I posted here because I wanted more exposure to my question. I am hoping someone else here has had this done and can help me. I have a excess clearance problem with the mains on my 425 Nailhead. The rods are .002" and I'm ok with that. But .003" MB clearance is too much. I don't want to take the crank out and have it re-ground. The machine shop that messed up suggested to me that I have my .010" undersize bearings "teflon coated" to take up this extra clearance. He said that if they end up too tight, I could adjust the clearance by using a coated one on one half and a un-coated one on the other half from an extra set. I don't know if I like that idea. What I need to know is, how much thickness will this add to my bearings and is this a good idea? The place that coats the bearings is called Polydyne and is located in Houston, TX. Thanks for any input on this subject. :bglasses:
Good luck with oil pressure Cause the coating will not do what you want it to. And the coating does NOT last, then what? They have got to tighten up the clearences on the mains.
are you experiencing noisy startup? or low oil pressure? just curious as to why you need to change? if it was a 455 the answer is obvious to me but a nailhead is a much more forgiving engine. smaller main journals etc..just curious. Bob
Bearing coatings are not designed to tighten up clearances. The bearing coating is normally only 1/2 of a 10 thousandth in thickness. There are coatings that can build up, but these are more designed for piston skirts. I would not worry about .003 on the mains, coat the bearings all the way through anyway, rods and mains, it's a great idea. And for the record, the coating is moly , not teflon. Your only other option would be to have the crank turned to .020 and change bearings. The nailhead does not need anywhere near the oil pressure of a 455 to live. That engine would live forever at 45 psi oil pressure. Jim Burek
Cold startup is fine. I only heard a slight noise once over the summer on our way to flint on a hot start. Later when I changed the oil, until the new filter filled it knocked pretty loud.....but after about 4 seconds when the pressure built up, it quieted down. [The nailhead does not need anywhere near the oil pressure of a 455 to live. That engine would live forever at 45 psi oil pressure. Jim Burek] I wanted to get it done in time for Flint last year so I let the extra clearance slide. I'm runnnning 10w-50 synthetic, and the pressure is good, even when hot. I guess the real problem is that I know it's there and it bothers me, but maybe I should just live with it. Thanks for the replies, guys.
You know if the main saddels are on the loose side of the spec. you can tighten that up to the tight side even if it is on the tight side i have reduced them to .0005 under the tight to get a clearace wright. mike
I used the Tefon coated grooved cam bearings in my last freshen up........ I have some pretty hard miles on it now and I still see 21 to 23 psi when idling hot (after driving for 30 min) at 700rpm..... I'd run them again.
I called Polydyne Coatings, and one other place, called Calico Coatings in North Carolina who will "moly" coat them. They both said the coatings adds about .0003" to .0004" (that's 3 to 4 TENTHS) per each bearing half, so that should add around 3/4 of 1 thousandth to the bearing. I don't know if that's worth all the trouble, or if the additional thickness really adds up to that much. Basically, we are not talking about a huge space here. I guess maybe I'm just being too critical, the '66 Buick manual says anything under .0036" is OK but .0005" to .0025" is preferred. If you use the rule .001" per inch of journal diameter, (2-1/2" dia.)that makes .0025" so then I only have 1/2 thousandth extra clearance. It's not easy being a perfectionist. :bglasses:
Doesn't matter how thick they coat um.. Because the excess coating will be removed by the crank/rods. However, the coating does fill the pores of the bearing, which helps reduce friction.:TU:
I didn't know that......I could probably accomplish the same thing by just pouring in a can that oil additive......I can't remember the name of it.
there's always STP! Remember how thick that stuff is! Actually I was kidding, sounds like maybe a heavier weight oil may help you out. Brian
Yeah, I used to build the race motors looser and run a thicker oil. (20/50 oil) with .003 on mains/rods. Now I go .002/.002 and run 10/30 oil. As long as the pistons aren't loose and rocking either way will work. With .003 there is usually some startup knock. Just run it easy cold and let it warm up before running it hard. Bruce
