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Big Bore Build - Part 1 - Cylinder (Read 182 times)
DragBikeMike
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Big Bore Build - Part 1 - Cylinder
06/30/20 at 18:37:55
 
This is the first in a series of reports outlining a big bore engine project. †The project will incorporate a 97mm bore with a modified cylinder head (my Stage III). †I will be building a spare engine so I can simply swap out engines and test.

I intend to judge the initial results based on cranking pressure, ease of starting, required fuel, drivability, audible noise, operating temperatures, seat-of-the-pants power, and timed acceleration runs. †If the results of the initial evaluation are good, I will continue to run the big engine to see how reliable it is.

The foundation for the big bore engine is a used lower-end assembly I purchased on eBay. †It is a five-speed unit form a 1997 LS650. †Getting that lower end to Hawaii was an interesting project in itself.

The cylinder is another eBay treasure. †I have no idea how old it is, but it was in very poor condition with a scored bore, bent & broken fins, cobby sealing surfaces, and years of baked on crud. †It will be matched to a Wiseco 97mm piston assembly, part number 4597M09700 (originally designed for the DR650).

The cylinder head is the take-off from my 2016 LS650. †It only had about 3000 miles on it when I replaced it with my Stage II head. †Itís in excellent condition and will be a good foundation for the Stage III mods. †The Stage III mods will include larger intake valves along with reshaped intake runners, and enlargement of the exhaust port to a full 1.79Ē to match my Mac header. †I will flow test the finished head and post the results.

Target compression ratio (CR) will be 10:1. †I expect that combustion chamber volume will be reduced slightly by the larger intake valves. †The cylinder will be trimmed or shimmed as necessary to achieve a static CR of 10:1.

Low-lift flow will be improved with the Stage III head, so I have chosen the Web 402 cam. †It has significantly less duration than the Web 340b (cam I am currently running). †The 402 should compliment low end torque. †The high end characteristics should remain similar to my current setup due to the enhanced cylinder head flow. †It will be interesting to see how it shakes out. †The 402 cam requires heavier springs with more travel. †Valve guides may have to be trimmed. †Valve-to-piston clearance must be verified. †The 402 is not a bolt in cam.

I will use acceleration benchmarks from my tight-quench engine and the 94mm Wiseco engine I am currently running. †The tight-quench engine was quite a bit faster than the Wiseco pop-top engine. † Comparative 2nd & 3rd gear timed runs on the completed 97mm engine should give us a good feel for how the big motor stacks up.

Letís get started.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #1 - 06/30/20 at 18:38:46
 
The Wiseco 97mm piston kit includes the forged piston, a ring set, a light-weight tool-steel wrist pin, and circlips.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #2 - 06/30/20 at 18:40:37
 
The 97mm Wiseco piston provides three significant advantages over the stock piston.

First, it is forged. †The forged material is significantly stronger than the stock cast material.

Second, it has a slightly taller compression height (about .025Ē) combined with a pop-top (.060Ē). †The taller compression height and the pop-top increase the compression ratio.

Third, the larger diameter piston increases the engine displacement. †An increase in displacement without a corresponding increase in combustion chamber volume will also increase compression. †Itís a double whammy, more compression from the pop-top plus more compression from the increased displacement.

So, by simply installing the 97mm Wiseco you bump up displacement by 43 cc and you bump up compression by about 1.2 points. †You end up with a 695cc engine with a compression ratio of about 9.5:1. †Itís all good.

Hereís a look at the stock cast piston next to the forged Wiseco. †Note the smaller, stronger, and lighter Wiseco wrist pin. †Also note the little mini pop-top and the nice valve reliefs. †Those valve reliefs permit installation of high lift cams with lots of overlap.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #3 - 06/30/20 at 18:41:31
 
Negative aspects of the Wiseco are weight and noise.  The 97mm Wiseco assembly is 11 grams lighter than the stock assembly.  That will probably affect balance a little.  I am currently running the 94mm Wiseco.  It is a whopping 52 grams lighter and it shakes a lot.  I suspect the 97mm vibration level will be significantly lower than the 94mm, probably close to stock vibration levels.

