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Honda CB250 K4 -72 racer mods

13821 Views 80 Replies 10 Participants Last post by  crazypj
I bought the Honda three years ago and have raced it for two seasons now. In 2015 me and my son didn't change it much, only changed silencers in order not to get black flag, and changed shift pattern in normal race practice. Only minor problems, had to weld a broken ignition coil bracket and weld an alloy tank leak. Improved rubbers under tank. Season ended with a problem with exhaust too low. Result nothing more than new foot peg, brake pedal and hand brake lever. Plus some frame welding. Winter spent with changing exhaust and making a GRP diaper (bellypan). Season 2016 it run well except that we got oil on our right knee, could not find out why. This winter bike will get an overhaul and some modifications. Chassi behaves well, previous owners had lengthened swingarm, reinforced swingarm mount. Modified steering stem and usual frame welding. Front disc brake works well and the rear Suzuki wheel no problem. The 30mm Mikuni VM pair perform well.
Thought it could be wise to check the engine so it was overhauled. Fairly good condition, only one broken piston ring and a broken valve spring. The improved camchain and tensioner was in good condition. The (not the fanciest) racecam was in mint condition. Another year I might polish the rockers. Right engine cover changed to a standard one because the oil lines to an external filter didn't fit with the new exhaust. Took some time matching head and carbs to inlets and exhausts.
We decided to change from half to full fairing, so yesterday we made new fairing brackets and fitted the fairing. As nothing ever is as easy that you expect a new tacho mount was needed. New ignition coil mounting parts had to be made.
Except for paintwork the bike is almost raceready.
Problem left to solve is a smart quick detachable upper T-bracket between screen sides and centre steering nut.
Any ideas?
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Hard to diagnose things from photos, but there are a few reasons why a circlip comes adrift. Could have been the wrong clips were installed, wrong clip diameter, wrong wire diameter so the clip doesn't sit all the way home in the groove, clip was damaged during installation or the rod is out of whack. A bent or damaged rod, or a small end that isn't running true to the big end can cause the wrist pin to move back and forth hitting against the clip. Eventually the clip and or groove will fail. The clip will erode the piston as shown in your photo and the pin will run up against the cylinder wall.
I didn't realize that was the clip still in there, I thought it was light reflecting off the end of the pin. It is odd.... the only time I have seen erosion like that on a piston, it has been from either a circlip or a piece of ring. You stated that the oil ring was ok and the second ring was intact. Pieces of the top ring wouldn't likely get in there without some sort of obvious trail past the lands and the two lower rings. Have a look at the top piston ring land and see if it shows any witness marks (above the pin area) from a end piece of the compression ring being snapped off during installation. How does the circlip groove look on the other side? Are there any odd marks on the inside of the pin compared to the pin from other cylinder? Something was bouncing around in there.
On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
I'm still curious about the four distinctive pits in the cylinder.
Again hard to tell from photos, but I assume the four pits line up with where the pin changes direction at the top and bottom of the stroke. If that is the case....When the rod is moving up or down, the debris will more or less stay up against the opposing side of the pin or pin boss, but it is still likely to leave witness marks (scoring) in the liner between the pits. When the rod changes direction, that's when the shrapnel starts to create the pits. Those eroded troughs in the top and bottom of the pin boss will force the debris out against the wall. That why you get more wear top and bottom. That area on the piston is not melted, it was eroded/peened by whatever was bouncing around in there.
Didn't the engine builder have an opinion? He took it apart, so had the best kick at the cat.
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So all 4 pits line up with the pin boss area of the piston at top/bottom of stroke... yes?

Who did the cylinder deglazing when that engine was assembled the last time? Whoever did it left out some longevity and hp. Maybe not much hp, but every tiny bit comes in handy if you are racing it.

What does your engine builder have to say about it? As mentioned, He is the one who gets the best shot at determining exactly what was bouncing around inside there and why. When a new engine was released, we used to keep a small stock of complete plug and play assemblies. If an internal problem ever did occur, we could ship out a complete engine and get the knackered one back unmolested. That way there was less risk of valuable information getting lost during disassembly.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.
