Good YZ490 con rods

[LWC82PE]

I was wondering if some of you guys can tell me what the best, strongest and beefiest con rod that is available for a 87/88 YZ490? I need one that’s really strong. Thanks

[Lozza]

"Strong" isn't an engineering term. In fact conrods are not very strong at all, every thing other than the pin bores are 52 HRc making them very ductile which is exactly what is required to combat compression and extention every revolution.They are designed for the piston mass and rpm that might give you a starting point. Bearings have a big influence on the red line rpm of a piston/rod assembly.

[Hoony]

I was wondering if some of you guys can tell me what the best, strongest and beefiest con rod that is available for a 87/88 YZ490? I need one that’s really strong. Thanks

is this for the Methanol burning Formula 500 speedway jobie? is it bending rods? Is a carillo rod available to suit?

[LWC82PE]

Yep, the top of the rod tore off at the gudgeon pin area. It has bent from left to right but i would say that is from when the rod hit something else. The rod smashed through the inlet port and the piston is disintergrated. Faaar out did it make a mess!

I have another low hour std bore 88 model motor but its missing stator/flywheel/carby. The aim is to source those parts and build that motor back to standard spec for normal fuel and then get a new pipe to suit that motor.

We then aim to resurrect the 'Hot' motor and want a good rod for that. That cylinder was on max oversize we might be able to salvage that barrel with a re-sleeve and some welding repairs. Ideally another barrel not on max oversize would be good and then we can bring that one up the the specs of the damaged one that needs a re-sleeve. That barrel has also had a thick plate welded around the base gasket area with extra braces around the transfer ports for extra strength i assume as the previous owners musto of had problems in that are. This motot is running an unknown internal rotor ignition with Hitachi CDI and i know its not standard 490 so thats why i need a standard ignition for the standard motor. Plus if the hot one blows in a race it i will be a lot quickler to swap over engines and not have to swap ignitions over and set up timing etc.

I dont really know why the top of the rod tore off, i guess thats what you get with racing and methanol. Possibly the small end bearing let go first and rollers came out an then the rod tore off? It was a Wiseco small end bearing that i put in there when the new piston went in. It did another 3 races since i put a different clutch basket in it because that one exploded but now the rod decided to let go. It was good the clutch basket was ok this time, but i reckon it doesnt like being push started and having that jerky force go though it. If it brakes again then we will try for a billet clutch because at the moment there isnt really any room for a kick starter.

[Hoony]

it could simply be that this motor is stessed way further than it was designed to for considering its running Methanol and the load on the engine when the car is backed into a corner at High RPM would be HUGE.

any idea how much horsepower this is putting out? does it still have the water cooled head?

[LWC82PE]

No i dont know the power its making, would be lots though i reckon Yes its got a aftermarket watercooled head on it.

[oldfart]

What's the top of the piston telling you .....have you any pics .
Are you able to get any parts X-rayed .
May have to look at getting an Alloy rod made up .

[Lozza]

I dont really know why the top of the rod tore off, i guess thats what you get with racing and methanol. Possibly the small end bearing let go first and rollers came out an then the rod tore off?

A pic would help a lot , sounds more like a piston problem than rod. Usually the piston break first and then that lets the rod flap about and cause all the damage. When the rod breaks the piston hits the head and normaly stays there in tact.
 From that tale I would say your well past the rpm/ power output the engine is capable of, Hoony has good point to a solid axle speedway car with big tyres will drive the engine on over run, with 0% throttle there is no 'cushion' for the piston. Try to ascertain where the failure occurs, ie into/out of corners, at WOT etc etc
Here is a good pic of a piston disintergration

[holeshot buddy]

leith what piston were you running
genuine or wiseco
its definatley a piston gone that has made the mess
i would say
some genuine pistons are cast
wiseco are forged much stronger
cast pistons crack skirts after a certain amount of hours
yz490s were renowened for piston dissapearing
out exhaust

eg lozzas piccy

[LWC82PE]

I will try take some pics tomorrow.
was a wiseco piston i fitted last year and only done a few events.
mess looks very similar to the above photo
doesnt appear to be a lack of lubrication or anything like that. Inside of the combustion chamber has a light tanny/black colour but not heavy carbon depsots or signs of overheating/pinging etc.
after what ever broke first you could see the piston had been slapping the head as there was grey flakey alloy around the squishband
these motors were quit common here in speedway and this car was the ex state champ in the 90's. The previous owner and his dad spent a lot of time developing it. Unfortuantely the father who was the brains of it all is no longer around to quiz him about it.
Thanks for the suggestions guys

by the way are sleeves available off the shelf for these or do you have to get them made up?

[Lozza]

The grey ali 'dust/powder' is a good sign the piston has gone south. The 'blow up of the month' pic it was the piston cracked where the circlip groove was allowing the gudgeon to fall out .................. 
Piston probably only hit the head twice, it must be racing in some 'vintage' class as I doubt it would compete to well against an RG 500 on fuel. A big bore Banshee/LC/RD 400 engine would would be a whole lot less drama.
 
A sleeve can be had for $90 US from ATV UNlimited but you will need a big lathe and a face plate to get it in and a lot of patience to get the old one out. The sleeve has an internal rib which prevents it being pressed out.

[LWC82PE]

After looking at it again we are pretty convinced the rod broke first and not the piston and that caused the piston/gudgeon pin to break. When the lifting the head off the  piston was at the top of the bore and a bit stick too.

You probably cant pick it up in the photo but there is evidence of the rod already being cracked as there is a light grey line along the middle of one side of the gudgeon pin. So that probably just let go from stess, old age, a lot more power than normal etc.

We had a 1951 matchless do this aswell and the top of the rod broke and the piston had similar damage and the rod mainly too k out one side of the piston skirt although the pin didnt break as its a lot bigger than the YZ one and the pin runs straight in the rod with no roller bearing.
I can get re-sleeving done no worries.

[Lozza]

Most definantly a rod failure, if it was a piston the small end eye would be in tact and the pin boss would not be almost in tact. I'll get a better answer on why this would happen.

[LWC82PE]

Thanks for that. I will see if i can get a good photo of what my boss reckons is evidence of a crack where the top of the rod has come off.

[Freakshow]

Steel Sleeves can be made up by Dave down there at west torrens, leith