80-81 1196 Big Bore piston kit

[WMarshy]

'78/79 piston on the right, 80+ on the left....

Wiseco piston on left, stock 80+ piston on right. Maybe hard to see, but the Wiseco piston isn't domed as high as the later models, but is considerably higher than the early ones.

Whats your point...think the compression would be higher than 10.5 (Wiseco)? So what, gotta run 93 instead of regular... Might want to clay the pistons to mare sure there there's enough clearance..

[ViperRon]

Compression ratio is a problem depending on the quality of gas but you can get additives to help in fact as bad as gas is sometimes is darn near a nessisty.



http://en.wikipedia.org/wiki/Compression_ratio

Compression test can somewhat check your results but volume check is the real definate of compression ratio. There are some rules for compression testing as every 1000 feet of elevation the guage reading will be about 14 lbs less


It is a great price on the kit I would check it over very careful and as suggested clay the pistons to check clearance in your particular engine. The bad part is it is not cheap to get the cylinders bored and you need to have a quality place do the work. Without a significant change in the cam, timming or carburation do not expect massive horsepower increases. If your tolerances are close be very careful in setting mechanical timming. If a valve comes to close the method I would use is double the gasket under the cylinder which will lower the piston away from head no machine work required.

[trbig]

Whats your point...think the compression would be higher than 10.5 (Wiseco)?

It was just a response mainly to Steve's.

The Wiseco sets are flat-tops I believe, and give a 10:1+ compression ratio (depending on which head you use)

From the guy's description of these cast pistons, they sound like maybe they're the same as the Wiseco.... somewhere in between the two stock pistons. What I would like to see is someone with an early model head/valves do a piston to valve clearance check with later model pistons in. If they work, THAT would be some bumped up compression numbers.

[WMarshy]

It was just a response mainly to Steve's.




From the guy's description of these cast pistons, they sound like maybe they're the same as the Wiseco.... somewhere in between the two stock pistons. What I would like to see is someone with an early model head/valves do a piston to valve clearance check with later model pistons in. If they work, THAT would be some bumped up compression numbers.

That would be nice to know! I'm almost certain I'm going to get a set of these. Almost cant pass this up.

[TomB]

The comp ratio is definitely an issue to check, as well as piston clearances, i.e. pistons may have a low enough comp ratio but touch on the squish clearance (or lack of) area of the head.

I've done big bore engines (not XS) by boring out blocks and using bigger liners then found the big diameter pistons touched or were extremely close to the edge of the head dome, not wanting to alter the head I machined the pistons. This was even though the comp ratio was right.

Other engines I've tuned I've ended up using two copper head gaskets to bring the comp ratio back down due to detonation.

I made my own copper head gasket for one of my XS's which works great, I used 0.9mm copper which upped the comp ratio a bit, a thicker copper head gasket could be made to lower it if these pistons ended up being too high ratio.

One problem I found with the higher comp ratio of my XS with the thin copper gasket is the starter motor struggles when the engine is hot, the engine is a well used loose free revving engine, but if you restart after just turning the engine off the starter motor stuggles. Obviously a good battery helps. Something to consider when altering comp ratios.

Tom



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[TopCatGr58]

Another post to show that I'm no genius!

I followed the link that ViperRon provided for learning about comp. ratios, and formulas...phew....I regrettably withdrew from Calculus and Physics classes in my last semester of college...was working 4 days and full time college the other 3 days, kinda left off a day off in there somewhere, and the stress was too much for my brain! I did pass Biology and Social Psych with A's though!

Anyways, I played with the formulas on the site where you need the bore, stroke AND the vc=Volume of the ending combustion chamber...the value that I didn't have. I used the known later model comp ration value of 9.2:1 to "TRY" to backwards calculate the vc. Came up with a value that when plugged back into the original formula yielded the OEM comp. ratio.

I then plugged in the newer bore value for the big bore kit(1179cc), and ran the numbers. Got a ~9.5:1 value. I then realized that the larger bore would also increase the resultant Vc combustion volume value..did a simple % increase of that value from the ~3% increase in bore size over OEM( I know the value calculation is much more complicated than that), but the resultant calculation didn't vary much from the 9.5:1 and so I'm thinking that it might be right or at least fairly close, both since the pistons are in the later larger comb. chamber sized HEAD, and would then "ASSUME" that when used in the smaller comb. chambered 78-79 head that the value "could" be closer to their advertized 10.25:1 ratio!?!? YMMV!

T.C.

