When to rebuild?

#1

so, in preparing my car, I’m trying to determine if my 300k mile engine is ready to go as-is or if I should try to rebuild/freshen over the winter. I’m no engine builder or mechanic, so spew forth any advice you have!

The reason I’m wondering about rebuilding is that, although the engine feels strong, I did find copper specs and flakes in the oil when I drained it last. I thought that may indicate wearing rod bearings.

Based on what I’ve read, I checked on a few other potential indicators that I need to rebuild. Here are my results:

  • spark plugs look good - dull grayish/light tan on all cylinders
  • power seems good and strong
  • passed emissions with no problem
  • no smoke (blue or otherwise) from tailpipe
  • doesn’t burn too much oil, although I really haven’t watched it too closely - one track day and I probably burned through 1/4 quart of oil (?)
  • oil / other maintenance has been done religiously since 100k miles (owned by family member)

So I did a compression test today (twice) and found these numbers:

141 136 142 142 146 145 (first pass)
150 145 149 148 149 148 (second pass)

The second set was higher - maybe due to the engine cooling / contracting? Cylinder #2 is lower on both tests, but well under 10% than the other cyl.

So… do I do a rebuild based on these? Do I need to do a leak down? Do I change rod bearings and call it good?

thanks!

#2

unfortunately the only way to answer this is to ask you questions. Is it within your budget to rebuild it right now? How many track days are you planning on running this coming year? How far do you have to travel to each event?

In making this decision, think about the sunk costs and potential further costs of having your engine blow while on the track during a race weekend.

Remember that right now it is late november, you could have the engine out of the car by the first week in December and into the hands of a shop by Dec. 15. All of this effort would translate to the motor being back into your hands by about Jan 15-20 and back into your car by Feb 10. So if you are in the mid-atlantic region, you could still make the openning event at VIR mid-feb. My point of putting this time table out there is to show that the process takes a long time so if you have an event every six weeks or so next year, you will likely miss one and possibly two.

Oh, my personal opinion, it is always better to take the pesimistic approach when it comes to racing parts, so replace early and check often.

Goodluck,
Jon

#3

There’s really nothing that says you SHOULD rebuild now, but it certainly could save some hassle (and expense in blowing a weekend). You can take the head to Joe Blow machine shop and do the bottom end freshening yourself and probably get out pretty cheap.

#4

here is some rebuild guidance
http://www.pelicanparts.com/bmw/techarticles/Borrowed/mult_engine_rebuild-1.htm

cheers,
bruce

#5

Thanks Bruce, Michael, Jon…

I did refer to that Pelican Parts site for some guidance and other work on the car. Here are my compression numbers adjusted for my 5500’ altitude:

165 159 166 166 171 170
176 170 175 174 175 174

As to Jon’s questions…

I don’t yet have a firm budget. This project was originally thought to be a "budget racer", but I’m quickly learning that to make a small fortune in racing, start with a large fortune! I guess what I’m saying is that I do not have it in the budget to rebuild, but if all signs point to the fact that I should rebuild, then I’d have to put it in my budget. I can always home school the kids instead of sending them off to college!:stuck_out_tongue:

Most of the races are far from Denver - so there’s a lot of travel involved, and therefore, I don’t anticipate running a whole lot of races due to time constraints.

What about you, Bruce, I think you said you are at about 280k - are you thinking of any freshening? Anyone else with high (250k+) miles just GnR-ing it (Gunning n Running!).

#6

Doug Gordon wrote:

[quote]Thanks Bruce, Michael, Jon…
I can always home school the kids instead of sending them off to college!:stuck_out_tongue: [/quote]
just make them get full time jobs and go to school - builds character!

[quote]
What about you, Bruce, I think you said you are at about 280k - are you thinking of any freshening? Anyone else with high (250k+) miles just GnR-ing it (Gunning n Running!).[/quote]
Once I get it back together I’m going to go through the assessment you’re doing now. I was planning to run it until it blows because I have the e30 M3 to fall back on if it is out of commission for a while.
cheers,
bruce

#7

Doug Gordon wrote:

Close… Mine only has 243K :wink: I know a couple of other SE racers with similar mileage running as well.

#8

Metallic particles in the oil are worrisome. That probably points to rod/main bearings on their last legs. At this point you may be able to get away with just a bottom end rebuild and freshing of the head. But if a rod bearing seizes at the track you could blow the engine.

#9

I agree with Jim–the metal is the only thing that would keep me from running it. The compression numbers are good-to-excellent. At least put a crank scraper in there to make sure you don’t lose oil pressure–could make those bearings last longer.

#10

Craig Geiger wrote:

[quote]Doug Gordon wrote:

Close… Mine only has 243K :wink: I know a couple of other SE racers with similar mileage running as well.[/quote]

Mine had 247K on it and was affectionatly known around the paddock as "Smokey".

I am pulling the motor this winter. I’m having the head refreshed and putting it on a newer, 180Kish, bottom end with new bearings. FWIW, my old motor had about 4 race/DE seasons on it. The rings are what eventually retired it.

JP