Oversize Pistons

[harvdog42]

Hello,

I'd like to know if Yamaha makes an oversize piston that would fit the XS1100 engine.

Are there pistons from another bike that could be used to give the XS1100 more cubic inches and still maintain stock compression?

Also, does anybody know what the minimum wall thickness should be if one were to bore out the stock cyclinder sleeves?

Thanks,

JIm

[Snow]

Checkout Wiseco bigbore kits, Pro-Flo near Pensacola, FL can get them to you for about $400 for the whole kit - they have two flavors, 1179cc and 1196cc. There are a few of us that have gotten Wiseco kits.

The Yamaha parts, if you can even get them, are insanely priced. If you do go through Pro-Flo, tell them Pete from the XS list sent ya



http://www.pro-flo.com/

[Geezer]

Re: Oversize Pistons

I wouldn't take the stock cylinders over 1.5 mm stock or they'll be too thin to prevent overheating and warping. The Wisco kits seem way too expensive to me but I just may be too cheap. For that much money it’s not delivering enough performance difference. If you don’t resleeve go at least 3mm over size, you’re not gaining enough to make all the work (and cost) worthwhile.

Another problem is that bike manufacturers don't publish the data you need to mix and match parts. Not only are you concerned with bore size and wrist pin diameter but you also have to consider deck height and piston pin offset. One thing I have found over the years is that bike manufacturers like to reuse certain features but you can’t count on it.

This is going to take a lot of research any way you look at it. If you can find a bike that you can get sleeves and pistons from you’ll be that much further ahead. It’s been too many years since I turned wrenches in a Yamaha dealer so I don’t recall all the models that might work but one good place to start is with be with 350cc (or so) singles. The Yamaha 650 twin might be another possibility.

Sorry for the long winded reply.

Geezer

Originally posted by harvdog42
Hello,

I'd like to know if Yamaha makes an oversize piston that would fit the XS1100 engine.

Are there pistons from another bike that could be used to give the XS1100 more cubic inches and still maintain stock compression?

Also, does anybody know what the minimum wall thickness should be if one were to bore out the stock cyclinder sleeves?

Thanks,

JIm

[harvdog42]

Thanks Geezer for the great reply.

I was just out looking a the cyclinders and it looks like there is plenty of meat (aluminum) around the stock sleeves to actually put in larger sleeves.

Do the stock sleeves just press out?

Another concern if one was to resleeve and rebore for larger pistons would be the head gasket. Wouldn't the stock head gasket opening be too small? Is it possible to bore out a stock head gasket?

Jim

[Geezer]

Toss the cylinder block in the oven and heat it up to about 375 F. and the sleeves should just fall out when inverted. They may require a little persuasion with a wooden mallet.

The head gasket isn't as bad as it looks. You have to cut out the original sealing rings and replace them with copper O-rings similar to what the old 2 strokes used but not as wide.

If you think the engine will be making excessive pressures (not normally aspirated) then you will need to have a ring machined around each cylinder (both in the block and head) and use a slightly taller copper O-ring. Look into what is done on turbocharged Z-cars. When it's all done you end up with a 5-piece head gasket and you'll want to use some sealer on the fiber part to help keep the oil in.

Geezer

Originally posted by harvdog42
Thanks Geezer for the great reply.

I was just out looking a the cyclinders and it looks like there is plenty of meat (aluminum) around the stock sleeves to actually put in larger sleeves.

Do the stock sleeves just press out?

Another concern if one was to resleeve and rebore for larger pistons would be the head gasket. Wouldn't the stock head gasket opening be too small? Is it possible to bore out a stock head gasket?

Jim

[TopCatGr58]

Hey there HarvDog,

The stock pistons are 71.5mm. The maximum oversize that Yamaha suggested was 1.00mm larger, up to 72.5mm diam. The 1179 Wiseco goes to 74.00mm. I had mine bored at a local motorcycle machine shop that builds racing bikes, and they had no problem with that amount, and didn't feel like it would cause any excessive amounts of heating. Since I couldn't get the regular oversized pistons, I went with the Wiseco. It comes with an oversized head gasket. I've ridden the bike to 3 rallies in the past 2 years since, and I don't perceive any excessive heating. I rejetted 3 sizes larger due to Indy Filters and 4-1 pipes. My compression values after breakin were on average 180psi at sea level, that equaled to about a 12.25:1 ratio, a bit more than the 9.2:1 of my 81 model, but have no problems running it, I just use hightest gas of 92-93 octane.

I think you're looking at a lot more trouble with trying to find mix/matched pistons and sleeves from other bikes/models when the Wiseco will fit and works well with very little effort aside from boring the cylinders....IMHO!?!? I'm no Yamaha Mechanic, just a shadetree survivalist that got my bike fixed as economically as possible! YMMV, GOOD LUCK.

