The cylinder head is the heart of the system.  The amount of air admitted to the engine is dependent on the ability of the cylinder head, and its associated ports, valves, and combustion chamber, to flow freely.  You can install all sorts of high flow components like air filters, carburetors, exhaust pipes and mufflers, but the engine’s ultimate performance will always be limited by the capability of the cylinder head to allow maximum flow.  It’s easy to add more fuel, but difficult to add the additional air required to burn the fuel effectively, and at the correct air/fuel ratio to achieve maximum power.  Therefore, power is limited by the amount of air introduced into the cylinder.

We currently don’t have a lot of data on the LS650 cylinder head.  We know that it has this funny plug that tends to leak, but I haven’t seen any info related to combustion chamber volume, port volumes, valve sizes, valve-to-valve clearance, maximum permissible valve lift, valve spring seating pressure or spring constant.  This post is intended to provide pertinent data on the LS650 cylinder head, so we can make informed decisions about modifications we intend to perform.  This data will help us plan for mods like high lift camshafts, increased compression ratio, larger carburetors, porting, increased valve size, exhaust systems, etc.

Valves

Intake valve head diameter: 33mm (1.30”)
Intake valve O/A length: 98mm (3.855”)
Intake valve face to keeper groove centerline 94mm ((3.700”)
Intake valve keeper groove diameter: .225”
Intake valve stem diameter: .2742”
Intake valve weight: 49 grams

Exhaust valve head diameter: 28mm (1.10”)
Exhaust valve O/A length: 91.5mm (3.600”)
Exhaust valve face to keeper groove centerline: 87.3mm (3.435”)
Exhaust valve keeper groove diameter: .223”
Exhaust valve stem diameter” .2737”
Exhaust valve weight: 43 grams

Springs

Spring free length (inner): 1.445”
Spring OD (inner): .825”
Spring ID (inner): .625”
Wire diameter (inner): .100”
Spring weight (inner): 14 grams

Spring free length (outer): 1.640” – 1.650”
Spring OD (outer): 1.155”
Spring ID (outer): .875”
Wire diameter (outer): .137”
Spring weight (outer): 36 grams

Spring assembly installed height (intake & exhaust):  1.300”
Spring assembly force at installed height:  52-55 lbs.
Spring assembly force at .950”: 136-139 lbs. (.350” lift)
Spring assembly constant: 24 lbs./.100”
Spring assembly coil bind height: .849”  (travel to coil bind 1.300-.849 = .451”)
Spring assembly weight (inner + outer): 50 grams
Spring retainer weight: 16 grams
Spring retainer keeper weight (per set): 2 grams

Let’s discuss the importance of the various data.

The valve dimensions are useful in purchasing replacement valves.  For instance, if you want to install a valve made from titanium rather than stainless steel, or a valve with a larger head diameter, the dimensions will help you find a valve that will fit the LS650 cylinder head.

Valve weight is necessary to calculate applicable spring rate for the maximum rpm you intend to use as a limit.

Valve-to-valve clearance is important on some engines, but as it turns out, not an important measurement on the LS650.  On some engines, for instance Harley Davidson, the intake and exhaust valves are situated close to each other in the at rest position.  During overlap, when the exhaust valve is closing, and the intake valve is opening, the valves can run into each other if the cam produces too much lift at TDC.  The LS650 geometry is such that even when both valves (intake & exhaust) are opened to .400” (something that will never occur), there is still .136” clearance between the valve margins.  We will never have to worry about installing a camshaft that causes valve-to-valve interference.
 
“Valve margin flush point with cylinder head surface” gives you an idea as to how much valve lift will result in the valve margin moving below the head surface.  If you want to setup the engine with zero deck height (i.e. top of piston flush with top of cylinder at TDC), then you must be concerned with collisions between the valves and piston at TDC during overlap.  

