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Honda Strokers

I've made a few three and four mm strokers similar in design to what you mention though not entirely without a spacer plate. They deliver the kind of power you might appreciate for GNC MX or TT racing. In fact one of the racers has won a few "A" class races in MX and TT on the same Honda.

Each stroker combination I've built has been unique enough that almost none of the design parameters are copied from one engine to the next. There's always different rider weight, or track length, budget or any number of other considerations to work with. It's because of this I can't tell you to simply drop in a +(X) mm stroker and use "X" piston with "X" other parts for the best set up. I can tell you a couple of general trends though. All stroker engines want to take advantage of a longer power stroke and inherent displacement advantage which translates into horsepower. The added stroke length is almost never only added to the power stroke alone, though for much improved bottom end it can be split in such a way so that it favors that part of the stroke. It's usually best to build a stroker motor from a barrel that hasn't previously been ported though if it has, the best results might be found if the transfer port height had not been changed very much or at all. The "drop in" stroker automatically adds transfer duration - sometimes more than is desired which could lead to a motor that accelerates rather sluggishly.

Actually, the price you mention for the crankshaft is about right but around here includes just that part. Relieving the cases, a new combustion chamber and porting adds to that total considerably. You'd be lucky to get away with spending just twice that amount. The options available to race motors is almost endless and can easily double the price of what is already twice as much as it seems going in. The crankshaft price is just the teaser. There should be a law against advertising stroker cranks without mentioning the actual "everything else needed to be up and running" price in the same sentence. I can think of at least one company that repeatedly sells the parts without mentioning to the customer what will need to be done before the part can be used - and they do it without shame. Buyers beware.

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A couple of other things have to be considered here - I'm thinking of the ratio of the piston stroke to the connecting rod length. For a 1986 "short rod" TRX250R with a stroke of 72 mm and a connecting rod length of 125.3 mm the ratio is 1.74:1. The later model '"long rod" engines have the same 72 mm stroke but use a 130.3 mm connecting rod which gives a ratio of 1.809:1. With this baseline data some comparisons can be made. It should be noted that the short rod engines produce more side load than ideal, resulting in premature piston and bore wear though partially due to the shape and size of the inlet port. Also, the short rod tends to be more of a "hot rod" engine in that it is more peaky with its power delivery. It should also be noted that CR250R engines produced before the TRX was introduced through current models all use a short rod combination.

A +3 mm stroke increase (to 75 mm's) using the long rod gives a ratio of 1.737:1 - almost the same as the standard stroke short rod TRX's. Even though it uses the long rod it becomes a short rod style crankshaft due to its stroke increase. A +4 mm stroke increase (to 76 mm's) yields a ratio of 1.714:1 creating even more side loading than the short rod standard TRX. It is because of this that longer connecting rods are used. For example a +4 mm stroker needs a rod length of 137.5 mm's to get the same ratio as a standard long rod TRX motor. Larger stroke increases require longer connecting rods to keep the stroke length to rod length ratio in check. A +6 mm stroke increase (to 78 mm's) yields a ratio of 1.762:1 with a 137.5 mm rod, putting its ratio in the middle of the short and long rod versions of this engine.

When both the stroke and rod length are increased it is almost always necessary to use a base plate spacer to keep not only the port timing "in check" but to prevent the piston from going so far above the bore that the top piston ring would be exposed. Some stroke length increases / piston / connecting rod combinations work well together, some do not. Some of these combinations are highly protected secrets of engine builders - and for good reason. It takes a lot of time and effort to develop such combinations. Many times connecting rods must be narrowed at the top or bottom to work in an engine. In some cases the big end must be reworked to reduce its bulbous shape to turn it into a flatter more appealing piece. When this is done it is typical to machine, shot peen and polish the rod to gain back strength which has been taken away with the reduction of material. These modifications can allow a rod to yield the same strength after the modifications as the same rod before they were done. Additionally, since such procedures yield strength increases good engine builders will perform shot peen and polishing modifications to all stroker crankshafts.

I have had some unusual experiences with some short stroker motors. For example I have a set of engine cases that will accept a 3 mm stroke increase using the 130.3 mm connecting rod with no modification to them at all, though only a couple of other cases I've put this same stroker piece in to have accepted it without clearance modification. The +4 mm increase parts using the same connecting rod will clear the cases (no trenching necessary) but the cases do need to be clearanced as mentioned above. Increases to the stroke which are greater than 4 mm's require trenching. When cutting clearance into engine cases for stroke increases I typically use .015" as a minimum though I have used .010" successfully. OEM machinery tends to have at least .020", in some cases more but seldom less.

Offset pins are interesting though they're typically only used in cases where the machinery must not be modified, for example, in stock racing classes. With parts like this the stroke can remain the same or be altered very slightly but the phase timing (relative to the ignition timing) can be drastically changed often yielding tremendous power gains and still "look" legal.

I've been around and around various 295's with head gasket problems as I'm sure many others have. These days I like to eliminate the problem by removing the head gasket - with no head gasket it can't blow! Careful machining / lapping of the deck surfaces along with Viton and Silicon o-rings as well as a stainless steel o-ring puts the problem to bed permanently.

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