Piston scoring

[Tim]

Ok, so tonight I took the top end of my XS650 engine off, in preps to break the entire engine down for cleaning / bead blasting and reassembly with a new ported head / lumpy cam etc.


I rebuilt the engine a few years back and have maybe 2,500 miles on it. I put in brand new 0.5mm over pistons in a fresh bore done by a top-notch machine shop.


On the right piston I noticed some scoring on the piston skirt and matching marks in the cylinder. Nothing of note on the left side.


Any suggestions on what would cause this? I didn't have plans to replace the pistons or rings or do any cylinder work at all given the low mileage. Probably running rich given the build up on the pistons. Rings look good to me - nice and shiny, no indication of blow-by or anything nasty. Compression always measured a solid 150+ PSI in both sides.


Sloppy rods maybe? Slightly loose bore on this side?

 

[620dark]

Maybe the piston pin is worn on that side and allowing the piston to wobble on the rod? Or, like you said, the rod's little end has opened up and is allowing the wobble?

 

[MotoMadness]

Could be a number of things. Contamination when assembled, carbon build up, cylinder hone not perfectly round. I had a GSXR where the prior owners machine shop did some shoddy work and the cylinder was not perfectly round causing scoring on the piston and walls.

 

[Tim]

I have complete confidence in the shop that did the work (Gord Bush Performance here in Toronto) but anything is possible. They machined the cylinders specific to each piston etc. - all the right things.

Didn't want to have to get into new pistons and a fresh bore - that's another $250 I didn't plan on spending and silly after only 2500 miles.

Been doing some reading and there could be any number of causes - poor oiling etc. so we'll see, but I'll likely have to just suck it up and replace them.

 

[crazypj]

That's excess heat and probably happened in the first ~50 miles?
Do you know what piston to cylinder clearance was used?
Put a flex hone down the bore and use 400 grit wetordry with some WD40 on pistons

 

[teazer]

PJ is right. It's a cold seize a.k.a. 4 corners that happens when a piston expands faster than the bore and sticks a little. probably when it was first built and tight but could happen on a cold start subsequent to that. We get used to EFI cars that run sweetly when cold and try to run bikes the same way. An engine must be warmed up before it's put under load.


If there's any aluminum on the bore, use muriatic acid on a cotton bud to clean dissolve it and then wash and oil the bore. That's an old race trick that works on any bore, but go easy with that stuff. It's eats skin fast too. It is usually available in pool stores or sometimes as a concrete cleaner.

 

[Tim]

So I might recover from this without replacing the piston? Not sure what the clearance was when they did the work. It was a good 4 years back.

Minimum it sounds like I need new rings. Might take the pistons/cylinders into Gord Bush and see if they can clean it up. I'm taking the cases etc. there for bead blasting anyhow. If they recommend a new piston then I'll go that route.

The engine was running perfectly when I took it out of the bike. I pulled it to put my newly built 750 in. Only reason I'm breaking it down is a stripped tappet cover stud that was causing a bad leak. Picking up the ported head/cam etc. Friday and will take the engine cases into Gord Bush next week.

 

[AlphaDogChoppers]

The engine was running perfectly when I took it out of the bike.

That tell you plenty, don't it?

A little bit of piston scuffing is not the end of the world, and probably happened in the first 500 miles. This is why a new engine should be broken in gently, in spite of what some dingleberries say about how you should run an engine hard to break it in.

I would not replace the pistons. I would also not use a brush hone. I would use a stone hone with a little WD40 or kerosene, but hone minimally. The stone hone will reveal high and low spots in the bore, something a brush hone cannot do. Just hone enough to give you fresh cross hatching to break in the new rings you will install. The slight scuffing on the cylinder wall will be below the area swept by the rings, so will not affect ring sealing. Also, sand the scuffed areas of the pistons with 400 WetorDry paper wetted with WD40. I also agree with the muriatic acid trick to remove any traces of aluminum that may be adhered to the cylinder walls. DO NOT get any muriatic acid on your pistons or the cylinder casting. <G> Muriatic acid dissolves aluminum. If there is any aluminum on the cylinder wall, you will see a bit of fizzing. Do the muriatic acid thing before you hone.

