My Universal M4-30 has been getting more difficult to start. After talking with a couple folks about a related issue I realized it was time to replace the glow plugs. I got a price from the regional Universal Marine parts distributor, $75 each plus shipping and a rather large packing fee. I've never had to buy glow plugs before but this seemed a bit steep for a fairly simple part made to fit a common Kubota tractor engine with a few marine parts bolted to it and a coat of metallic blue paint. Yesterday I went out to the boat and pulled one of the plugs. After scraping off the blue paint I found that it was made by NGK and the part number. They can be had through Amazon for less than $7.00 each. My local NAPA store had them for me this morning, $35 for 4. I can see paying a fairly high mark-up on a low sales volume specialty item for the marine market but nearly 10x the fair retail price for something so common place?