Not that it matters in the grand scheme of things, but I still believe something was thrashing around in there. At this point you'll never know what it was. Something got in there during assembly. The extent of pitting does suggest it was perhaps something more substantial than 1 circlip. I will wager a testicle that the area around that circlip is not melted. You mentioned that the pin was still a slip fit in the piston. That would be less likely if temperatures got that high. The piston in that photo doesn't show any melting of the crown or down the side above the pin boss. There is nothing that would cause that area to melt without the rest of the piston going he same route. The area around the pin is thicker than the crown, so if things were melting there wouldn't be much of a crown left. Get someone to take a macro photo of that area of the piston and post it or look at it under magnification. It will show that area has been peened, blasted, eroded away, not melted. The clip is just caught up in the rearranged aluminum.
Here is an example of a BMW piston with similar erosion.
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for whater reason a small chunk of piston alloy,that fractured loose fom the area outer to the dcirclip groove
maybe the wrist pin reached a back and for hammering that stress cracked off a small chunk
of course up and down goes the piston and at the change of direction the recip,the minor bit of metal gets wedged and rolled up to go the other direction
after couple hundred 100,000 recip motions the balled up metal is eroding iron liner material
for sure there was never anything close to melting temps
A chunk of piston alloy wouldn't erode the cylinder wall to that degree. How would a piece of cast aluminum small enough to come from that eroded area and small enough to travel through the inner diameter of the pin cause 4 divits that size? If there was a chunk of piston pin missing, I would assume he would have mentioned it.
Mike hasn't answered the question about who assembled that engine originally (as in who did that "cylinder deglazing"). That first photo (post 32) highlights the deglazing and suggests whoever assembled the engine was very inexperienced. The image in the photo even leaves room for speculation that the so called deglazing was done overtop of the pitting/scoring. Is that an illusion..... or did he use 60 grit stones and the crosshatch is that deep? Did the previous owner throw together a sick engine? If so, you would think that the pits in the wall would be covered in combustion crap, but........ Again, it's a low resolution photo, so who knows. Some history on the engine might help.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.
If you want, you could post a few more pictures of that offending piston. Especially one or two showing the circlip retaining groove on the opposite side of that piston shown in your previous post. The more photos the better...
Hard to tell anything further from that photo. There do appear to be some marks at the edge of the groove and in the groove itself that look odd. As in how did they occur if the clip was still in there? If the clip has been removed in the past, they sometimes get pick marks, but if done properly and the clip is removed using the area that is relieved (for that purpose) there shouldn't be any marks in the groove. How does it look to you ( using strong readers or a magnifying glass)? Are there any marks in the groove that couldn't have occurred if the clip was still there? Although.... if the clip wasn't in there, I would expect more damage would be likely and maybe more blow by combustion crap in the groove given the condition of the rings etc. It would also contradict what your engine guy said.
Bottom line as I see it. Either something got left in there during the last rebuild, although it would take some doing to get something hiding in that area.
If I took that engine apart ... there wasn't any shrapnel and both clips were installed, I'd be wondering if some previous owner threw it back together with those pits in the wall because they didn't want to spend the money on a cylinder.... but didn't you have the engine apart at some point before this?
How carful was your engine guy when he disassembled the engine? Is he positive the clip was in there? Always good to start with a clean bench and stuff rags in the case around the rods before you pull the cylinder completely.... even if you are planning on splitting the cases. Better chance of catching whatever was in there assuming anything was left of it. If he didn't split you cases, hopefully it's not hiding in the bottom of the cases.... although odds are are it would just lay there.
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Then the engine was apart in 2016. Notes says cylinder cleaned and honed. Top piston ring was broken at this cylinder. Both top rings was replaced. So it has two race seasons since then. Everything seemed ok except some blue smoke from the other cylinder until last race when it came in blowing heavily from the engine breather. The guy who made both these strips have many years experience in racing and maintaining these Hondas, so I assume that he knows what he's doing. .
