750 SF2 piston and cylinder wear

Paul Marx said:
Maybe one of the engineers on here could comment on wear in the middle of the liners?
Paul
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Ok, here you go. I use mostly a couple of different publications as support material for analysing piston failures. I don't remember ever seeing just one publication that would cover all of the already known failure modes.

This one is from a publication of Kolbenschmidt - Motorservice Rheinmetall automotive - when the scoring is on the middle of the skirts (especially on the pressure side):
"
Dry running/Fuel damage:
• Over-rich engine running
• Combustion defaults (misfiring)
• Insufficient compression
• Defective cold-start device
• Oil dilution with fuel
"

This publication is mostly suitable for big sized engines with relative low output if compared to the size of the engine, four stroke but both petrol/gasoline and Diesel engines. Some other publications are more suitable for engines with higher output per displacement.

The back side has bigger marks and certain shape on them because it is the pressure side, conrod causes a force piston being pushed against the opposite side of the rotation on power stroke.

I would have checked the oil if it smells fuel and/or if it has clearly lost its viscosity.

I would have expected more all around and/or four courner marks if the bore was too tight.
However, generally when analysing piston damages, in some cases the appearance is much dependent on engine and piston model.

-Jouni
 
During all 4 oil changes since I've owned the bike the oil has never smelt of gas, nor was it ever thinned or contain any gas film/swirl. I constantly checked the B8ES plugs after a spirited ride (or during one) and they usually were burning nice and clean. The plugs would foul during a short ride or from just idling, and the piston crowns dont have excessive carbon buildup. The last compression was 150/160, and the bike rarely backfired or misfired.
I have to decide whether the problem stems from poor oil pressure/clogged passage ways in the crank, but have been told that the pistons would still receive oil from below, or if the problem is from too tight clearance. Since the cams and followers were coated with oil, it tells me that the oil pump is working. Measuring the clearance now won't reveal what the clearance was after the rebuild. I am reluctant to split the cases since it's involved and may not be the cause. I'm getting other opinions that say too tight clearance. The piston wear only on the front and back sides could be from an out of true bore, but that's just my guess.
I'm considering sending off a last oil change sample to be analyzed.
 
piranha-bro2 said:
Pistons aren't round. They are narrower at the sides than front-to-back. So highly unlikely to have any wear marks on the sides!!
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Yes, but only when they are cold. Gudgeon pin and reinforcements on piston for it are guiding most of the heat expansion to happen on the direction of a gudgeon pin.
 
Sounds a bit that the first set of pistons may have failed because of a reason that is still present. Did they had any pattern or were they too scored?
Nothing else than just too tight for both sets???

Personally I would just strip down everything and inspect everything - especially the oil pump and check and clean all of the oil passages (to be cleaned after every 100 000 kms anyway), measure the bores at least on three different heights and crossed that is at least six measurements per bore + the pistons, and check what oil was used. And around 250 other things plus the carbs and fuel system - I count on industrial maintenance for everything decades old and according to it everything not checked is already broken.
B8ES may be a bit on the hot side for a 750, though B9ES is quite far colder. Some other makes do plugs with heat range somewhere between these. Idle jet sizes may need to be checked for current fuels.

-Jouni
 
I took the cylinders and pistons to another machinist who measured the bores and said they were true and just a little on the tight side of .003 clearance.
He honed the cylinders to eliminate the wear marks and said that they are still in spec for the same 80.6 mm/ 3.1693" pistons that were in there. He couldn't say exactly what caused the piston wear but thought it could be from a rich mixture at low speeds giving gas wash, and he also advised using a modern blended 20/50 oil instead of mineral. Since the original pistons had no wear marks, I think it's doubtful that the oil passageways got clogged up after just 1600 miles since the rebuild and multiple oil changes. I would like to inspect the oil pump but think it must be working since the cams and followers have no wear or discoloration. I would split the cases if it was an easy procedure but after looking at the manual, its beyond my mechanical ability, and it might be for nothing.
So the plan is new pistons, gaskets, modern oil, and lean out the carbs for low speeds.
Thank you everyone on this Forum for your input.
 
Would a slightly tight bore cause it to nip up, partial seizure? Next obvious question: what jets are in the carbs, what air filters, are they restrictive, float height. Blended oil? Have a look at the oil video here. It's a bit academic anyway; everyone recommends mineral oil for Motocycles that run wet clutches when running in. Something is missing here, and I dont like mysteries with no proper answer. Please look into too much fuel and bore wash. Flooding carbs.
madrid

Thread 'Cool Videos'

There seems to be nowhere to post cool videos other than the joke thread, whereas some videos are far from a joke. Forgive me for posting non-Laverda content, but you all need a break from the shed once in a while, no?  :D

Winter storm near Bergen, Norway:
[youtube]http://www.youtube.com/watch?v=LFWkwnmdKCc&feature=player_embedded[/youtube]

Amazing starling murmuration:
[youtube]http://www.youtube.com/watch?v=eakKfY5aHmY&feature=player_embedded[/youtube]

Right, back to work!  8)

 
If it were bore wash to that extent, your oil would be black, thin as wind and stink of fuel- if it’s not, it’s not bore wash!
I have had a new GPZ900 do the first 3000 miles from new with the choke stuck on- no long term problems, just ran massively rich.
If the original pistons didn’t look like this then you’ve got to wonder what’s different this time around. Remember you stripped the motor to find the knocking noise, that’s not bore wash. Something was knocking under load and your pistons are worn out in next to no time? You need it looked at as a whole - just taking the barrels to an engineer won’t tell the real story. Dont be in a rush to slap this one back together, I have seen motors rebuilt with the same problem built in . There must be someone near you you can take it to? Doesn’t need to be Laverda specialist, just a decent bike mechanic.
 
