say on the box labels to fit them with a 0.002" clearance.

[406flyer]

2 minutes ago, SlowDinoDog said:

0.005" piston clearance seems like a lot. My Wiseco 250 2T pistons say on the box labels to fit them with a 0.002" clearance.

That is what the factory manual says also.  I believe it was setup this loose from the beginning.  I would have loved to gone back in time and measured it the first time around. 

[SlowDinoDog]

Hopefully, someone with experience with CR125s can add something.

My attitude is small bore 2Ts start eating piston and rings after just a few 10's of hours, even if you do everything right.

Don't get too hung up, you'll be doing it again...we just gotta learn what works to make them last a bit longer.?

[406flyer]

4 minutes ago, SlowDinoDog said:

Hopefully, someone with experience with CR125s can add something.

My attitude is small bore 2Ts start eating piston and rings after just a few 10's of hours, even if you do everything right.

Don't get too hung up, you'll be doing it again...we just gotta learn what works to make them last a bit longer.?

I believe you to be correct but I would think you'd get double the hours if it's actually in specifications?   I'd pedal the pos a long time ago as it's rained on every trip with plug fouling.  We thought it was nephew riding it to easy but my brother in law rode it like he stole it on the last trip and it would eat a plug quicker than the nephew.  

I would've put him on a nice kdx myself but he's Honda blind.  

[SlowDinoDog]

9 minutes ago, 406flyer said:

I believe you to be correct but I would think you'd get double the hours if it's actually in specifications?   I'd pedal the pos a long time ago as it's rained on every trip with plug fouling.  We thought it was nephew riding it to easy but my brother in law rode it like he stole it on the last trip and it would eat a plug quicker than the nephew.  

I would've put him on a nice kdx myself but he's Honda blind.  

The most reliable would be a larger bore 4T.

The cylinder in my '05 CRF450X still looked great with 300-400 hrs ART.

My TE150i cylinder shows wear after 38 hrs.

Edited March 5, 2021 by SlowDinoDog

[KnotAPro]

heck! take all the questions out of the fix it game. plug and play with all new OEM Part(s) via R/M for around $480 plus change. New OEM Cylinder and gaskets, new OEM Piston Kit. Get 5% back in RM Bonus Cash for other Honda parts and accessories. Check the Ring gap and smooth the lead edges of the ring as normal. Then he'll be back to throwing dirt on his riding buddies. 

[406flyer]

We just put a new OEM Honda piston and ring into it.  The cylinder is the question.    I know the .004" isn't going to foul plugs but it isn't right.  Curious if anybody ever had a cylinder replated that wasn't in spec.  

450 is out of the question.   Nephew just turned 16 and his dad has a 05' cr450f in a box and is soured on the idea of a 4t.  I watched my nephew on his dads 00'cr250 last year and it was a bit much for him in single track.  He did pretty good and then I watched him kiss a tree for no real reason.  Never seen him wipe the 125 out.  He has a pretty good handle on that bike.  

  He wouldn't hesitate to buy a new cylinder if we can stop the plug fouling.  Kinda getting near the end of dumping money in a bike like this.  It's the only flaw of the bike but it's a pain.  

[mlatour]

6 hours ago, 406flyer said:

That is why I was into the crank seal as it always looked oil fouled and not fuel fouled.  It looked fine.

'looked fine' but did you perform a crankcase leakdown test?

If in doubt replace it, especially if repetitively fouling plugs at such short intervals.

[406flyer]

14 hours ago, mlatour said:

'looked fine' but did you perform a crankcase leakdown test?

If in doubt replace it, especially if repetitively fouling plugs at such short intervals.

   I didn't mention that I replaced it.  I pulled the old one out and it looked like new and put a new one in.  Years ago I pulled some seals that were nasty but this one looked good.  

I was going to rig up some stuff to leak down test it but the tear down was spur of the moment once the seal looked good.  

We'll probably just run it with top end this year and if he has it next season he'll send out the cylinder or buy a new one.  

Thanks for all of the ideas.  

[406flyer]

Compression Test wide open still 140.   No difference.    

Old cars I work it doesn't make a difference either.  

He's going to buy a new cylinder or have it replated.  Millenium is out as they did it last time out of spec.  Any recommendations for US chrome or any others that won't take more than a couple weeks?  

[SlowDinoDog]

I had a good job done quickly by PowerSealUSA a couple years ago...pre-covid lockdowns.

They can get a variety of pistons. I told them which piston I wanted and they bought it, fit it, and shipped it all to me and it worked great with a good ring gap clearance.

[turbo dan]

I doubt the clearance is related to your problems.  The piston is tapered toward the top, I believe the factory clearance is measured near the bottom of the skirt.

Unlikely the replating shop just screwed it up.  I've never had a problem with one.

Edited March 12, 2021 by turbo dan

[406flyer]

I'm not blamming the plug fouling on the clearance.  While were replacing the piston it doesn't make sense to me to install a new piston into a bore that is over the specified clearance.   Looks like the shops us chrome and Powerseal are 4 weeks out.   I'd just get a new cylinder, not sure what he's gonna do.    

If you take a bore guage and measure the cylinder,  it has zero taper or out of round.  This equals zero wear yet it reads a 1/2 thousands over the maximum service limit then they screwed it up when the bored it.  Maximum service limit is almost .0015 over the recommended standard.  We have .004 piston/cyl clearance with .003 being max service and .002 standard running.     This is why it kicks at the low end compression.  yes it will run fine for a while but it's already worn out when it goes together. 

Pistons have been measured from the skirt since the beginning.  Everyone measures from there.  

[406flyer]

New cylinder measures 2.1250" Exactly like the manual specifies

Piston Measures 2.1230"    

.002" clearance like the manual specifies.  

They overbored the cylinder when it was re-chromed.  Based on the numbers it should run a lot more hours between rebuilds than if we were to keep putting pistons in the lose bore.  

I'll do a compression check once it's broken in.  

Moral of the story.  Measure your shit and down take anyone's word that it is right.  

Anyone want to buy a cylinder?