Fresh top end SEIZED

This is the middle piston. It has severe scoring on both intake side and exhaust side. It seized tight. I was barely moving when it stopped. This could have been alot worse! Cyls all look good. There's some galled up aluminum on the middle cyl that I will clean off with some acid. I'll give them a hone on the mill. The mag and PTO pisons show minimal wear. Just a bit of the coating rubbed off. uploadfromtaptalk1457134196488.jpguploadfromtaptalk1457134216562.jpg

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Mag and PTO... Wash is the same on all three. Piston crowns are totaly intact. uploadfromtaptalk1457134555837.jpguploadfromtaptalk1457134589716.jpguploadfromtaptalk1457134611761.jpg

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whats the skirt to wall clearance??, looks like they were tight to me with the scuffing on the top by the rings on 2 of em. stock is .002-.0025" when you go aftermarket they need MORE clearance like.00325- .0035" because they expand more then the stock powder forged units do.
wisecos grow even more(usually .0038"-.004".
 
The manual says .0028" - .003" with a limit of .010" for factory pistons.

Spx recommends 5-6 thou for anything over 66mm bore. Maybe I got .0045 and not .0035 I can't remeber now haha. I remember they were half a thou tight. I will measure again and post back.


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Double checked my clearances again and I'm getting closer to .004"

My crank guy here in town said minimum 5 thou for these pistons. I guess they are a bit tight.

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If any of you fellas have an opinion on this I would sure like to hear it before I put it back together and mangle another piston.

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That's a bummer. I think this is because of the lack of oil bleeding. I would replace that center piston and rings, put it back together, bleed the oil line, and check the center carb boot. I would also verify your oil cable setting, pull oil cable for max oil delivery, and put some premix in the tank. Then try it again.

Good luck with it.
 
OldIron said:
That's a bummer. I think this is because of the lack of oil bleeding. I would replace that center piston and rings, put it back together, bleed the oil line, and check the center carb boot. I would also verify your oil cable setting, pull oil cable for max oil delivery, and put some premix in the tank. Then try it again.

Good luck with it.
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Thanks mate

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That scuffing that Mr Viper mentioned is not common with SPI pistons even after 100 miles or so. I had the misfortune to see a number of SPI pistons with that same coating (moly-whatever) on them after 100 - 200 miles while I chased an issue on a Rotax 670. They all looked barely used every time in the cylinder that didn't have a problem. I agree with Don, there must be a clearance issue. Incidentally we never did find the issue with that engine and finally swapped a 583 back in the sled. It has been running ever since.
 
.004" should be plenty of clearance, hone the cylinders and make sure you have exactly .004", replace the center one(piston), and run it. I still don't buy the oilpump thing because you ran it for like 15-20 mins before hand but... it wouldn't hurt to bleed it and it only takes a couple mins to do but doubtfull its locked with air or all 3 would have been wiped out not just the center. Check those other 2 pistons well make sure the rings float nice and free in the ringlands as you can see where they were too tight at piston and ringland area and scuffed some, if loose they should be fine to reuse.

did you have the cylinders replated prior to this?
 
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mrviper700 said:
.004" should be plenty of clearance, hone the cylinders and make sure you have exactly .004", replace the center one(piston), and run it. I still don't buy the oilpump thing because you ran it for like 15-20 mins before hand but... it wouldn't hurt to bleed it and it only takes a couple mins to do but doubtfull its locked with air or all 3 would have been wiped out not just the center. Check those other 2 pistons well make sure the rings float nice and free in the ringlands as you can see where they were too tight at piston and ringland area and scuffed some, if loose they should be fine to reuse.

did you have the cylinders replated prior to this?
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The middle cylinder was replaced at some point. It was definitely replated. As for the other two I'm not sure. I will make sure to get them to 4 thou. I'll check the chamfers on the ports too maybe I didn't put enough there.

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Pretty interesting. Getting ready to finally put my viper back together. Thinking all oem might be safer. What made you not use oem pistons?

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03viperguy said:
Pretty interesting. Getting ready to finally put my viper back together. Thinking all oem might be safer. What made you not use oem pistons?

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Well I have a buddy that works at a big distributor here and he gets me the pistons for 38$ each. I've used spx pistons many many times without any issues. I think they are great and affordable. Oem parts are way too expensive up here.

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