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Low compression concern on rebuild


RD02

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Hi everybody,

I took my ‘73 2002A for a drive Sunday. Back to the house with 900 miles on the rebuilt motor over the past few months. I know that’s not enough for full break-in. With the motor hot I did a compression test and was a little surprised to find #1 was 10 to 12psi lower than the others. 

#1-146  #2-157  #3-162  #4-155

Back at 300 miles, out of curiosity I did a cold comp check just on #1, it was 155. So it’s gone down. 

Did a leak down test,  #1-75psi in/53psi= 29% leakage

                                    #2-75 in/66= 12%

                                    #3-75 in/64= 15%

                                    #4-75 in/62= 17%

On #1, lots of air heard from the oil filler. Very little from the others.

Gave #1 a squirt of oil through the plug hole, comp test jumped to 165psi.

 A recent oil check showed it had lost almost a full quart of oil in 300 miles. 

 It puffs out bluish smoke when hitting the gas after a deceleration.

Spits oil on the inside of the air filter cover. 

So, the E12 head was redone with seats and valves ground. 

       New exhaust valves, springs, guides, late model seals. 

       Reused stock cam, rockers, shafts.

       Head was surfaced and pressure tested

   

       Block was surfaced, bored and honed for 90mm 9.5:1 cast pistons.

       New piston pins.

       New Deves rings, gaped per IE guide sheet. 

       New small end rod bushings, big ends rounded up.

       Crank ground.

       All new +.25mm bearings. 

       Everything balanced. 

 

       Timing is set on the flywheel ball at 1500 rpm. 

       Motor has been breaking in per IE instructions. 

       Oil used, per machinist’s recommendation, 40wt Chevron Delo 400 (opinions?)

       

I’ve searched and read several previous posts about low compression and rings not seating. Ever recommendation from using BonAmi, cheap oil, break-in oil and just drive the crap out of it.

I couldn’t find anything with reasons why rings won’t seat. Are there instances were they just don’t seat even if, hopefully on my part, everything was done right?

I think that’s about it. Looking for some guidance. 

 

Thanks, Robert

 

 

 

   

       

 

 

 

Edited by RD02
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Are you using a synthetic oil for the break-in?  If yes, don't.  If no, why don't you continue to drive the car another 1,000 miles and see if the compression improves?  It may be a bit different process with an automatic, but vary the revs.

 

Regards,

 

Steve

Edited by Conserv

1976 2002 Polaris, 2742541 (original owner)

1973 2002tii Inka, 2762757 (not-the-original owner)

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5 hours ago, Simeon said:

How did you align the piston ring gaps?

Simeon has a good point, if the ring gaps were aligned instead of staggered it may account for the low compression and oil pumping so to speak, Mike McCartney had a story in the restoration guide about that topic!

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Admittedly, your description suggests both unseated rings and leaky valve guides/valve seals.  Yet at  only 900 miles, it may be premature to reach for the Bon Ami.

 

Deves flexible cast iron rings should seat as quick if not quicker than stock (harder) OEM chrome plated rings, and I would imagine that also holds true for the oil control rings.  They tend to be forgiving when used with less-than-perfectly-prepared iron cylinder walls too.  Of course, rings that are the wrong size for given pistons and cylinders is another matter - and clearly significant.  Simeon's point regarding the wisdom of staggered ring gaps is a good one; however, piston rings are not typically static while riding up and down against rings lands and cylinder walls, and after 900 miles, I would expect the gap positions to be a non-issue.  Naturally, if things were tight fitting when installed, maybe they need more time to find the so-called sweet spot.

 

Straight 40 weight Chevron oil ought to be fine for an engine run in, but being unfamiliar with it, perhaps it has an elevated detergent level that is counterproductive for much the same reasons as not favoring synthetic oils for break in.

 

Assuming that everything was properly measured and assembled, i.e., everything fit, two things come to mind.  I would consider draining the oil and replacing it with an oil specially designed for engine break in.  This includes changing the filter, which I would carefully examine for engine particulates.  At least it will do no harm and at best it will facilitate a quicker break in period.  Next, you might consider contacting your machinist and/or whoever built the engine.  They might offer some insight that is not that obvious to those of us without personal knowledge concerning all of your engine's particulars.  If you built the engine, you might re-evaluate how you measured your pistons, piston to cylinder wall clearances, ring sizes and  . . . ring gaps.  (Just stating that rings were gapped per a guide sheet, carries with it many assumptions and does not fully explain gap size and how they were measured or adjusted.  Although it is unlikely, it is not impossible for rings to be installed in the wrong ring land, inverted and even forgotten.)

 

Your mention of 90mm pistons suggests non-stock, oversize.  One assumes these were new, or at least checked for roundness and wear.  Is it possible that your pistons are forged (rather than cast or hypereutectic).  If that is the case, cold running clearances could explain some of your symptoms - especially when the engine is cold or if your engine is running at a low operating temperature, e.g., below 160F.

