1991 CR125 - Top End Pics

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1/11/2019 1:42 PM
Edited Date/Time: 1/11/2019 1:50 PM

Hi

Took barrel off today and the pics are below.
New to this 30yrs on from schoolboy motocross, so would appreciate your expert views on the findings? Many thanks
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1/11/2019 1:56 PM

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?

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1/11/2019 2:08 PM

Thanks Sandman768. They cant really be felt with a finger nail in the cylinder. It looks worse it the pics to be honest as they are somewhat maginfied in the pics. I will post somemore pics shortly.


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1/11/2019 2:52 PM
Edited Date/Time: 1/11/2019 3:51 PM

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1/11/2019 4:18 PM

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.

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1/11/2019 5:19 PM

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.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

1/11/2019 6:05 PM

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.

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1/11/2019 6:05 PM

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.

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1/11/2019 8:19 PM

DynoDan22 wrote:

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.

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.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

1/11/2019 10:37 PM
Edited Date/Time: 1/11/2019 10:38 PM

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.
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Even more impressive is the view from the inside:
90-97 HPP
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98/99 HPP
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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-tricks-tuning/




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1/12/2019 2:24 AM
Edited Date/Time: 1/18/2019 1:09 AM

Thanks all but not for good reading, considering I was told this engine had been rebuit not so long before I bought it. Was only opening up for a look.

Some good tips there Nino and a real useful link which is very much appreciated.

Looks like the poston is stamped 54.5, as can be seen in pic below.

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Some more pics below of top and bottom of the cylinder
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1/12/2019 5:52 AM

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....

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1/12/2019 9:28 AM

sandman768 wrote:

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

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1/12/2019 10:03 AM

Rocky739 Thats class lol.. Could see all the new gaskets on all the engine, got excited and took my chance and bought it.

I suppose the more you look the more you find. See below pic of wired power valve lever.

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1/12/2019 11:09 AM

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

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1/12/2019 2:27 PM

sandman768 wrote:

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

Ageed 100%, not the type to cut corners and will want everything tiptop. Unfortunatley for the posters on this forum this will probably need a lot more posts looking for further advice and guidance.

Looking forward to this steep learning curve, and appreciate the help offered.

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1/13/2019 11:16 AM

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.

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1/14/2019 5:10 AM
Edited Date/Time: 1/14/2019 5:12 AM

Taking a closer looking at the two pics below, it looks like the piston may have made slight contact with the head? See the un-carboned area on the piston top (as circled in yellow). This appears to match the shape of the slightly lighter mark circled yellow on the head (need to zoom in to see this).
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The cylinder head was mounted with alan key bolts through the head (pic below) and not studs and nuts as it should be. Not sure if this could be a contributory factor?

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Would my best option be to purchase a newer second hand cylinder? Am I correct in saying a cylinder from 91 up to 98 would bolt straight on?

Thanks

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1/14/2019 6:24 AM

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!

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1/14/2019 6:08 PM
Edited Date/Time: 1/14/2019 6:11 PM

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...

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1/14/2019 7:32 PM

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.

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1/15/2019 2:52 PM

sandman768 wrote:

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...

No play in the main bearings. The engine has been appart and had them done.... apparently! But we have heard that before somewhere lol

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1/15/2019 6:44 PM

Pazaz wrote:

No play in the main bearings. The engine has been appart and had them done.... apparently! But we have heard that before somewhere lol

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...

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1/17/2019 2:50 AM

Looks like a split is in order then. Cheers peops

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1/18/2019 1:14 AM
Edited Date/Time: 1/18/2019 1:24 AM

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!!!

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