JE878 gone bad

[misterdimwiddy]

Reading my way through '4 stroke performance tuning' by A. Graham Bell and stumbled upon the following equation to calculate piston acceleration at TDC.

a=0.000457*N^2*S*(1+(S/2L))

where

a = acceleration ft/s^2 (horrible imperial units)
N = rpm
S = stroke (67.5mm or 2.657")
L = rod length (5")

Putting some numbers into this, and if I have done my calculations correctly, the indication is that over revving has an enormous effect on piston acceleration because rpm is a squared term in the equation.

Revving an extra 1000rpm to 9k results in a 27% increase in acceleration which then means that using F=ma (Force is proportional to acceleration), Force on the rod (small and big ends) also increases by this % amount.

For a given level of acceleration, if piston mass increases (as the JE does being 40g heavier than stock), the force F increases even further.

Need to double check my calcs but I reckon over revving to 9k with 448g JE pistons combines to give a 39% increase in force on the small end compared to a stock piston at the red line.

Even with a perfectly healthy lubrication system, this has to be a major threat to effective small end lubrication (much smaller bearing surface area than the big end) which doesn't even have pump pressure to maintain the seperation between rod and pin surfaces.

Pickup on my rods/pins is not dreadful but I am now fairly confident that the damage is my own fault through (usually unintentional) over revving and that keeping to 8k (which then only results in a 10% Force increase due to the 40g extra piston weight) should give me a reliable motor.

Bore and ring wear has resulted from running the JE top end with a knackered crank back in April.

Thinking of using a programmable ign unit (again thanks to Sandune51 who pointed these out to me) to enable me to set a reduced rev limit. You could simply say 'don't over rev it' but on track I find it difficult to focus entirely on the tacho and it revs like a good'un with all the goodies attached.

May all be Sunday morning nonsense but at the moment it seems a plausible explanation to me.............

Awesome, now we're talking, love your work now factor the pressure taken to force the oil form the gudgeon pin surface and you are on the way to finding out the best oil to use for high RPM! So is the pin designed to rotate in the piston or is it static?

To keep to your red line, fit a rev limit light (my choice) or a rev limiter. The light should be mounted in your peripheral vision allowing you to keep the track in view and not over rev…..
Rod

Still questioning why the rod would need to rotate in the piston and the rod especially when interference fit pins are used in the piston when circlips can come displaced from their groove. This leaves rotation in the rod alone.

Sounds as though at extreme revs the small end can deform axially under load and this then grips the pin which if fixed to the piston causes piston rock and bore wear. If the pin can rotate on the pin then there is no problem in this event (unless the piston deforms as well).

I did find some info on F1 rods having a pressure oil feed up the length of the rod to the small end.

You have convinced me that an ign unit is the way to go Cobba. Relatively low cost in the scheme of things. I also have Dyna coils on my shopping list.

All good interesting stuff.

Peened rods are back on TUesday and hopefully rebored liners so I can start putting the motor back together.

[dandywarhol]

I'm under the impression that the pin will pretty much always "rotate/move" in the piston as the alloy piston expands. There's quite a lot of surface area on the 2 piston bosses - more than the rod itself.

Some pins are locked to the rod by a pinchbolt so the only bearing is the piston bosses - others run directly in the rod or a bush in the rod small end eye and the pistons have to be heated to pit the pin. But they're still located by circlips/plugs due to the high temperature/expansion of the piston compared to the gudgeon pin.

[coxylaad]

cobba what programmable ignition modules are you talking about? sounds interesting.

Can I ask a question - sure there are bigger capacity twin cylinder engines that rev higher than the TRX lump, how do they manage?

[dandywarhol]

Yep - Aprilia and Ducati twins rev higher. Engine stroke determines max. revs (along with other details). A short stroke, coupled with lower mass, means the piston acceleration/max speed is lower, therefore less stress on the components, .

[coxylaad]

so I am taking from that, that the TRX is a long stroke twin then?

[misterdimwiddy]

so I am taking from that, that the TRX is a long stroke twin then?

That is an interesting comment.

My VTR1000SP1 revved to 10k redline and has a 100mm bore and 63.5mm stroke.

RSV mille is 97 by 67.5mm so same stroke as the TRX but revs to 9500.

Both are much more modern machines however than the old TRX motor design.

Can't find info on mass of pistons for these two but I can't see them being much different from a JE; maybe even more? An extra 9mm bore is a lot of metal.

There again the SP1 was not built as a 'normal' road bike so materials are probably a bit special.

More evenings to be had searching for back ground info.

[misterdimwiddy]

Peened rods ready for reassembly;



Bolt heads and nuts were peened with them but I'm chucking these and putting in new.

Barrels will be back next week and I am going to have 40g's removed from within the pistons. 10% piston mass increase can't be a good thing for small end lubrication even if I stick to the 8k redline.

[dandywarhol]

Good plan - never reuse big end fasteners. I didn't realise the rods are the same as the mark1 Tdm 850 - 3VD

[Rod.s]

Peened rods ready for reassembly;



Bolt heads and nuts were peened with them but I'm chucking these and putting in new.

Barrels will be back next week and I am going to have 40g's removed from within the pistons. 10% piston mass increase can't be a good thing for small end lubrication even if I stick to the 8k redline.

