Thread: Opinions on a Cylinder
05-06-2015, 08:19 PM #1
Opinions on a Cylinder
I have a '97 GTX 787, with about 320 hours. A compression test was last done two years ago, and showed close to factory spec (on a cheap walmart tester), other then some carb issues it ran fine last season.
I pulled it out of storage, and noticed one of the head bolts sitting in the bottom of the hull. It probably snapped last season, and fell out on the drive home (It was in place last fall when it went into storage).
I've pulled the head, and I'm going to have to pull one jug to drill and remove the stud. I'm going to replace every gasket I have removed, and pick up a new set of head bolts.
I'm just wondering if I should go a little farther while I'm there. I'm told I should replace the rings on the side of the jug I'm removing. I'm just debating honing both cylinders and replacing the rings on the other piston as well.
Normally I'd bore and replace the pistons just because I was down that far, but I just ordered a new impeller and wear ring, and don't want to waste a whole lot more money this season.
TL;DR: Based on these pictures, should I replace one set of rings, or hone and replace both?
Also, what's the best way to remove dirt and oxidation off the water jacket area of the head?
05-06-2015, 11:03 PM #2
- Join Date
- Jan 2006
- Patterson La
its a 787!!! bolt it back together and rock on.
If it were mine, and I was pulling one jug, I would replace both pistons. But you would be fine with only doing the rings. you would also probably be fine not touching anything since there is no scarring, or seizure. Did you pull the raves and look at the sides of the pistons?
That old with that many hours, and the rotary shaft seals don't leak oil into the case?
05-07-2015, 01:15 AM #3
when you get the barrel off, check the ring end gap and if you have the means, the piston to bore clearance. This will tell you what needs replacing. Anything less is just guesswork
If it was mine, rings on both pistons would be a absolute minimum since you have gone this far
05-07-2015, 02:22 PM #4
Thanks for the quick response!
Would new pistons mean boring the cylinder?
The more I think about it, the more I want to put new pistons in it. I decided to go for a new ATV this year instead of trading out to a 4 stroke seadoo, and I think I'll be more then happy with this one for a few years with the pump rebuilt.
The only problem is my mechanic is backed up right now, so he wouldn't be able to bore my cylinders for two weeks. I might look around to see if anyone else in town can bore them, as I want to find someone with a bead blaster anyway. If I could hone them myself and put the same size piston back in, and get the same results, then I'd happily do that at this point.
I'm going to remove the jugs tonight, my mechanic won't order me any parts until I check both pistons. I have a standard set of callipers, so I'll measure clearance at the top and bottom of the jug (Though a middle measurement would be more useful). I'll throw up a few pictures when I do.
Where would I check to find a rotary shaft seal leak? I have never had a problem with the rave valves, so I didn't remove them (I will tonight).
Thanks for all the advice!
05-07-2015, 03:00 PM #5
With those hours I would do a complete teardown and re-assemble with new gaskets and other parts as necessary. If you look at the piston top and head you can tell that the cylinder with snapped bolt has also burnt a little bit of water too. Probably what contributed to the slight scarring at the ring gap cause it was washing the lube away. I happen to have a 112 hour 787 torn down right now. I did a complete inspection and cleaning. I will be re-assembling with new crank seals and new gaskets. I have also honed the cylinders and chamfered the port edges. Better to catch any loose crank bearings or bad bearings on the CB shaft BEFORE it comes apart and windows the block.
05-07-2015, 03:11 PM #6
I'd bolt the head back on a recheck compression. I wouldn't do a damn thing till it scatters. Seriously 787 parts are not expensive and I wouldn't put a fresh top end on a tired crank. You can tell if the rotary valve seals are leaking if it smokes real bad on a first start after sitting. Seriously pull the raves, clean them, bolt the head on, check compression, and send it! I had a rfi that looked way worse that your cylinders and had a rotary valve seal leak that ran like a champ. Its a 787. Not a 4-tec or some crazy high compression race motor spinning over 7k. This is my advise if you don't pull the jugs. If you yank the jugs you have opened a whole other can of worms that others have covered.
05-07-2015, 09:01 PM #7
The jugs will definitely have to be pulled. I have a hard enough time drilling bolts when I can reach them, and none of my drills will fit inside the hull to drill this one out. It's also conveniently snapped about 2-4mm into the jug, so I don't dare try to weld a nut to it.
I've probably only put 15-20 hours on it since it tested ~160 PSI in both cylinders. I'm honestly scared of breaking another head bolt if I try to torque them back on.
I'm going to educate myself on rotary shaft/valve seals. Last season she got progressively harder to start for the first run of the weekend. I assumed it was needle valves leaking and flooding the engine (I have new valves and carb kits sitting on my work bench), but from what you guys are saying, it's much more likely to be those seals.
Right now I've decided bare minimum is a hone and set of rings in both cylinders. Heavily oxidized bolts and chafing my arms on the edge of the hull made for slow progress tonight. I unbolted the exhaust manifold, but still need to pry it free. After that I'll be able to fully examine the pistons, but for the money I think I'll probably just change them out. In either case every gasket and seal I have disturbed will be replaced.
05-07-2015, 09:11 PM #8
As for 'do I have to bore it for new pistons', IMO you'd be crazy not to, but you need to take some measurements first.
A simple vernier check of the bore size (or see if you can see anything stamped on the crown of the piston) will tell you if its been bored previously.
If its standard bore, with those hours, you can guarantee that the bore will no longer be round and parallel
05-07-2015, 10:47 PM #9
I've removed a bunch broken off like that with my little 115v MIG. I just stick a nut over the hole then start welding to the bolt 'til it's good-n-red. Then drag the cherry up onto the nut and continue filling. The weld won't stick to the aluminum. When your done smack it good a couple times with a hammer and twist it on out before it cools too much. Also if it's oily you should clean it with carb cleaner then use a propane torch to sweat the oil out. If not then your weld will have a hard time sticking to the bolt. Stainless is very hard to drill too.
05-09-2015, 10:16 PM #10
Removed the pistons today, and I believe they are the stock size. The uncaged bearing was a nice surprise, I now wish I would've watched a video before I removed them. I don't think any of them slipped by the rag I had jammed under the piston, but I think I'll pull the engine from the ski to check. I'm pretty sure my front engine mount is toast anyway.
I have a Lincoln 140, but my garage is wired poorly so it always runs cold. I may try to weld first, but since I'm only set up for flux core wire, I likely won't be able to make a strong enough bond as the bolt is set into the hole. I may try my plasma cutter, it melts steel out of aluminum nicely but I've never done anything that small and long before.
I also just picked up a blasting cabinet today, so I'm going to try my hand at soda blasting.
The pistons I removed are in very good shape, so it doesn't make sense in my mind to buy new pistons without boring the cylinders. As long as I don't have to mess with shaving rave valves, I think I'll just send my jugs off to be bored.
As long as the turnaround time on the jugs isn't terrible, I should have everything put back together by this time next week!
Users Browsing this Thread
There are currently 1 users browsing this thread. (0 members and 1 guests)
By dlajoy1122 in forum 2-Stroke PerformanceReplies: 14Last Post: 08-27-2013, 05:33 AM
By chris1200 in forum Yamaha PWC Performance (2-stroke)Replies: 10Last Post: 07-17-2009, 10:27 AM
By jazgeek in forum Polaris Open DiscussionReplies: 11Last Post: 01-13-2008, 01:58 AM
By meeper in forum 4-Tec PerformanceReplies: 11Last Post: 09-09-2005, 09:50 PM
By Prez in forum 4-Tec PerformanceReplies: 9Last Post: 08-04-2005, 09:44 PM