That's exactly it, an HB .61 PDP. It came free with a ragged Hawker Huricane someone wanted me to get out of his way. He thought it was worn out due to the dykes ring feeling like there is no compression while sitting. I tore it down went to order parts and got educated by mecoa! The muffler was aslo a freebie, A friend had it lying around at random. Told me it if fit it was mine. I did have to order a carb from perry though as the old one was missing too many parts. Great engine though, just cant seem to slow it down enough to get her to STOP pulling planes on the ground.