Unused original stock R3Y cylinder heads with spark plug hole on the top rather than the end. Designed for 18mm spark plug. Will fit on other R3M engines but will look slightly different. The exhaust port opens out to a larger size internally but this is not an issue. The height of the head where the studs go through is 5mm shorter on this head compared with other R3M cylinder heads. Depending on the studs fitted to your engine, you may need to extend the thread by a couple of mm or add a washer under the nuts.