11-14-2013, 06:30 PM #1
Oversizing a 787 engine, rave issues.
I got an 1996 xp 787. I just broke a rave valve the other day and ended up with a damaged piston. I got an set of pistons on my shelf, but they are 1mm oversized. I was wondering when oversizing, should the rave valve be trimmed too in order to not touch the piston, or is it ok with 1mm oversizing and should this be done with 1.5 mm oversize. Also going with an 1mm over, should there be something done with the carbs? I doubt it, but just someone who has oversize their engine can give more advice on this issue.
11-14-2013, 08:49 PM #2
I think first over is OK after that the rave valves need to be shaved. I have always had the machine shop check clearance on the rave valves when going oversized.
11-14-2013, 09:05 PM #3
11-14-2013, 09:13 PM #4
- Join Date
- May 2008
- Elephant Butte New Mexico
I've read a few threads where this issue was discussed, and some mentioned specs, try using search
11-15-2013, 08:45 AM #5
YES, you need to check clearance on the valves on ANY oversized piston. Valves will need to me machined.
If you have made no other changes, ( aftermarket exh, flame arrestors, head, etc) you will not need to make any jetting changes. I would suggest rebuilding your carbs. Be sure to use genuine mikunk brand parts, and replace the needle and seats.
11-16-2013, 09:57 AM #6
The easy way to check rave valve clearance is 3 layers of electrical tape on the rave. Turn the engine over by hand and trim the rave until the tape does not scrape off
11-16-2013, 04:25 PM #7
- Join Date
- Oct 2012
5 layers of electrcal tape for sparkplug gap
1 layer for piston clearance
2 layers ring gap
2 layers for pump wear ring to impeller gap
Had to do it........you know im joking
Actually a great method for checking rave clearance
11-17-2013, 11:06 AM #8
- Join Date
- Mar 2006
I have taken apart many high hour 787s where the blades were making contact to the stock pistons (rings). The blades will hob into the cylinder casting reducing the clearance. If you are using new castings and new blades, you will not see this with OEM parts.
11-19-2013, 11:25 AM #9
I saw one that had a broken spring in the rave cap do this once, the spring broke, so the rave jumped up and down at every piston stroke... it eventually deformed the rave housing/cylinder until it would touch the piston... One would think you would hear that going on and miss the power... but that's probably why he was able to buy the ski so cheap in our case.
When the motor went back together with new rave housings, springs, diaphragms, etc, the clearance was checked with the tape method, measured the tape to make sure it corresponded to the clearance spec of course. I trimmed them with a half round file to spec and it ran like a top.
11-19-2013, 05:44 PM #10
I always just put a flat edge in the cylinder and slip the feeler gauges under it. For 1mm you will definately need to clearance the RAVE's. I have also seen them wear into a cylinder and catch a ring. Normally those are the really gummed up ones that nobody ever performed proper maintenance on.
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