Exmoor beast said:
If it were bore wash to that extent, your oil would be black, thin as wind and stink of fuel- if it’s not, it’s not bore wash!
I have had a new GPZ900 do the first 3000 miles from new with the choke stuck on- no long term problems, just ran massively rich.
If the original pistons didn’t look like this then you’ve got to wonder what’s different this time around. Remember you stripped the motor to find the knocking noise, that’s not bore wash. Something was knocking under load and your pistons are worn out in next to no time? You need it looked at as a whole - just taking the barrels to an engineer won’t tell the real story. Dont be in a rush to slap this one back together, I have seen motors rebuilt with the same problem built in . There must be someone near you you can take it to? Doesn’t need to be Laverda specialist, just a decent bike mechanic.
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Yes, I think that it's a basic engineering problem that needs sorting from the ground up, so crank out, check crank and rods, cases...
Paul
 
Exmoor- Good point about the oil- at a 1300 mile change it was pitch black and thick, doesn't overly smell of gas, but I'm getting it analyzed for content. I don't fully understand how the pistons are lubed, but know that oil is splashed up from the lower crank area, and I can see a pan below by using a boroscope. I see the series of 3 oil passageways on the center of the engine case, and one more near one of the studs, which I'm assuming lube the cams and followers?
So in essence, could it be clogged passageways somewhere inside the crank that are starving the pistons? I'd also like to test the oil pump if that's fairly easy but the Green Book is sort of vague w no testing info. Since the cams and followers were coated in oil w no wear, I'm assuming the pump is good, but I could be wrong.
 

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Two of the passages go to crankshaft main bearings on both sides of the timing chain gear.
These two bearings feed oil to the centrifugal separators on the crankshaft.
Next the oil goes to the big end bearings and splashes to the bores.

-Jouni
 
MotoJouni said:
Two of the passages go to crankshaft main bearings on both sides of the timing chain gear.
These two bearings feed oil to the centrifugal separators on the crankshaft.
Next the oil goes to the big end bearings and splashes to the bores.

-Jouni
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Which I mentioned in the 2nd post of this already long thread ...
>>>>>>>>>
Assuming a lubrication issue, potential reasons could be crank oil slingers full of 'silt' and blocking the oil holes in the crank pins that feed through the big ends and exit to feed the underside pistons and cylinders.
>>>>>>>>
The only way you can measure oil pressure (that I know of) is to tap into the feed post-pump ... SFC Elettronicas and twins modified for an oil cooler, have threaded outlet bosses that can be used with a gauge.

Unfortunately there's no way you'll get a boroscope into the slingers to determine whether they are full of silt or not. The 'pan' you mention is a splash plate between the crank and sump. I have no idea how much oil vapour in a twin motor with no oil going through the big ends makes its way into the cylinders, but (as mentioned once again) I dismantled an SF motor that had zero oil through the pins and rods and it had seized so badly that one rod was bent beyond 90 degrees. The big end was still freely rotating ... so that would tell me that there's no spalsh of any merit and oil vapour just doesn't quite cut it. That was zero oil through the rods - slingers won't suddenly go from oil to no-oil ... it'll be a gradual starvation scenario.

You can observe oil feed by removing a cam end cover and spinning the motor on the starter, block one of two exposed spindles with an M6 bolt and see if oil is exiting the open spindle hole - I'll bet my bike on there being plenty of oil in evidence.

All this is academic - for peace of mind on a 'problematic' motor with an unknown history, I think splitting the cases would be wise. it's not as scary as you would imagine. With the crank out you'll be able to look into the dreaded slingers and see how close to 'high tide' the silt in the slingers is. If they're not full and not yet into the crank pin cavity - and you're really careful - you can even clean them out using a few aptly shaped probes to clean them out. It's not ideal but can be done if you really want to avoid splitting the crank. If the silt is into the crank pins you really have no choice.

On the subject of fuel wash in the cylinders, with my 39mm FCRs the SFQ was running very rich for quite some time while I was fine tuning. Plugs were as black as, piston tops the same. No sign of piston oil starvation.
 
TeddyG said:
Wolfgang recommends .003" clearance which the machinist claims to have done.
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that is correct clearance for a Ross piston, works out at 0.076mm

I only use Ross Racing Pistons which I buy from them direct in lots of 60 each order
Once here I have them ceramic coated on the crowns and dry film coated on the skirts and then bore and hone the barrels to 0.065mm clearance
 
Thanks guys and pardon my ignorance- I see 2 holes at the bottom ends of the connecting rods but don’t see any dirt or silt in them, and looking at exploded diagrams of the crankshaft and crank case I see no “slingers”. Stupid question, but would pouring kerosene into the crank case clean out any silt, or blowing in compressed air into these holes? Looking at the Green Book directions for splitting the cases appears intimidating so I’m resisting doing it.
I received the bike at 15k miles and I could tell it never had been serviced and probably never had the oil changed, so hence all the problems I’ve encountered at the current 18k miles. I’m going to wait until I gather enough info and nerve to split the cases.
 
I don't see any post to indicate the ring end gap. If the gaps are too small the rings will expand and possibly score the bore.
FWIW Top rings normally should be gaped as follows;
Top ring 0.004 in to 0.0053 in PER INCH OF PISTON DIAMETER. the later clearance for high performance engines.
2nd ring 0.005 in PER INCH OF PISTON DIAMETER.

Refer to Wiseco site for confirmation.
 
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