1991 CR125 - Top End Pics

There are much more experienced members on here than me but I will give it a go... I like to monitor & learn. It looks like to me you have ingested some dirt/sand at some point? Or have a leaking airboot to airbox? The piston & cylinder scratches..... Can they be felt with finger nail in the cylinder?

Definitely has ingested dirt/sand. Is it still stock bore? Check for any markings on top of the piston. If stock 54mm bore, looks like you can get a first over piston and take it to be bored to fit.

It appears it has worn through the nikasil coating. You can't bore it, it must be sent in and recoated. Some will say sleeve it but I would stick with the nikasil. Millennium Technologies can fix you up.

That cylinder has a steel sleeve in it. Previous owner probably blew it up and had it sleeved. Rear skirt of cylinder was ground off which is a popular mod. I'd have it bored and a new piston fit.

Yep it has eaten some of natures sandpaper... If the nikasil is 100% intact it could be used with a new piston and probably run, but to be right getting it recoated is the best option, I have had good luck with powerseal USA but there are others that also do reputable work.
Do remember that whatever went through the top end went to the bottom also... The rod and mains also are probably a bit long in the tooth. If your gonna race it I'd go ahead and put a new crank and mains in it as cheap as the OEM Honda parts are.

After putting my glasses on and looking at it, I believe you're correct Dan. Ports appear to be dressed up to match the sleeve.

This cylinder has already been sleeved. I personally would get a new cylinder with Nicasil plating right away. Get a quality piston like Vertex or Wössner at 0,05mm of play and off you go.

On HPP engines one good upgrade is to get the HPP valves & guides off the 98/99 model. These seal better than all previous HPP valves.

Here's a pic from the exhaust side:
left is 98/99 valve....to the right it's the 90-97 valve. You can clearly see the blow-by when closed which is what makes them less effective.


Even more impressive is the view from the inside:
90-97 HPP


98/99 HPP


Speaking of the HPP-maybe also have a look at my setup guide i put up on Thumpertalk here:
https://thumpertalk.com/forums/topic/1207620-honda-125-hpp-setup-tips-t…

Some good advice and photos from Lucknino! Every 2 stroke for sale has : "new top end"........even if they don't know what year their bike is....

Bwahaaaa! Ain't it the truth!
I subtract 500 dollars off my offer if it says that!
Also if brake hose is on the outside of the fork I subtract 200. Lol

Anyone who has bought an older bike has been down this road, start a build and do it the right way!

Looks like the previous owner put a steel sleeve in it and didn't have the ports chamfered. There are sharp edges on those ports and they are catching the rings. Your lucky it didn't end up worse.

As far as I'm aware any HPP cylinder and head combination will work together (91-99) They all had different porting and squish specs every year though, so the combinations are endless... I think the 95 was rumored to have the best porting and I know they don't show up on Ebay as often as the other years...
Best thing about Honda's is the sheer volume of parts for sale since so many were sold.
I would do some more CSI work to determine if the piston really was hitting the head and if so why, lots going on there... none of it good!

Grab the rod & see if it moves up & down...lots of play could allow for piston to make contact with the head...looking at the head, it appears bottom end bearing came apart at some time, those are needle bearing marks in the head... I would say a full teardown is in order...I would not reuse that head...

My suggestion would be to scour ebay for a cylinder and head. I would not use that head and I don’t like sleeved cylinders unless there are no other options. I bet you can find both a head and cylinder on eBay at a reasonable price.

Check the crank too. It may be time for a crank rebuild.

All of this is pretty standard for an older bike. I am on my 3rd older bike rebuild and they have all had some surprises. Most of them have been painful but all of them have been fixable.

Bwahaaa! If the same crack mechanic did the bottom end the while he did that top I couldn't get it split fast enough! Cheap as the crank and mains are combined with the fact it ate about a pound of topsoil and a 125 is like a grenade with the pin pulled even when rebuild by a reputable mechanic...

As i already mentioned i'd get a new cylinder and head right away! No need to fuss around with that poor sleeve job nor with the head which is in pretty bad condition anyway.

Take care not to mix cylinders& heads of 90/91 with other years as those had ROUND TOP pistons and according domes in the heads.

92-97 are all "equal" and there you are free to mix-n-mtch heads as well. Minor changes over those years.

98&99 are different as the sideports got larger/wider at the base of the cylinder. Also the 98/99 cases are different! So you want to stay away of 98&99 cylinders. Heads are ok though.

If it has a 54,5 piston it's oversize! Stock pistons are 53,93-53,97mm.
---> In order to be able to run a bigger piston they needed to shave some off the HPP valves&guides. So if you install them in your next cylinder with stock bore they won't fit good! As mentioned above try to get hold of 98/99 valves&guides. If not definitely avoid the 90-92 valves as those are too restrictive in top-end (while still having too much blow-by when closed). Those years HPPs are to avoid.

It indeed has touched the head...which to me indicates a bad lower-end bearing....

Anyway - seeing the state your top-end is i'd also open up the lower end and check for state of bearings&crank....i personally would toss everything right away (!!) and start over from scratch installing all new OEM parts. OEM crank only with 125cc Hondas!!! Costs little and is by far the longest lasting/best quality!!!