Partzilla Live Q&A with John Talley – February 26, 2021
Partzilla Live Q&A returns after a brief hiatus, with John Talley answering your motorcycle and ATV repair questions.
Engine rebuilds and crankshaft parts came up a few times during the session, particularly for Yamaha powersports vehicles, and as always, the ever-popular TRX 400 Honda ATV. Check out excerpts from Partzilla’s latest live Q&A session below, and tune into our YouTube and Facebook channels every Friday at 3pm Eastern for the next edition.
Partzilla Live Q&A – Engine Rebuilds, Crankshafts, Bearings
I'm getting ready to do a top end rebuild on my Honda TRX 400 EX. Is there anything in the valve train I should automatically replace?
John Talley: I’m betting the valve train might be ok. When you pull it down and take out the seals, you’re definitely going to want to replace the valve stem seals. But just get in there and feel it. If you put the valve in there with the spring off, and it rocks around a pretty good bit, you probably need to do something about that. Plus, it’s very difficult to measure the inside of the actual valve stem bore, but you can take a micrometer and just take a peek at the diameter of the valve stem itself to make sure it’s within spec. And for that, you’re going to need to reference your service manual, not your owner’s manual. One more thing. Look at the wear pattern on the face of the valves and see if it’s the correct thickness, and in the right orientation as far as the inner and outer. You want to head toward the center of the valve face itself, and that’ll give you another metric by which you can decide whether you need to replace the valves or not.
Is there anything to do after rebuilding a Yamaha Raptor engine to get the oil up quicker so it doesn't run without oil for a bit?
John Talley: Well the biggest thing when you’re doing an assembly is to actually use assembly lube on all your different bearing surfaces. Beyond that, what I would typically do is I’ll crank the engine over five, ten, fifteen times with the spark plugs out just to prime up the oil system, especially if I’ve gone all the way down to the crank and everything is completely disassembled. It has to re-prime, so to speak, even though the oil pump is hopefully sitting in a pool of oil down at the bottom of the engine. But if you did not use any assembly lube, then that’s what you need to do. Just let the engine crank over and then fire it up.
I'm replacing a crankshaft on a Yamaha R6. Do I have to replace the bearings on the case or just the ones on the rods?
John Talley: I assume you’re talking about the bearings on the crankshaft itself. If you’re having to replace the crankshaft, go ahead and do the bearings. Don’t try to reuse anything, especially anything that you may have to press out. … just the amount of labor it takes to get all the way down to the crankshaft, I’m going to go ahead and replace just about everything. I wouldn’t try to reuse any of it.
Want to see more live repair questions answered? See all of Partzilla’s Live Q&A sessions here.