[fusionking]

Hey Topcat! I think you ought to re-check your figures again. To start with a compression check reading can vary from things like cam timing etc. 180 psi is a very low figure for a 12.25 to 1 engine. should be in the 200's anyway especially with a stock cam. Wiseco's chart for your pistons shows 10.25 to 1 , a much more believable figure considering that it is living on 93 octane. Even if it was water cooled it would need more like 110 octane to run over 12 to one. To run 93 you'd have to back the timing so far off (like 0 ) it would'nt be worth it. Actual compression ratio is figured from combustion chamber volume vs. swept volume and a comression reading doesn't really enter into the equation. Garry P.S. I'm glad they are 10.25 to 1 cause it makes them a streetable solution that I would very much like to do on my bike soon as this is very much in my capability and I also really like hearing about how great this has worked out for you!!!!!!!!

[harvdog42]

Oversize Pistions

Hello T.C.

You're probably right about mixing and matching pistons and sleeves when there is already a kit out there that'll do the job.

I'm a little concerned about the increases compression you mentioned since I am running a supercharger on my bike.

Are the Wiseco pistons designed to give higher compression? Or was that a result of getting your head decked?

And did you notice any performance increase by going to the bigger bore?

Jim

[Geezer]

I'm still uncomfortable with going 3mm over in the stock sleeve. This was a common thing to do in the 1960s but cracked and warped sleeves were also common side affect. The only way to do it right is to go with larger sleeves.

Geezer

Originally posted by TopCatGr58
Hey there HarvDog,

The stock pistons are 71.5mm. The maximum oversize that Yamaha suggested was 1.00mm larger, up to 72.5mm diam. The 1179 Wiseco goes to 74.00mm. I had mine bored at a local motorcycle machine shop that builds racing bikes, and they had no problem with that amount, and didn't feel like it would cause any excessive amounts of heating. Since I couldn't get the regular oversized pistons, I went with the Wiseco. It comes with an oversized head gasket. I've ridden the bike to 3 rallies in the past 2 years since, and I don't perceive any excessive heating. I rejetted 3 sizes larger due to Indy Filters and 4-1 pipes. My compression values after breakin were on average 180psi at sea level, that equaled to about a 12.25:1 ratio, a bit more than the 9.2:1 of my 81 model, but have no problems running it, I just use hightest gas of 92-93 octane.

I think you're looking at a lot more trouble with trying to find mix/matched pistons and sleeves from other bikes/models when the Wiseco will fit and works well with very little effort aside from boring the cylinders....IMHO!?!? I'm no Yamaha Mechanic, just a shadetree survivalist that got my bike fixed as economically as possible! YMMV, GOOD LUCK.

[TopCatGr58]

Simple Math!?

Hey FusionKing,HarvDog, Geezer,

Thanks for the correction. I guess simple math doesn't work for those stats. I first looked at the value for what the XS was supposed to create at sea level, around 142psi +/- 14, took that and divided by 14.7 for 1 atmosphere at sea level and it came out to around 9.66, not too far of the 9.2 value?? Then I took the compression values I got from my test of 180psi, divided by 14.7 and got 12.24, so that's where I got my "Gestimation" of my acquired compression ratio, but since I didn't study physics I apparently erred in my calculation process!! But that is probably good news for HarvDog since he's running a supercharger ! ! ! With a more realistic 10.2:1 ratio, it might work for him.

I remember seeing SLEEVES on the Wiseco site, but not sure if they are oversized/larger than stock, or just replacements for the stock ones in case the originals were too badly out of round to bore properly!?!? I would think with the supercharger that he would be getting more pressures and higher temps than if it were normally aspirated, so going with the larger/bigger/thicker sleeves would be the safer way to go.

I didn't have the head decked or shaved, it wasn't warped, so I was able to go back with it stock with just a decent cleaning. As FusionKing stated above the Wiseco stats report a 10.2:1, so they are designed to provide a little increase over stock value!

The bike does have a considerable amount of power, however, it was 9 years earlier that I had last ridden the stock powered bike so it's kinda hard to remember how it felt as compared to the big bore power feeling!! Hope this helps in your decision making!?

T.C....simple shadetree survivalist, no A.S.E certification!

[Waldo]

May 29/03
I rebuilt by xs11 Midnight Special (81) with a big bore kit. I wasn't able to acquire any of the stock Yamaha Oversize pistons. I did not have to change the jets or anything else although I do have a 4 into 1 pipe. The company that made them was Wiseco. Please email me if I can be of further assistance. Waldodabiker@Hotmail.com.
Waldo

[Geezer]

Re: Simple Math!?

You want lower not higher compression with a blower so there is more room to blow in a charge. Check the stats on new turbo model cars as compared to normally aspirated ones.

I think my old blown Packard Clipper was a whole number lower in CR than the standard model.

For a blown XS11, I'd start with 2 base gaskets and maybe and aluminum shim if it's still too high.


Geezer

Originally posted by TopCatGr58
Hey FusionKing,HarvDog, Geezer,

Thanks for the correction. I guess simple math doesn't work for those stats. I first looked at the value for what the XS was supposed to create at sea level, around 142psi +/- 14, took that and divided by 14.7 for 1 atmosphere at sea level and it came out to around 9.66, not too far of the 9.2 value?? Then I took the compression values I got from my test of 180psi, divided by 14.7 and got 12.24, so that's where I got my "Gestimation" of my acquired compression ratio, but since I didn't study physics I apparently erred in my calculation process!! But that is probably good news for HarvDog since he's running a supercharger ! ! ! With a more realistic 10.2:1 ratio, it might work for him.