Dyke Rings Change Things?

[Vintageman]

It seems to me if you have an engine that did not have a Dyke ring stock, using a piston with a Dyke ring could change port timing and if all else the same raise compression. On the later point if the piston body is the same exact shape including dome it could raise compression. I have a set of older Suz 250 wiseco pistons with dyke ring, I noticed the dome was not as tall as OEM piston dome. Curious if this was a mistake or done to ensure compression remains the same as stock. I am hoping it changes the timing making the exhaust port have a little less (milder) duration.

What do you think?

[Alan H]

The Dykes piston rings don't affect port timing at all. They give slightly reduced ring/barrel friction on the upstroke. Very popular 'back in the day' less common nowadays as they are more expensive to make than 'normal' or keystone rings.
Really, only useful on racing engines where the last bit of power makes a difference.

The piston rise and fall, crankshaft or piston pin offset, and port positions alter port timing, so unless there is a height difference from piston pin to piston top, the timing will be the same and so will the compression.

[Vintageman]

there is a height difference....

The example pistons I am talking about have the same piston pin height to top edge of piston. What is different is the location of the top ring. On a Dyke piston it is significantly higher (i'll measure, but maybe 2 mm). So to me this ring will expose the exhaust port at a different crank angle. I realize we consider the top edge of the piston when estimating port timing be it a ring there or not, but to me the ring edge is the better true seal (piston has smallest diameter at the top). So I would think there could be a slight timing difference or at least the dyke expose the port much cleaner and with a more of a "snap".

Also as I mentioned the dome is not as high reducing compression , but since the Dyke is higher up it seals the gap between the piston and cylinder versus the standard ring which is mm below the piston edge. I think there is a difference. How much I am not sure.

Others have experience or opinions here?

UPDATE. I really need to check pin to heights with Dyke ring. I attached from Jennings book what I think he says Dyke ring could affect timing, but like said the real height is what counts. Still curious about compression influence


"On some engines fitted with Dykes rings, the top piston ring and not the piston crown controls the opening and closing of the exhaust and transfer ports. With these engines, the exhaust duration is calculated using the same formula, however dimension C (the deck clearance in mm) must be very carefully measured using a depth gauge otherwise your calculations will be several degrees out. In engines where the Dykes ring actually determines the port opening and closing, dimension C is the distance the ring is below the top of the barrel at TDC. Referring back to FIGURE 3.5 you will note that the Rotax kart engine appears to have mild porting for a road racer. This engine, in fact, has a single Dykes ring located very close to the top of the piston. Dimension C is 1.8mm, so what looks like motocross porting is truly road race porting. In this case the exhaust duration is 201°."
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[Vintageman]

When I compared T350 pistons OEM and old wiseco with L ring at top, had them side by side on same wrist pin, the top of L ring was actually lower than top edge of OEM piston by 1/2 mm maybe a little more. And, the dome was not a steep a stated before.

Well I was hoping L ring would have been higher. The wiseco will make porting open time a little longer. But the less dome peak would drop compression.

[D&D CHASSIS]

Kinda splitting hairs, don't you think?

[Vintageman]

I measure and it is 0.060" lower. That is the top of the of the L ring edge is 0.060" lower than the top edge of the OEM piston. It is more complicated than that for the wiseco piston top dome material is about 0.060" higher at that the top of the L ring, but that material is back a ways from the cylinder liner edge -thickness of the L ring top + some more (I'll try to get a good pick). It is significant distance and once the top edge of ring is past EX port she'll be a flowing through that space rapidly.

I can see why one is to consider the L ring top edge for timing and the top edge of the piston when the ring design are like to OEM type

0.060" is one thick piece of hair. I raised my T305 Ex port 0.080" (to ~31.2 mm if I recall) and was more than I liked. I was hoping to find the wiseco L ring versus OEM was other way around so I could have taken back some of that 0.080".


I have some OEM GT380 pistons and older wiseco for the same with dyke L ring. I am going to dig them up and see if same deal.

Subtle difference? I bet it is noticeable if you chose to use these wiseco pistons on these bikes which only have ~ 33/32 mm Ex port heights stock.

[Suzukidave]

I had a set of the old type Wiseco pistons with the L shape rings in a GT750 and after opening up the engine again years later i was very surprised how gummed up and stuck the rings were in the ring grooves . I guess as commented on these type rings must be for racing with engines that get disassembled regularly . I have read that the Keystone type rings are better at self cleaning .