[crazy steve]

...One problem I found with the higher comp ratio of my XS with the thin copper gasket is the starter motor struggles when the engine is hot, the engine is a well used loose free revving engine, but if you restart after just turning the engine off the starter motor stuggles. Obviously a good battery helps. Something to consider when altering comp ratios..[/COLOR]

That's a very real issue; I know that my '93 Sportster (with the compression ratio bumped to 10.5:1) doesn't like to turn over as easily as it did with the stock 9:1 ratio. You can mitigate this to a degree by installing longer-duration cams (reducing cranking pressure), but that will change the power curve a lot, not to mention the added expense of new cams. Expect a somewhat shortened starter motor life...

Another thought is compression releasers; many of the big-inch, high compression V-twins use these on one or both cylinders for starting. They now have automatic units; you 'release' them manually for starting, then they close when the motor starts.

[Mathh]

Expect a somewhat shortened starter motor life...

I burnt the stock starter motor of a Honda CBX1000 when starting for the first time after I rebuilt the engine, due to the slightly higher compression. i had to swap the stock 0.8Kw starter for a 1.0Kw one. But then on the cbx everything is UNDER dimensioned ...

[WMarshy]

Any thoughts about being able to turn 8500 rpm with these pistons? Cast ones are a little heavier right? I'd definitely be replacing crank bearings as part of the build. I imagine the crank will have to be rebalanced as well.

[trbig]

When a crank is balanced, it is put on a machine by itself and balanced, and has nothing to do with the choice of pistons. The pistons are all going to be the same weight and counteract each other with two going up and two going down. You're replacing stock cast pistons with cast pistons. Put new crank bearings in and go.

The biggest part of it is to choosing a good machinist that will bore these old cylinders, but has been around the block enough and knows enough about boring to take into account that as the bit used heats up as it cuts, it expands a bit. So what ends up happening if they aren't careful, is the bore at the bottom becomes wider than the bore at the top.

[TomB]

knows enough about boring to take into account that as the bit used heats up as it cuts, it expands a bit. So what ends up happening if they aren't careful, is the bore at the bottom becomes wider than the bore at the top.

A really decent engine tuner will hone the bore after boring, measuring piston/bore clearance at the top, middle and bottom of the bore until correct.

When asking an engine tuner/machinist to bore your block take your pistons along for him to reference from, and tell him what clearance you want. 2 though for road engines and 3 to 4 thou for racing engines is normal, the 4 thou probably relates to the forged racing pistons as well as the higher engine temps of racing.

Cranks are dynamically balanced using weights a certain percentage of rod and piston weight added to the crank when spun up.




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[WMarshy]

I already have a good machinist lined up that does good work. I've had him do engines for me in the past and will do as much or as little assemby/work as I want. I might even get the chance to look over his shoulder through the entire process if I have the free time. He's a one man band and does top to bottom, inside to out work on everything from old Franklin's to Cat dozers. I'm going to approach him with some questions and see what he says. I want to see if he has a flow bench for heads. I'm not sure if he does port'n polish but would be interested if there is anything that can be had. Street performer cams would have to come the following year...

[TopCatGr58]

Original pistons are forged to go along with the radiused high performance shot-peened rods and chrome-moly rings. All that attached to a one piece forged crankshft attributes to the longevity of these air-cooled motors, even with the abuse they have recieved over the years.

You're replacing stock cast pistons with cast pistons. Put new crank bearings in and go.

I reviewed the many motorcycle magazine review articles on here, and they all say standard 3 ring pistons....they do talk about the crank bearings, but don't mention anything about the pistons being forged.

I remember there was at least one fellow on here that actually worked as a Yamaha mech and such, would like to hear from them whether they can confirm the Forged vs. Cast pistons manufacturing??

Motoman, I can't recall your history exactly, think you raced off-road bikes and such, but not sure....just asking if there is any documentation or such that you can share with us to verify that the pistons are forged?

T.C.

[WMarshy]

I bought one!

So, I made the plunge and bought the kit! Its the first step in the right direction...I think. And if your wondering, I bought flowers for SWMBO prior to asking if I was "allowed". Lol

I also looked at what crank bearing cost on ebay... yikes. I wonder if they can still be bought at the dealer..?

[jimbyjimb]

You crazy kids have a serious need for speed, talking about big bore kits and lopey cams and port/polishing and such. I thought this bike was beastly enough as is, especially with the tractor gearing.

Then there's the turbo bike that got posted, for God's sake.

You guys are nuts! Thanks for the good read, it's facisnating to see what you can do with these little engines!