Before you assemble, wash the parts thoroughly with detergent and hot water, NOT solvent. It is common practice to use something like carb cleaner to clean bores after honing, but this is a very poor practice that does not get the particles of metal and abrasive to release from the cylinder walls. After washing, Spray a little WD40 on things to prevent flash rust.

I suspect that your machine shop did the cylinders on the tight side of the spec, which is how I think an engine should be built. It does require careful break-in when an engine is built that way, but you get the longest ultimate engine life provided that none of the parts scuff during break-in.

Now, as far as bead blasting your cases go, I would suggest you consider soda blasting instead. We recently bead blasted a set of Honda 360 cases, and my intern spent an entire afternoon cleaning the beads out of all the nooks and crannies. This was after he thought they were clean. Then after hours of additional cleaning, I still found glass beads hiding in various places. Very, very bad. I will never bead blast cases again. In the future, it will be soda blasting which I can wash away with water. A dedicated soda blaster and bags on blast soda are now on my shopping list of things needed for the shop.

 

[teazer]

Before you put any hone in that bore, dissolve the aluminum that's been deposited. Not doing so will clog the stones and/or give you a larger bore than you need by the time it's round. If the rings are free in the grooves, I probably would leave them alone too.

Just dress off any high sports on the pistons, clean up the bores with acid and oil it well.

 

[Tim]

Thanks for all the input guys! My last XS engine I did have soda blasted, so might go that route again as suggested. The Honda CA95 cases were bead blasted by Gord Bush and seemed to be clean (which I will re-confirm and examine more closely before closing the engine up ).

 

[Tim]

Here are some more pics - looks like both sides are worn. Overheating etc.? My first ride on the bike was likely quite lean (it was a cold March night and my pipes were glowing a bit ) - I was a little eager to ride after years of building.

So maybe I just killed them in the first few miles. But the wear on the pins makes me concerned about the rods too. What do you guys think? Use this crank and buy myself new pistons? I have another crank here as well (likely not very good however - came from an engine with holed pistons and had been sitting for likely 10+ years - maybe that's good?).

And to confirm, the '50' on the rings tells me these were 0.5mm over right? I'm trying to recall what I did 4 years ago Going to take these and the cylinders back to the shop that did the work and get their opinion. More than likely will buy new pistons/pins/rings and do a fresh bore to 0.75 or 1mm over and try breaking it in again.

Makes me cringe at what my 750cc engine looks like inside

 

[reds]

Take it to Gord and have him mic the pistons and bores. Did you check the plugs after your first ride? after assembly, when was your first oil and filter change? What kind of break in did you do prior to riding it? For first rides, did you vary speeds and rpm or just cruise? Did you bring it up to temp before givin'er some throttle to see what the new motors got?
Just let Gord know all the details and if everything mics out fine, have him clean it up with a light hone and re-ring it if it's just a daily rider. Change the pin and small end bearing too. A scuffed piston is not the end of the world and prob happened within the first few hundred km

As for the rods, check for play in the big and small ends. If any thing looks off post a pic or take to Gord

 

[Tim]

This was all done almost 3 years ago now, so I don't remember exactly when I did what while breaking it in. I put 2500 miles on the engine over a couple of years and then replaced it with a different motor. Now this engine is going into another bike and I tore it down to clean it up properly and put a taller 5th gear in it.

Will be heading to Gord's next week and see what they suggest.

 

[reds]

Yeah, wasn't really expecting you to answer all those questions. Was just more for yourself or anyone reading that's breaking in a new motor to think about when doing so. As for babying a new motor during break in, That's a crock. once it's up to temp don't be afraid to give it some gas. Just don't be hammering down the 407 with the throttle pinned! Vary your speeds and rpm. If you're going down the road doing 50km, don't be afraid to give a little throttle and blip it up to 80km and just bring back down after. It's good for seating the rings. These aren't old 2mm thick cast iron piston rings you are running. Modern rings are harder and need the gas pressures expanding them to the bores letting them seat properly.