When selecting a camshaft, you need to be interested in what TDC lift the cam will produce and be aware that if that TDC lift exceeds the flush point you may have to increase the depth of the valve reliefs in the pistons.  If you intend to use a piston with a raised dome (pop-top), like the Wiseco, the problem may become worse.  From what I understand, the LS650 piston doesn’t come close to the cylinder deck at TDC (I recall one forum member stating, “you can measure deck height with a tape measure”.  Very humorous analogy.).  However, most tuners in search of good horsepower numbers try to run around zero deck with a resultant .030” to .040” quench clearance (the head gasket compressed thickness), a condition I intend to pursue.  If that’s the cool aid you want to drink, then you must pay attention to where the valves will be during overlap.

Valve margin to seat OD gives you some sort of idea how much seat material is available for installation of oversize valves.  Usually, oversize valves require installation of oversize seats, but it’s my understanding that sometimes there is enough material in the standard seat to allow installation of slightly oversize valves in the existing seats.  Given the fact that there is about .060” between the stock valve margin and the edge of the seat, it looks to me like it might be possible to install valves that are 1mm or 2mm larger.

Exhaust & Intake valve margin to margin distance allows you to evaluate whether there is room for larger valves.  Looks to me like the LS650 has plenty of room in this department.

Port diameters are useful in evaluating installation of carburetors and exhaust pipes.  It doesn’t make a lot of sense to install a 45mm carburetor on a port with a 42mm opening unless you intend to modify the port.

Port volume is useful to monitor port modification.  Keeping the flow velocity as high as practical is a noble endeavor, since that will maintain good throttle response and drivability.  If you get out of hand with the grinder, and increase port volume too much, the drivability will go in the toilet.  You might end up with a killer top end at the expense of being able to ride the bike to that lonely road where it’s safe to go WOT.

Knowing what the stock port volume is will allow you to monitor your modifications.  I imagine small increases in port volume are perfectly OK, for instance, cleaning up casting imperfections and blending in steps at valve seats may increase volume a few cc.  The resultant increase in volume and possible decrease in velocity should be more than offset by the improvement in flow.   However, if you start making dramatic changes to the port, like raising the top of the port to improve the angle of attack to the valve, you might want to build up the bottom of the port to restore volume to something closer to the stock volume.  That will help to maintain velocity.  I assume more port volume will result in higher power at higher rpm and reduced low and mid-range power.

Combustion chamber volume is necessary to calculate compression ratio.  The advertised ratio on the LS650 is 8.5:1.  With a 57cc combustion chamber, a 1mm head gasket (7cc volume), 652cc displacement and an assumed deck height of zero, the compression ratio should be about 11.2:1.  But then there’s that negative deck we were discussing earlier.  That negative deck along with the head gasket become part of the combustion chamber, so instead of 57cc the chamber is more like 87cc (when the gasket volume and unswept cylinder volume are added in).  So, you end up with 87+652/87 = 8.5.  Fiddle around with those numbers for awhile and you will figure out that the unswept height of the cylinder is about .130”.  Add the gasket thickness to that and you end up with .170” quench clearance.  Ah yes, the tape measure could certainly be used to measure that.

Checking valve-to-valve clearance is accomplished by setting the valves at the desired lift and then running numbered drill bits between the margins.  In the case of the LS650, I had to open the valves all the way to .400” before I could get them close enough to each other to take a reasonable measurement.  The valve-to-valve is never gonna be a problem on this engine.

Valve retainer to seal clearance is measured with the retainer & keepers in place.  Pull hard on the retainer to close the valve and seat the keepers, then measure the distance from the underside of the retainer to the guide seal.

To measure spring installed force, set up a good bathroom scale (the strain gauge type is preferred) on a drill press.  I use a large metal plate beneath the scale to make sure that it is uniformly supported.  Then place an ample metal plate on the scale platform so that the spring force is not concentrated in a small area, the plate distributes the load.  Place the spring assembly close to the center of the plate.  Zero out the scale.  Then apply force to the top of the spring and retainer until the spring is compressed to the installed height (in this case 1.300”).  Read the scale to record force at installed height (55 lbs.).  Then continue to compress the spring to a height representative to full open (I chose .950” which equals .350” lift) and take a reading (this spring exerted 139 lbs. @ .950”).  The second reading not only gives you an idea of how much force is available at full lift, but also gives you a second data point to be used to calculate the spring constant (84 lbs. increase over .350” compression works out to 24 lbs./.100”).