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Discussion Starter #1 (Edited)
So I just got a new to us atv for the kids, starts (pull start), runs good.......checked compression 105 psi cold. Put some oil down the spark plug hole and got 125+ psi. Time for new top end. Took it off today everything looks good, no gouges, nice smooth bore. Just trying to figure out if its stock piston/stock bore? It has 5W stamped on the top of piston and I measured the bore and got max measurement of 71.86mm using a cheapy digital caliper. Stock is 72mm bore so I assume 5W is stock polaris piston & size?? What are you guys getting for psi on new motors/top ends? Going to take it to local machine/motor shop and see what they think..........think I can probably get away with hone and stock size piston?
 

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Sounds like stock piston diameter. Only way to know if you need to bore it out is to get a new piston and ring set and take measurements. The cylinder needs to be measured for out of round and diameter in several locations. Can't be done properly without a set of snap gauges.
 

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Why would you rebuild an ATV that ran well unless you just wanted to do a project? I can see just doing a $20 set of rings plus a top end gasket kit but not the machine shop expense on an older good running ATV.

We have 325's and thankfully they do not wear parts very much but maybe 2 strokes are not like that.
 

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Discussion Starter #6
I look after my stuff and make them right when I buy them. It started and ran yes but it clearly needed a new top end. The service limit is 115 psi. mine was 105 and it should be 130 ish fresh. Its a 13+ year old 2 stroke on its original top end, its time to make it right. For $200+ parts and machining its well worth it to me. And I have all gasket and seals to do the bottom end too. 2 strokes are a much different beast than a 4!
Cheers
 

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Discussion Starter #7
Cylinder and head are at the machine shop, just waiting for piston and stuff to come in so they can start on it. I went first over .020 SPX piston.

Once I pull the recoil/mag and primary off can I change the crank seals from the out side? Or do I have to split case to do that?
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Good call on the rebuild. Ive had and rebuilt a few of these. Once that engine gets down to about 95 psi its done anyways. Would of became real hard to start. After top end rebuild ive always gotten around 130 on the compression. If you want you can also replace the crank seals. No special tools needed and will take about hour and a half.
 

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Discussion Starter #10
1.5 hrs? Do you have to split cases? I think you can change them once primary and recoil/mag is removed pull out old one and push in new one??
 

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Been a long time since I've been in one of them, but I seem to recall you being able to change the crank seals without taking apart the bottom end. Good plan doing them too while you've got it apart.
 

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You do not have to split the crank case. I was actually surprised how easy they were to change. Its been a year or so since ive rebuilt one but buy aftermarket seals. I think I bought tusk brand for about $12 for both. Much cheaper and never had a problem with them.
 

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Manual says you need a flywheel puller is this true.........need to get at the crank seal?
A puller design to work with that size/shape would be ideal especially if it is stuck on tight but have pulled flywheels without a puller like on mower engines. We are going to have to pull the one on the 325 engine soon.

Key is to remember for every force that is an equal and opposite force. A puller will insure the force is on the end of crank. I may start soaking with with PB Blaster this weekend.
 

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Yes you need a puller. Im a mechanic by trade. So sometimes I forget that most people dont have the same tools as me. Most auto part stores have tools for rent for very cheap.
 

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Good info on getting one through a parts store.

Got ours stripped down to the flywheel and it has three tapped holes to attach a wheel puller. Did start the soak but sure much will work its way in. Was going to make a run the HF for a puller and the hardware store for a bolt to pull the primary clutch. At least before the ice storm hit today we got both unbolted which was a chore.
 

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Discussion Starter #18
Well top-end is done SPX coated piston, bore 1st over, head cleaned up too. New crank seal in on PTO side......atv is basically to the frame except for tranny. Need to get it out too, to replace seal. Don't have socket big enough to get swingarm bolt out 1-3/4".......maybe tomorrow after trip to Canadian Tire.
 
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