Given the season, I'll try not to sound like a grinch, but.... I'm not sure it's safe to assume he knows what he is doing. As previously mentioned, the photo of the cylinder wall shows a finish that was very poorly done. Even for a first timer done at home in the garage it would be considered a fail. The repeat ring failure suggests that the ring lands were worn beyond the service limit or perhaps there were problems with ring end gap that caused ring flutter, but ring land problems are more likely. Things that should have been measured and addressed. The top of that liner looks strange and makes me wonder if there is a bit of a ridge that the top ring is smacking into. The brokers rings from the latest occurrence could also likely be from the ring ends snagging those two upper pits or shrapnel flying around in the pits. Cylinder wall finish is critical if you want to squeeze any sort of HP and longevity out of an engine. It's worth spending some time doing some reading on the subject. It will fill pages and you will get all kinds of opinions. One of the best sources is Grumpy's Performance Garage. You can find current info there and what was "true" 30 years ago isn't necessarily true today.
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That is a really unusual wear pattern. It appears that the pin has repeatedly hit the liner at BDC and TDC which are the times in a cycle where the rod is more or less stationary. That in turn suggests that the pin must have moved laterally (side to side) to impact both sides and then to not create a groove down the liner.

So maybe it is related to a harmonic effect of some sort. What sort of revs does it run to and did the rider notice any particular revs that it used to vibrate worse than others?
But.....if you look closely, the pits appear cratered from being repeatedly struck with something small, the pits are not rounded in a way that you would expect if it was the pin and Mike said there weren't any witness marks on the pin. Granted the pin is harder than the wall and would wear less. The pits also get deeper as they get closer to the end of the stroke, so an undamaged pin running at right angles to the bore couldn't make pits that look tapered outward like that.
Don't forget there are 4 divits. I think the answer to you question to Mike about the divits relation to the oil ring will be yes ( but that would only apply to the top 2 divots.) The reason I say that it is... if you go back to the first photo showing that piston, the pin boss is eroded right up to the oil ring so therefore the top of the divits would line up with the bottom of the oil ring Y/N? I did wonder about a ring getting snagged when the barrel was put on, and that would normally leave fairly distinct witness marks, but thinking about it... on my second cup, some of the witness marks, if any, could be erased by the erosion between the pin boss and the oil ring. Only problem with that theory is that the oil ring and second ring were fine according to Mike. If a top ring was snagged during assembly and broken off, you would think there would be some trail either across the two lower ring lands or in the cylinder wall starting from the very top. Breaking a chunk of compression ring off during installation and having it find its way into the pin area seems unlikely. You'd think it would just head south into the case. Not trying to be contrary and sometimes it takes more than one set of eyes to sort these things out. Plus it's fun figuring them out when it's not your engine and your not paying (sorry Mike).
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From what I'm seeing in the photos, the pin boss is eroded downward (below the pin), I'm pretty sure that when measured, the lower extent of that erosion on the piston will lineup with the lower edge of the pit in the wall at BDC and vice versa at the top. Unless I'm missing something, all that can tell us is that something was rattling around in there. Based on the depth of the pits, if either the oil ring or second ring played a part, then there would be damage evident on the rings.
Yeah, but that doesn't explain why the wear is most extreme at BDC and TDC......
My theory on that is in post 30. The eroded area on the piston boss which is more or less tapered, hurls the debris against the wall when the piston changes direction and that creates the craters seen in the bottom of the pits.
The distance between the wear marks are about 68mm. Stroke is 50.6mm and wrist pin diameter is 15mm. This should indicate that something hard has hammered the marks in both piston and cylinder. As both circlips was in place, it must have been something else. The top ring was in a dozen pieces, some of them only 2mm long. If a piece could have went down without leaving any marks on the piston, it could have caused the damage. There was no signs of the problem before the last race. In the race the engine did some 170,000 revoloutions at between 8500 and 11500rpm, which might be enough to cause the damage.
Now I'll try to figure out why a RG250 has one of the pistons and head looks like it has been sandblasted.
You could start by......Google images of piston detonation and see if you find a photo that matches.
Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.
Does the seized big end belong to the same cylinder that has the blasted piston and head?
You know the original one will magically reappear now....
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