Anyways, Hopefully it works out good for you

 

[AlphaDogChoppers]

Tim, I really don't think a re-bore is necessary. I am not sure from the pictures if that is actually wear on the wrist pins, or just discoloration. Can you feel any wear? Check the clearances, I would bet those wrist pins are just fine. It DOES look like things got a bit hot, and running lean could have done that on a new, tight engine.

If your piston clearances are still good, just clean up the scuff marks and put it back together. (With new rings, just because.)

Break in methods are a constant source of discussion and conjecture ad nauseum. Fact is, modern ring profiles and materials are probably fully seated within the first hour of running. More important is to avoid RPM and engine loads that can cause piston scuffing. The rings will seat no matter how you run it unless the bore job is badly done.

A lot depends on piston manufacture as far as how pistons should be fitted. I have built engines that the manufacturer called for as little as 0.0004" clearance. Forged pistons, while stronger than cast pistons, expand more with heat and need more clearance. Modern Harley engines are built with less than a thousandth clearance. They can run hot and tight for several thousand miles before they finally loosen up, but then they are good for 100,000 to 150,000 miles if not abused during the break-in period. If run hard during break-in, 20% or more of ultimate engine life is sacrificed.

Another point of argument is whether to use synthetic oil or not during break-in. I don't have an answer that I am happy with. On one hand, I have heard of engines that took too long to break-in when run from new with synthetic. On the other hand, synthetic oil gives your engine better protection, and tolerates heat better. I have decided that I want to give my engines maximum protection during the critical break-in period. I keep throttle opening under half throttle, and I keep RPM modest until I have at least 100 miles on an engine. I never had an issue with blow-by breaking an engine in that way. I also do 3 heat cycles at fast idle before I put any load on a new engine. This makes sure the gaskets are all seated before you build any pressures in the engine. Mostly important with a high-compression build.

 

[Tim]

Definitely cannot feel anything on the pins.

I was definitely more careful with my 750cc engine, rolling on and off the throttle gently, not holding at a fixed RPM for any length of time and taking it easy on it for the first couple hundred miles at least. Of course I probably blipped the throttle a few times

Will get out the 400 wet/dry and some WD40 and clean up the pistons later.

 

[crazypj]

Looks like you forgot to oil small ends on assembly?
Engine needs to be over 2,000rpm so oil can be centrifuged off flywheels, being 'too gentle' causes problems.
That looks like it was bored tight, the lean mixture and first ride probably cased all the wear.
Doesn't look like there is any 'real pick' up on cylinders and the rings don't look worn
Spot of assembly lube inside small end , 'polish' pistons and put it back together, I would run it for another 5~10,000 miles before spending a wad of cash (I would only re-build if I was selling it, I'm weird like that )

Edit, I just took a look on desktop monitor and I can see the alloy smeared on cylinder
It will be easy enough to remove though

 

[Tim]

So muratic acid to clean the cylinder walls, 400 grit wet/dry and WD40 on the piston skirts...

Do I need to re-hone and replace rings? I don't have a honing tool so it's either buy one for $18 if that's the right tool or I'll get Gord Bush to re-hone them quickly and pay them $40 or whatever they'll charge. I might just get them to clean the cylinders/hone them as they likely have everything ready to go and might do them for me cheaply.

Just don't want to reassemble with questionable parts. For $225 or so I can have the cylinders bored/honed and buy brand new pistons/rings with coatings on them, so spending $30 on honing/muratic acid and another $60 on fresh rings ($90 total) vs. $225 for all new parts / fresh bore.... well, I might just invest the extra $135 and be done with it.

 

[teazer]

So muratic acid to clean the cylinder walls, 400 grit wet/dry and WD40 on the piston skirts...

Do I need to re-hone and replace rings?

Yes to the first and no to the second unless you see damage on the rings or they are sticking.

 

[Tim]

Rings look and feel perfect. Engine always had 150 or better compression. Will clean and reassemble.

I'm using a copper gasket so I can open it up anytime I suppose to have a look