This shows the two pistons from the bottom.  Note how the stock cast piston has a full circle skirt while the Wiseco forged piston has a partial skirt.  Iím not really sure if the partial skirt has any advantage over the full skirt, but Iím confident the stock cast piston runs a lot quieter than the Wiseco.  You can run tighter clearance with the cast piston.  The full skirt combined with the tighter clearance results in far less piston slap and the associated noise.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #4 - 06/30/20 at 18:42:31
 
You canít build a 97mm engine without a 97mm cylinder.  So, to me, the logical place to start was boring out the cylinder.  If I canít get the cylinder bored, I canít build the engine.  Iíve always wanted to try and bore a cylinder but have also been told that itís not a job for a lathe.  Lotís of folks have told me that you must use a cylinder boring machine.
 
My used cylinder was a real jalopy.  No big loss if I screw it up.  Why not give it a try on the lathe?  Learn a thing or two.

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Re: Big Bore Build - Part 1 - Cylinder
Reply #5 - 06/30/20 at 18:43:15
 
Broken & bent fins.  Corrosion.  Beat up sealing surfaces.  It was nasty.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #6 - 06/30/20 at 18:44:10
 
A rigid setup would be needed to maintain a straight bore.  That would require a heavy-duty boring bar.  Some 1.5Ē aluminum hex stock did the trick.  This thing was solid.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #7 - 06/30/20 at 18:44:54
 
A 1.5Ē torque plate was modified to attach the cylinder to the face plate on the lathe.  It worked well.  It was easy to get the cylinder running true and it was solid.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #8 - 06/30/20 at 18:45:40
 
A workpiece this size must be turned at a low speed.  The speed was set to 200 rpm.  Feed was set at .15 mm per revolution.  That speed and feed produced a decent finish.  It took 5 minutes to make one pass.  The depth of cut was about .004Ē per pass.  For the final pass, the feed was reduced to .075 mm per revolution to achieve a better finish.  If you ponder that feed and speed stuff and the length of time for each pass and the depth of cut, you will probably figure out that I was standing in front of that lathe for a very long time.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #9 - 06/30/20 at 18:47:34
 
These short videos of the hobby lathe in action will give you a feel for how the thing performs. †Itís a pretty old lathe and itís pretty beat up, but it got the job done.

https://www.youtube.com/watch?v=bvnpV2cQ36A&feature=youtu.be

https://www.youtube.com/watch?v=X1I3fn-3aY8&feature=youtu.be
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Re: Big Bore Build - Part 1 - Cylinder
Reply #10 - 06/30/20 at 18:48:19
 
Since the cylinder was in the lathe, might as-well clean-up the cobby base gasket surface.  Just a quick kiss with the tool bit and it was in good shape, perpendicular to the bore too.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #11 - 06/30/20 at 18:49:42
 
You want to stop boring about .005Ē under the finished size.  The Wiseco 97mm piston skirt is 3.8175Ē and I was shooting for .0025Ē clearance (a bit more than Wiseco recommends), so my finished size goal was 3.8200Ē.  I stopped boring the cylinder when I reached 3.8140Ē.  Plenty of wiggle room.

Now it was time to hone the cylinder to achieve the proper finish and size.  You need a way to hold the cylinder without clamping it radially.  If you clamp the cylinder radially it will distort the bore and you will ruin the cylinder when you hone it.
 
The cylinder also must be clamped between torque plates to simulate the stresses imparted on the cylinder by the normal clamping force of the cylinder studs.  On this job, I found that to be particularly important.

The solution was a cradle that attached to the torque plates.  Now the cylinder could be clamped in the torque plates and suspended in the cradle.  A drip pan was set up under the cradle to catch all the slop.

This is not a job for a spring hone.  You must use a rigid hone with a micrometer feed, and you must continuously monitor the size as you hone.  Use coarse stones to get within about .0015Ē of the finished size, and then switch to fine stones to bring it home.  Lots of honing oil is in order.  A mixture of nine-parts kerosene to one-part motor oil does a nice job.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #12 - 06/30/20 at 18:52:37
 
Periodic measurements with an inside micrometer and dial bore gage assure you stay on track.   If you start seeing a taper develop you can favor the small area of the cylinder with the hone.  Turn the cylinder over periodically and switch honing direction (top to bottom, bottom to top).  Go slow and continually take measurements.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #13 - 06/30/20 at 18:53:26
 
Back up those measurements with a feeler gage check to make sure itís right.
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Re: Big Bore Build - Part 1 - Cylinder
Reply #14 - 06/30/20 at 18:54:10
 
The drip pan is a must.  Wet honing is messy.
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