N* engine teardown part #2

[jhall]

Hi all,

Here's some pics of N* main bearings and making the measurement for head flatness.

I'd like comment on the main bearings. It appears I need to replace these (and the rod bearings for good measure). I don't want a knock in the near future.....

[Scotty]

I wonder what creates scoring like that on the bearings? I believe that once you take out a bearing whether it be a main or rod bearing you need to replace it as you will no longer get the correct 'swish' when you reinstall them, but I am curious what other have to say about that thought.

Personally, if I was in there, I would want to replace them regardless. The problem with that idea is, do you need to check clearances with plastigage to ensure correct clearance with the new bearings or is there a brainless service bearing that would work? <<< using a service bearing makes me nervous without checking proper clearances

[Scotty]

Yes, I did mean crush but I managed to relay what I meant to you...

Another bbobynski golden tid-bit of information, I was under the impression that the tangs held the bearings in place, wow was I off... I didnt realize that crush was to stop spinning I thought it was for the purposes of obtaining the correct clearance. Its pretty amazing that the tang could spin, have you seen any with tangs that spun? What happens to the tang, is it just flattened and smeared out? Thanks, Mike

[Topcat9pet]

Most of my engine building experience has been with general aviation aircraft engines. There was no FAA requirement to change the bearings in piston engines as long as they were within spec's. I don't know of any shop that would ever put the original bearings back into an engine during major overhaul no matter how good they looked.

Mostly because no one wanted to tell the aircraft owner,who was seldom knowledgeable about the engines in general and less so about FAA requirements, that you put old bearings back in the engines. Most people who can afford to fly their own aircraft have an attorney on retainer who can be even more difficult to reason with.

Good Luck.

Cleatus

[jhall]

Hi guys,

Thanks very much for the response.

bbobynski - Thanks very much for the generous info. From my past experience with working on older model engines, I have to admit I'm a little nervous about using the old bearings, but I'll go with the advice.

For good measure, and my own piece of mind, I plan on using some plasti-gauge to check the general clearance on the mains.

The heads look good. I can just get a 0.002 gauge under the straight edge at two points. A 0.003 won't go.

The more I look at this thing, the more I'm impressed with the engine design. It looks like some of the concepts used in two cycle outboard engines have been incorporated. The cooling jacket, cylinder relationships - even the head gasket design.

I am curious about the rebuild market for these engines. OK, you get big miles from the motor and then it goes bust. I seems a real shame to waste something that can go this far again with a rebuild. Maybe the labor to refurbish exceeds the cost of simply replacing it with a new one??

[Dloch]

With regards to the main bearings, mine had the same look and I did reuse them again.

When rebuilding hi perf endurance motors if the bearings looked good and were within the clearance you wanted for that motor they were reused. These were marine motors that lived there life at 7600 for hours..... When putting a motor together the first time you go through great pains to get the clearances correct. This may involve buying standards, .001 over & .001 undersize bearing and mixing and matching to get the clearance you want. If you just threw them out at each rebuild, generally after 2 races for a minor, you would have to go through the whole time consuming procedure again to no end.

If a crank or a rods were replaced then you have to go through the procedure. Generally the crank and rods, as long there were no cracks detected with non destructive testing, and the lenghts checked out from the previous rebuild they were all reused. The pistons are tossed after 4 races no matter what.

The valve train and rings are replaced after 2 races. That was what is considered a minor rebuild. The major, 4 races included the pistons and rod bolts.

As Guru said your mains look good put em back in and don't worry about it.