Honing wear

[jonno]

Had a rebore and new piston fitted to my TS1 sprinter barrel.Its steel lined and had a CR250 piston fitted originally,the new piston is a wiseco. The thing is the new honing marks around the inlet and exhaust ports were polished out after one meeting,I stripped it down as it nipped up,no wonder.It wasnt a problem with the CR piston. Why would it wear so soon,rings too hard? I use rock oil synthesis 2 race fully synthetic.

[Warkton Tornado No.1]

Personally, I like Wiseco despite the fact that they are forged (as opposed to Top Tuners preference in cast) but doubt that would be the problem, though what bore clearance did your borer set? Too tight or too slack can exasperate wear. Ring gap is another factor that needs consideration.

Presumably your machinist has made the bore absolutely perpendicular to the deck....

Wiseco also have alternate rings for most of its piston sizes to suit different bore materials. You should be running rings to suit cast iron. The full catalogue is easily accessible on line.

I’d doubt that the oil is a problem as Rock Synthesis is good @ 33:1 (as I can vouch) but I have been amazed @ the quality of Exol Optima @ 50:1 after I tentatively started using it in a non-race tuned engine. How many Manufacturers produce non-castor based two stroke oils specifically for air-cooled race engines?

[Fast n Furious]

Cast and forged pistons have different coefficients of expansion.
The problem is in obtaining this information so you can make an informed decision as to what your bore clearance should be.
Forged pistons are not really better. They just have different problem properties compared to cast.
If there was any worthy advantage to using Forged, the Japs would have used them in their 2 strokes and the mass production cost difference between cast and forged would be negligible to these big boys.
Kawasaki did use Forged in their early 2 strokes but even they gave up using them by the mid 1980's.

IMO. the forged, with proper bore clearance in a properly cooled cylinder will stand the stresses for longer.
I usually change pistons because the skirts start to crack away around the pin, which is normal in a tuned cylinder with a cast piston. The trick of course is being able to catch it before it breaks off!

[Warkton Tornado No.1]

Cast and forged pistons have different coefficients of expansion.
The problem is in obtaining this information so you can make an informed decision as to what your bore clearance should be.
Forged pistons are not really better. They just have different problem properties compared to cast.
If there was any worthy advantage to using Forged, the Japs would have used them in their 2 strokes and the mass production cost difference between cast and forged would be negligible to these big boys.
Kawasaki did use Forged in their early 2 strokes but even they gave up using them by the mid 1980's.

IMO. the forged, with proper bore clearance in a properly cooled cylinder will stand the stresses for longer.
I usually change pistons because the skirts start to crack away around the pin, which is normal in a tuned cylinder with a cast piston. The trick of course is being able to catch it before it breaks off!

The Japanese, do indeed, opt for the cast pistons in their water cooled barrels which make up the large percentage of modern two stroke engines.

On occasion, I have used a cast piston alternative to a forged Wiseco & it blew up in practice & the problem of deficient, cracked pistons tends to occur primarily with cast pistons, which probably could be detected prior to use if we all had unlimited resources.

The trouble is that certain tuners hide behind their reputations & this is when you get the excuses which can be likened to ‘smoke & mirrors’

In Jonno’s case (& I respect the fact that he does not wish to name the supplier) I believe he is being fobbed off with the excuse that the exasperated wear is ‘due to the short piston’
I stand to be corrected, but that smells of the kind of whiff you get when there is a cattle market.

Which is worse, a long piston that often has to be cut or a short(er) piston that tends to maintain its original structure obtained though whatever process made the billet or casting?

Short pistons have been used successfully for many years in Lambretta conversions which are virtually all over-square & I’ve yet to see that being the cause of the kind of wear he describes.

[jonno]

That is what I cant understand,the CR piston didnt cause wear like the new one.Only needed a touch up after 2 seasons of sprinting.

[Warkton Tornado No.1]

That is what I cant understand,the CR piston didnt cause wear like the new one.Only needed a touch up after 2 seasons of sprinting.

Let’s sum up what we have here:

The oil & mix ratio (& presumably fuel) hasn’t changed.
The cylinder & bore lining material hasn’t changed.
The crankshaft assembly hasn’t changed.
Presumably the little end bosses are not tight on to the small end eye/bearing.

So, what is left? What hasn’t been tied down?

Bore clearance: There are fundamental differences between forged & cast pistons but both can be used successfully in the application you have & personally I apply the same rules as far as bore clearance. For a nominal bore of, say, Ø66 mm the clearance that I would apply would be 0.0035” in an un-plated air cooled cylinder despite any recommendation that the manufacturer might recommend. That would be for any application of road or race. Any tuner of Lambretta cylinders would be familiar with either that ‘rule of thumb’ or be able to back up their reasons as to why & be prepared to 'publish' their applied logic.

The fresh bore needs to be perpendicular to the bottom end assembly as in the deck & crankshaft axis & in line with the con-rod plane.

Ring materials vary for different bore materials.

Ring end gap can vary dependent upon selection.

