Help Diagnose My Pistons from a Burn Down

MountainMax

MountainMax

New member
Joined
May 19, 2003
Messages
747
Age
23
Location
Churchill Falls,NL, Canada
Could you guys give me some opinions on why this burn down Hapened? It's from my 1980 SRX440 modded to the nuts. My plugs read fine, nice tan, my egt's were about 1100 degrees, running 110 Octane Race fuel with Dominator Race Oil, Coolant Temp well in the green. Timing is also advanced as per SRX Snow Race Yamaha Manual. Oh, yeah they were New Wiseco Pistons that were coated at Swain Tech with ceramic on top and anti friction race coating on sides. I had good cylinder clearances and ring end gap. I Only got aobut 6 runs down the 660ft track (@75MPH) then i lost power as compression dropped and this is what I found.
 

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it might just be me but i havent heard anything good about weisco pistons on here or by the local guys.

the local guys i know usually change pistons if they are useing weisco every 4-5 runs for some reason.
 
You could have cold seized it.Iwould make sure you are getting good flow with your cooling system.I dont know what yamaha racing manual says for thermostat opening temp But i had a kawasaki Interceptor that did the same thing.The interceptor's thermostat is suppose to open at 106 degrees ,yes 106 degrees.My 81 srx would run all day at 170 degrees with stock pistons ,the wiseco pistons that I usedwere alot lighter wall which would heat up quicker and also fail faster.Hopefull all this 2 finger typing helped you in some way
 
what was the piston to cylinder clearance before and after coating the pistons ? looks too tight......
 
squish has nothing to do with piston to cylinder clearance. I have had some pistons coated before and lost some clearance. had to remove some of the coating. I just run uncoated now with no problems.... measure the bore and than the piston. what's the differance ? I run .0055 with no problems.
 
Oh sorry, I misread what you wrote, there was lots of clearance here, actually i measured it and it was near the max allowable tolerance, I also measured the ring end gaps.
all was good.
 
looks to me like a piece of the ring broke off on the exhaust side. it couldnt have been a cold seizure or lack of oil or the damage would be on all sides of the piston. did a piece of metal break off the spark plug or flake from the exhaust port on hard runs? i would most likely expect those types of things.
 
No ring broke or flaked off, and the plugs are nice and tanned color, nothign broke, no port damage, only damage was on the exhaust side of the two pistons and the rings melted into the pistons which lost compression.
 
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MountainMax said:
Oh sorry, I misread what you wrote, there was lots of clearance here, actually i measured it and it was near the max allowable tolerance, I also measured the ring end gaps.
all was good.
Click to expand...


you need to run a looser tolerance with a forged piston, you will want ATLEAST 0.004" because they expand more.
 
mountainmax.....what are your cylinder clearances.....#S Also where are you running the engine at? what rpm? Is the engine speed within the range of the pipes and the port timing?? If the port has closed when the acoustic wave returns to the port then the side of the piston is going to be struck by that wave that was intended to push the air fuel charge back into the cyclinder. This puts a lot of extra heat on the skirt of the piston.... an old issue of sno tracks had a great article explaning powervalves and pipe acoustics...
 
I can't measure anything now as it's blown apart, when i put new pistons in there I will measure the difference. Im running about 10300 rpm the spec olav said for my pipes/porting is 10200 so I have a slightly softer spring on the way also. Porting was also done to Aaen specs, as were the carbs and his race pipes........
 
The weights im running are already very light at the tip, can't grind much more off them, I wanted a higher engagement RPM so i ordered one with a stiffer engagement and sligtly less total. hope to get it closer to what I want.....
 
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