Edited by avoirdupois
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28 minutes ago, avoirdupois said:

 

Assuming that everything was properly measured and assembled, i.e., everything fit, 

 

Your mention of 90mm pistons suggests non-stock, oversize.  One assumes these were new, or at least checked for roundness and wear. 

 

Taking these two points further and to address a common problem I've found.  The machinist will use a single piston when measuring/boring all four bores.  The problem is that there is variation from piston to piston.  Doing the above mistake leads to a situation like the one you describe.

 

It could be other things, but the point I describe tends to be relatively common.

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Curious that the recommended break-in oil is a fleet service diesel oil with little to no ash (an assumption on my part based on their vague description online as being safe for DPF & CAT), loads of detergent and presumably no zinc. I would have thought the recommended oil would have been something like Valvoline VR-1 20W/50, as that is about the only oil one can find with any ease that is near what would have been on the shelf in the 1970s. I bring this up because none of the parts you mention should be made with esoteric material that were non extant in the 1970's, and if the machinists did their work to period correct listed tolerance etc, one would think "the oil of the day" would be what you wanted to use. Break-in or otherwise.

 

I broke my engine in with VR-1, doing about 100 (no kidding) WOT 4th gear dyno runs. Its got 3k+ on it now and cylinder pressures are with 5 psi, leak down 2-5%on each hole. Built ver similarly to yours, using nearly the same spec parts. 

 

Suggest you follow the wisdom of avoirdupois & AceAndrew and add some high load + high rev, then easy driving cycles. Check again at 1500+ miles.

Edited by 1974_Verona
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I'm following this thread, because I have a local friend's  car with similar issues.

 

The basics:

 

IE cast 9.5:1 pistons (90mm)

E12 head with old type guides and (presumably new, yet old type) seals - my own car has old type seals/guides and burns no oil FWIW.

165 psi in each cylinder (cold / dry test)

mixed / iffy leakdown numbers

leakdown passing rings in to crankcase

oil burning - especially on overrun / engine braking - then during re-application of throttle

Mileage on fresh engine - about 1000 miles +/-

 

car runs like a scalded cat. DCOEs, tii distributor, etc.

 

My short term plan is to pull the head and fully inspect guides and seals before going deeper. The oil consumption bothers me more than the leakdown numbers. 

 

Tried 40wt oil and several dozen HARD runs incl engine braking, high rpms... now running thicker oil again, so the owner can enjoy it without the stink, while we await winter weather teardown. 

 

ring gaps were checked before assembly. Running Deves rings.

 

Interested to see what comes up, and to compare notes eventually!

 

Edited by wegweiser

Paul Wegweiser

Wegweiser Classic BMW Services

Nationwide vehicle transport available

NEW WEBSITE! www.zenwrench.com

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Just for curiosity put an IR or Laser thermometer on the individual exhaust runners and see if there is a variance that corresponds to that cylinder. Could be that cylinder is running richer than others (significantly). 

 

I am not sure what IE's brake in recommendations are but I run my engines hard right out of the box (once I confirm no leaks or temp issues and drain the initial oil). and I have never had any issues. 

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Quote

IE cast 9.5:1 pistons (90mm)

wasn't there a thread on tolerance variations on these pistons?

 

Yes, most machinists only measure one piston because they are supposed to be damned near identical if they are new.

Yes, I measure all 4 for diameter, ring land width, blah- de- blah, and I measure all 4 bores before I assemble. 

Because my only paycheck is finishing a race, and position is the bonus.

 

Likewise, I am in the 'high pressure' camp of engine break- in.

 

t

 

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"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Wow, thanks for all the feedback!

I did the assembly, my first complete engine, finding I’ve got a lot more to learn. 

The machinist I used was recommend by a local German car repair shop. He’s has 30 years of experience machining and assembling European engines so I trusted his work and followed his recommendations. 

 

The pistons are new and are cast. The piston set included new Deves rings, pins and retainers from Ireland Engineering. 

Top compression ring gap was set to .014”, second was set to .012”. Oil rings were  pre-gapped by Deves, work sheet shows a range from .010 to .016”.

Each four pieces of the oil rings were set 90 degrees apart around the piston. Second compression ring was set 180 from the top oil ring and the top compression ring 180 from it. I did not pay attention to the gap orientation versus the higher or lower side off the block.

 

Ace, I’ve made the common mistake you’ve posted. I do know the machinist did not bore the cylinders until he had the pistons in hand. There was no mention of installing each piston in a certain cylinder. I also made the mistake of not verifying the measurements of pistons and bores myself. Enthusiastic ignorance on my part.   

 

Conserv, no, no synthetic oil. I used the Delo 400 oil recommended by my machinist,”because it’s got a lot of good stuff in it”.

