I have a stock bore/stroke 1991 FXR. Was warming it up in the driveway the other day and had a valve guide explode on me. Both intakes bounced the metal chunks back and forth until I got it shut down. Heads tood some damage..........but I think I can clean them up. I lucked out on the jugs. They seem to be fine. Pistons got pretty beat up on top. I would clean them up and re-use if it were'nt for one of them having compressed and pinching a ring (won't slide easily anymore).

So a friend of mine hooked me up with a set of EVO pistons out of his FatBoy. When I got them home to compare I noticed that his pistons are quite a bit shorter than my FXR ones. He said they are stock pistons.........but I'm wondering if they are stroker pistons instead? I threw a wrist pin halfway in each of them, so that I could line up the height evenly. The difference is around 0.165"

I'm assuming I'd lose quite a bit of compression if I run his pistons. Any thoughts?

 

Ouch! Sorry to hear of your problems and best of luck getting back on the road. One of our experts will be along soon, but it looks to me as if those Fat Boy pistons are not suitable. I suspect there must have been a change in cylinder height between your bike and his, or connecting rod length perhaps? Bear in mind some tuners talk of using different thickness head gaskets to alter compression ratio, that .165 is way outside the realms of being usable!

 

The pistons won't work. If your engine is basically stock and going to stay that way just get some cast flat top pistons. The reason I say flat top is you will probably have to cut the haeds to clean them up properly and that will raise the compression ratio. Even if the rings would move freely you should replace those pistons as every one of those injuries are a potential starting point for a failure. Be sure to check out the wrist pin bushings for clearance to make sure they didn't get pounded also. Hope this helps.

 

The part numbers are marked on the top of the pistons if anyone wants to identify the two....one is an 83 which would make it a very early Evo, other looks like 88.

 

Google the part numbers on top of the pistons...one is a stock 1340 Evo...the other is a 1200 sportster

 

There is your answer......

 

Craziness. The guy that gave me the pistons runs a cycle shop locally. He must have forgotten what they came off of. I tried to google the part# but couldn't find anything. Thanks for the info on that. If I'm gonna run new pistons, how much gain would I get out of going .10 over with everything. I need to have my machinist do a new valve job on my heads anyhow...........and I need to buy new pistons..............so I was thinking about boring a bit. Cylinders have mild scuffing on the sides from possible piston slap.

Secondly........anyone have any idea what would cause something like this to happen? Here's what I had going on:

Stock displacement 80" EVO. Andrews EV-3 cam. Adjustable pushrods. Motor was/has been running awesome for years. Stock lifters (I replaced them maybe 3 or 4 years ago).

I pulled it out into the driveway and started it up. While it was running I noticed gas leaking out of my fuel connection to my float bowl. Shut it down and ran a new line. Started it back up and all hell broke loose. The gas was draining right on top of my tappet block (rear block.......which is coincidentally the head that had the valve guide break). Could gas have worked it's way down into the lifters and done some crazy ****?

 

Duplicate post.

 

That's interesting. You've reminded me that when I had my original Evo motor stroked the builder told me he used Sportster pistons. That difference in height looks about right!

Kelly, I recommend you have your cylinders honed/rebored, so they match your new pistons. If you have a local indy or dealer who can do that for you, get them to recommend what size, after they check and measure what your cylinders are currently. Best not to guess!

 

Valve guides can fall apart with age or a sudden inrush of cold air when the motor is really hot, usually happens to exhaust guides when the exhaust falls off out of the head, yours might have been just old and wore out.