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I've used aluminum center seals in all sorts of multi cylinder Kawasaki and other engines for decades. Built my first ones in the early 1980's, after the Yamaha design, and no problems. As far as I am concerned, they are the way to go, with some 'engineering' along the way. I always cut the OD of my aluminum seals to the same exact OD the main bearings on either side of them are, so crush is same for the seal and bearing, taking into consideration that the cases can be worn. That is where the O rings help.
A good check of both the main bore diameters, and OD of the seal is mandatory, to make sure there aren't crush issues past what a main bearing does to the stock, clean, unworn main bore.
I always use O rings on the OD's, they help allow for a light misalignment of the seal to center race, and they help seal the seal to the case. I have also used Three-Bond 1104, and, 1194 on the seal OD's that do not have O rings, never had a binding/growth issue, never had a seal/crank fail from adding the sealer.
I like to add a Yamaha feature, a dowel pin to the upper case (Yamaha cases load into the bottom case, not the top), along with a holder hole in the OD of the center seal, to help let it stay aligned, and in place. Yamaha race type twin and four cylinder horizontally split two stroke cases use a ball bearing receiver to hold the main bearings in place, and a dowel for the center seal as well.
I always build my seals like the Yamaha ones, and do not waste cut (back cut) the area between the outer area of the seal 'lip" to the edge of the seal. I see some of the currently available seals have radical undercuts between the seal area and outer edge of the lip, and I feel this does not help seal integrity, only makes for a seal that, if stays in contact with the inner bushing, can over load the side of the seal lip, to failure.
An aluminum center seal can ride against its inner bushing/collar, with no adverse effect, as long as the seal itself isn't worn excessively, allowing too large a seal to bush clearance on the other side of the seal.
Crankshaft alignment is also absolutely critical in allowing aluminum seals to live. A crankshaft out of true a half, thousandth, can side load an outer seal lip to serious wear/failure.
I have also used other outer seals in some applications, building an aluminum holder that carries two outer seals smaller than the OD of the stock main bore, on either side of the holder. This works well, too, but, is not as long lived as a full aluminum center seal is.
The original H1R's used a two seal setup for center use, back to back, two lip, positive pressure against the crank. This differed from H1 E# 8801 and later. The early H1 street bike seals were proprietary, they used crankcase pressure to push the seal lips against the crank bushing, and were not in contact when the engine was stopped. ALL factory H2R cranks, and later stock street bike H1, after E# 8800, and, all street H2's, used positive pressure seals. These seal lips were always in direct contact with the center bushing, engine running, or stopped.
Just info to consider.
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