Honing & the type of resultant cross hatching can vary & is significant & integral toward obtaining a good set-up.

Piston lengths vary but that being used as an excuse for the significant wear experienced can be put in the box marked “B S” as far as my opinion goes

I’m racking my brain to try & think of any other factor....

[jonno]

Heres my bore clearance thread. Ring gap was good too.Will see how things go next weekend at East Kirkby .

http://www.ilambretta.co.uk/forum/viewtopic.php?f=7&t=8600

[Fast n Furious]

Suggest you measure the "Gudgeon pin offset" if you have the means.
It could be that the OEM used an offset and the Pattern didn't (or visaversa)
Differences in piston mass will also have an affect.

[jonno]

Looking on the tinterweb found a pick of similar wear which apparentley was down to poor lubrication. Running a vforce 3 inlet,and have heard the vforce 4 solves the 3s problems.which were what does anyone know?

[Fast n Furious]

Can't see how the reedvalve type affects lubrication?

[jonno]

Well I've been warming up the cylinder on the drive cooling it down running the engine a bit more etc. This is the result. 0.09mm bore clearance 0.4mm ring gap,which includes the now shiny area. Was hoping to race at the weekend,but if this is the result of the engine basically running on tickover,whats the risk of a monumental seizure? What the f%*k do I do now?

[Warkton Tornado No.1]

Opinions vary on bore clearance, but I'd be happy with what you have stated.

Do you have all of the product part numbers for the Wiseco piston & ring(s) that you are currently using as well as any for the original Honda piston & ring(s) that you replaced?

[Fast n Furious]

I'd feel lot happier with 0.05mm of bore clearance and 0.1mm of ring gap.

[jonno]

The wiseco seems right as a replacement so presumably the rings are right too.The rings edges arent smooth now.Seems like the rings are too hard ?

[a-teamlambretta]

too many holes ?? expansion and contraction of steel liner ?? , rings clattering over the holes ect ??

[a-teamlambretta]

too many holes ?? expansion and contraction of steel liner ?? , rings clattering over the holes ect ??

forgot to ask , whats the exhaust side like ??

[Warkton Tornado No.1]

The wiseco seems right as a replacement so presumably the rings are right too.The rings edges arent smooth now.Seems like the rings are too hard ?

I’ve suggested previously that ring materials vary & have the Wiseco catalogue to hand if only the part numbers were available to me.

Personally, I like Wiseco despite the fact that they are forged (as opposed to Top Tuners preference in cast) but doubt that would be the problem, though what bore clearance did your borer set? Too tight or too slack can exasperate wear. Ring gap is another factor that needs consideration.

Presumably your machinist has made the bore absolutely perpendicular to the deck....

Wiseco also have alternate rings for most of its piston sizes to suit different bore materials. You should be running rings to suit cast iron. The full catalogue is easily accessible on line.

I’d doubt that the oil is a problem as Rock Synthesis is good @ 33:1 (as I can vouch) but I have been amazed @ the quality of Exol Optima @ 50:1 after I tentatively started using it in a non-race tuned engine. How many Manufacturers produce non-castor based two stroke oils specifically for air-cooled race engines?

Wiseco make pistons specific to applications by sharing the basic forgings across a range of products as well as their rings. Therefore, obviously, rings intended for, say, chrome bores will fit pistons intended for other bores & so on.

I'd feel lot happier with 0.05mm of bore clearance and 0.1mm of ring gap.

As for varied opinions on bore clearance, the scuffing to the honing that we see in the images would be even worse if that clearance were tighter. Having personally utilised the same type of a Wiseco piston (not intended for Honda CR) in both cast iron & Nikasil bores, quite rightly, different clearances were applied. Having disclosed that information to a certain tuner, I believe that ‘my’ choice of piston is still being utilised @ the very top level of sport, so I feel somewhat justified in my reasoning...

[jonno]

There is a code on the piston box of 2276CD.Which I'm guessing is the ring part number.The bag for the piston rings has a code 2520CD.I'm guessing the difference is for nicasil and iron ?

[jonno]

The exhaust side was the first time this happened,was worse with the inlet affected too but the rest of the barrel was ok. This time after getting the deglazer on it its all the way round.Its not affected the top end of the barrel, I suppose because the piston is straighter in the bore at the top.

[Warkton Tornado No.1]

There is a code on the piston box of 2276CD.Which I'm guessing is the ring part number.The bag for the piston rings has a code 2520CD.I'm guessing the difference is for nicasil and iron ?

This has proved to be a bit of a wild goose chase but we're here to help & you have my empathy (for what it's worth!)

That information would appear to be relevant to the liquid cooled Yamaha YFZ350 Banshee Ø64 mm piston.

That must be an old part number for the piston as it’s not in the current catalogue, but the ring part number is current.

2520 CD - Alloy steel, ferrox coated, chrome faced, internally notched.

Not very ‘Honda’ are they, your new parts? It doesn’t look as if your ‘tuner’ made you aware of the switch....

I would question compatibility of the Yamaha piston with what might be a Honda liner.