Have you checked the bearing locating surfaces for out of round? If you've over revved the engine then the bottom cap can "stretch". It would be worth while getting a bore gauge and check the vertical dimensions. Yamaha doe not give you the dimension, but, they do give you the bearing clearance, so a quick bit of math will tell you if there is an issues.

Also with the gudgeon pin, remembering that the rod top holes only feed the area that the pin goes through the rod, excess is passed between the rod and the piston clearance. I might be worth your while checking that the pin has the correct clearance in the piston for the oil to circulate.

Really enjoying what you are doing

Rod

[Rod.s]

Still questioning why the rod would need to rotate in the piston and the rod especially when interference fit pins are used in the piston when circlips can come displaced from their groove. This leaves rotation in the rod alone.

Sounds as though at extreme revs the small end can deform axially under load and this then grips the pin which if fixed to the piston causes piston rock and bore wear. If the pin can rotate on the pin then there is no problem in this event (unless the piston deforms as well).

Do you have a photo that shows the entire pin in clarity?

[misterdimwiddy]

Peened rods ready for reassembly;



Bolt heads and nuts were peened with them but I'm chucking these and putting in new.

Barrels will be back next week and I am going to have 40g's removed from within the pistons. 10% piston mass increase can't be a good thing for small end lubrication even if I stick to the 8k redline.

Have you checked the bearing locating surfaces for out of round? If you've over revved the engine then the bottom cap can "stretch". It would be worth while getting a bore gauge and check the vertical dimensions. Yamaha doe not give you the dimension, but, they do give you the bearing clearance, so a quick bit of math will tell you if there is an issues.

Also with the gudgeon pin, remembering that the rod top holes only feed the area that the pin goes through the rod, excess is passed between the rod and the piston clearance. I might be worth your while checking that the pin has the correct clearance in the piston for the oil to circulate.

Really enjoying what you are doing

Rod

It's tricky Rod because I am rebuilding this time with a mix of new and used parts (funds are not endless) but a lot better understanding of where problems may lie when running as high state of tune with the TRX. Ideally all parts would be new but I am where I am.

The rods I have had peened are from a stock used motor so my hope is that that has not been over revved significantly and has seen only 'normal' road use. I will measure up the big and small ends anyway.

When you ask for pics of the full pin, do you mean the new JE pin or the damaged ones that I first posted showing damage to the centre bearing area?

Making enquiries today about DLC (diamond like coating) for the JE pins before they go in. It's only 2-4microns thick but sounds as though it offers great benefits for small end reliability (reduced friction, wear resistance). I'll let you know what I find out. Chemical Vapour Deposition I believe is the application method but have no idea if it is horribly expensive. Upper operating temp limit is 450°C so should be OK for pins I reckon.

I'm dead keen to start reassembling so that I can get to Brands in January to run the motor in. No hurry though and shouldn't rush it as I am enjoying learning all of this new stuff.

Not a lot of info online about where to safely lose weight from the JE pistons so I will have to go it alone and hope I get it right.............

[Gordo]

Found this on a hot rod site. Your pistons might have the same


If you do have to touch any of the pistons, do it in the same place that the manufacturer did. There should be a small hole drilled into the balance pad underneath the pin bores. Don't monkey with them anywhere else.

Gordo-

[cobbadiggabuddyblooo]

which manufacturer, Yamaha or JE.
Is stress relief needed after cold working the pistons????
I know this proccess was undertaken when machining was performed on the billet trees I had made up...
Just like you had your rods shot peened to close any of these minute tears in the rod surface..
I'm sure someone used to working with metals may be able to answer this...

[misterdimwiddy]

After a long search for information on piston lightening I am going full circle now and leaving the pistons alone.

Watched 'how its made' on forged pistons and there are a lot of heat treatment and machining steps. My fear is weakening the piston or distorting it by machining so I will live with the 40g extra and limit myself to 8krpm.

Have gathered some info on gudgeon pins for those considering lots of engine mods;

Found a place in Milton keynes that can do DLC coating on gudgeon pins but need to find out whether JE (787-2350-14-93C) pins are carburised or nitrided. This determines the temperature at which the DLC coating is applied without reducing the final pin hardness.

Have asked JE how they finish them but have a feeling they may not want to tell me such detail. Their info says case hardened high-Nickel carbon steel but nothing about heat treatment temperature.

Pins have to be microfinished before DLC application and eventual OD increases by approx 3microns. Bonding is at an atomic level and application temperature determines the coating longevity.

Friction coefficient is 0.1 dry (steel on steel dry is 0.7) and there is great benefit from reduced sensitivity to effective oil lubrication. This is what draws me to the process.

Then (DOH) on the JE site I read that they do DLC coated pins in this size/material (787-2350-14-94/98C)

Will try TTS to see if they will have my new pins back and get me a pair of coated ones.

Lots of modern (auto stop start) car engines use this coating on pins because of the challenge of maintaining lubrication.

Should have a budget cost for DLC coating tomorrow.

[cobbadiggabuddyblooo]

Question came to mind.. If replacing the rods without splitting the cases, how do you torque the rods up when placing back on the crank?? Can you take a couple of photos please on how you manage this as there isn't much room to dance around in there so It would be great to see how this is achieved ..