 

Sydney, I’d bought Macartney’s book and looked back at the story you mentioned, “one employee, with German precision, was lining up all the ring gaps in a nice straight  line”.

 

avoirdupois, lots of good info. As I said, I failed to verify measurements. My first move will be a visit to the machinist. It’s been several months since I got the engine. Hoping  he keeps a record file. 

 

Verona, I will certainly follow there’s and all of you guy’s suggestions. 

 

Weg, very similar for sure. Will continue this post as I go along. 

 

Anthony, interesting point. The Solex 32/32 DIDTA is on my to-do list. Before I bought the car the carb was replaced with a reconditioned one, and far as I’ve noticed some of the jet numbers don’t match what’s in the Haynes manual. I don’t have either thermometer, but what variance would you look for?

 

I followed IE’s break-in recommendation, picture of it below. After 500 miles, yeah, I started pushing it more and more. The engine starts good, cruises great at any speed, the highest so far was a good run at 85mph. I just checked the gas mileage, it was 18 mpg, premium grade only.

 

Ignition is stock, black coil kw12v, distributor jfud4. Plugs are Bosch WR8DC, are these ok or does someone have a recommendation?

 

Dang, I’m so tempted to pull the head and pan and pop the piston out. But, for now I’ll follow up with my machinist, change to a different oil, change the filter, drive it, and check compression. I’ll keep posting as I go. I’ll try to add some pictures below. I’ll take all your additional questions and comments enthusiastically.

 

You guys and this site are the best and hugely appreciated. 

Thank you, Robert

 

Probably 100 miles worth on the cover

06744D8B-6401-47C6-8A5C-D955F81DB905.thumb.jpeg.ee4d997066dc5fe6e0037859b16a16d2.jpeg247B779B-1F48-464D-A164-E72F68A0C08C.thumb.jpeg.ac180b04f0f7bcffa21b644b668f6f31.jpeg01309FBF-9983-4337-9376-3E760B97E9F3.thumb.jpeg.04145c5b6b932b55ac9b1b542d964dba.jpeg5B40EE88-90E9-42A7-B8FC-784EC7D0EEF5.thumb.jpeg.6234a0a6ca1f54ce96b52b9ba4484630.jpegFC0B17CF-F1BB-4378-940A-CC78355D61F8.thumb.jpeg.b573c7dfc8264c5c2373d169ef8de738.jpeg

 

 

 

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More thoughts.

 

Your fuel mileage does not seem low, but not abnormal, especially without knowing more about your driving conditions,vehicle weight and even things as mundane as tire pressure..  Hard test driving may result in less fuel economy.

 

It might be helpful if you posted pictures of the other spark plugs for comparison.  I concur with T-man (and others) that this cylinder is running too rich a fuel mixture.  It may be that the other cylinders and spark plugs could benefit with more air and less fuel too.  You wrote about ignition timing, but with respect to the experts, the engines may have been mass produced but that doesn't mean each engine is identical.  This becomes even more significant considering the many changes to your engine since it was born -  and changes in fuel, etc.  (In other words, using the timing specs is only a baseline.)

 

It is doubtful tat the fouled plug in your picture is defective.  That said, consider swapping it with a plug that has a more normal appearance.  Are all gaps the same?

 

For the time being, you might consider running the valve cover blow-by though a catch can or downdraft tube.  At 100 miles, that much oil is bound to add to your fouling issue.

 

Your varying compression and leakdown numbers are disappointing, but far from apocalyptic.  As stated earlier, Deves rings tend to be far more forgiving that their chrome plated and/or gapless competition.  It may be that there is a sizing issue, but carbon tends to fill voids and rings that are too tight have a strange tendency to wear until they fit.  (Correlatively, gaps that are too big do not for an engine that operates long enough to form carbon rings that are too tight have been known to break and wear piston ring lands,)  Since these rings were with the pistons, it is assumed that they fit.  (Cylinder wall taper is easy to overlook, but that does not mean it is significant - anywhere, including cylinder two.)  Looking at things from the top down, have you rechecked your valve gear, especially on cylinder two?  As noted, valves that don't fully close, are hardly a recipe for good compression.  Good silicon-bronze guides are always welcome, but  -  even the best guides - will not limit oil leakage if the corresponding valve stem is worn, tapered or even slight bent.  The same holds true for valve guide seals.  They tend to works well, but can only control a reasonable amount.  Some of the above is hard to identify with the head in situ; however, you can double check to determine if the number two valves are tight or even too tight.

 

Another thought concerns the cylinder head.  Was it resurfased and how much material was removed or left.  Too much metal removal can alter valve timing (retard) and create an engine with slightly different operating characteristics that the stock setup.  On a rare occasion, some shadetrees may resort to tighter valve lash as a means of adapting to the situation.  This approach seems unwise.

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