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BMC 1.5 rebuild experience


Allan Edie

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I have recently finished a rebuild saga that owners of BMC 1.5 might be interested in, especially if they are considering a rebuild.  The experience is documented in a blog at:

https://www.blogger.com/blogger.g?blogID=5669388339113821275#editor/target=post;postID=4460763766630206706;onPublishedMenu=allposts;onClosedMenu=allposts;postNum=0;src=postname

Enjoy,

Cheers,

Allan Edie

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Sorry guys, I couldn't figure out what the problem is, so here is the text:

To rebuild or repower was the question.  The BMC 1.5 in our sailboat still worked perfectly, but with 7000 hours on it, it was time to consider options.  Because I had stockpiled some high quality parts in anticipation of a rebuild, and because the only local engine rebuilder was about to retire, I decided to do a preemptive overhaul while he was still available.

 

I stripped, pulled and delivered the engine to the rebuilder myself.  An interesting project that, but relevant here only in that my costs were less than many owners might experience for the same level of work.   Some $4500 or so and several months later I re-installed the engine, it started instantly on the first try, and I was pleased.

 

During ~200 engine hours in the first summer season, the engine worked perfectly.  However, it consumed ~1.5 liters of oil/100 hours.  Not acceptable as far as I was concerned, not too bad according to the rebuilder.  I attempted to reduce oil consumption by upgrading valve seals and altering crankcase ventilation - neither made any difference.  I concluded that the problem had to be rings - perhaps compression rings put upside down.  The OEM scraper rings I supplied with the new pistons were not marked, so I was suspicious.

 

In the end, having exhausted other options, I bought a used engine stand, adapted it to bolt onto the engine bed on the boat (wouldn't want it getting loose during rough weather at the dock), pulled the engine and pulled it apart to check the rings.  All rings had been installed right side up, and as far as I could tell from wear patterns, they had seated ok, although the oil rings not perfectly.   However, the ring gaps were dramatically larger than specs. - ~200 - 300% of minimum spec on most of the rings.  In the rebuilder's defense, I supplied the rings with the new pistons, and by the time the rings needed to be fitted, the engine was long overdue to get out of his shop.  The last thing he wanted at that point was for the thing to sit there for another month while I got into another frustrating interaction with a supplier.

 

In any case, all considered, I figured that the oil consumption problem was likely due to excessive blowby due to oversize ring gaps, possibly exacerbated by incomplete seating of at least the oil rings, if not the compression rings as well.

 

So, after ball honing the cylinders, I replaced the rings with a new set, perfectly gapped, and put the engine back in place.  The following season, over another 200 or so hours, the engine worked flawlessly, but the oil consumption was, if anything, worse than it was the first time.  I was not well  pleased.  The engine went back on the engine stand at the end of the summer season.  I found that the rings, the second time around, had not seated properly - both compression and oil rings showed inconsistent and incomplete wear patterns on the sealing surfaces.  After much head scratching and digging for information, I concluded that I must have been insufficiently aggressive with the ball honing, or with the initial workout I gave the engine, or both. 

 

On to round 3.  Again, I installed yet another new set of rings, perfectly gapped, again after ball honing, only this time ~50 strokes instead of 20.  I also replaced the standard top oil ring with a modern three piece ring instead of the older single piece cast ring.  When I first started the engine, as soon as I knew that I had oil pressure and cooling, I took the engine to ~2500 rpm in gear while moored at the dock.  I figured this to be ~75% of full power output which is what I understood to be the approach used by at least one rebuild shop which uses a dynamometer during initial ring seating.

 

It now finally appears that oil consumption is very low, undetectable after ~20 hours of running subsequent to the initial 3 hours of ring seating.  About time.

 

My conclusions here are:

 

1.  Good luck finding a shop that actually does a rebuild properly.  The fellow that did my rebuild had many decades of experience, did all his own work personally, and came very highly recommended by a car rebuild enthusiast whose opinion I trusted.  When I pulled the engine to check his work, the cylinders were as perfect as I could measure using professional tools, but the ring gaps were grossly outside specs, presumably because he did not check the gaps, or because he did not want to delay the work to obtain a new set, or because he did not consider the over large gaps to be a problem, or perhaps more than one of these.  In any case, in spite of my best efforts in choosing a shop, the rings were installed grossly out of spec, and I suspect that that was a large contributor to my initial oil consumption problem.

 

2.  The initial load on the freshly rebuilt engine needs to be serious right from the initial start.  The failure of my second ring installation was almost certainly due to my not being aggressive enough.  I warmed up the engine under low load before ramping up power over the next several hours of operation.  The third attempt did the job by immediately (i.e. within less than one minute) taking the engine to ~2500 rpm, in gear, tied to the dock, which I figured was something like 75% of maximum power in my installation.  In retrospect, I doubt that my first attempt was anywhere near aggressive enough either, although the seating seemed to be noticeably better the first time around.  However, the two honing jobs were dramatically different - the first starting with coarse stones to remove the ridge at the top of piston travel, the second being ball honing only after the cylinders had been polished by the initial seating attempt.  Apples and oranges as far as the microscopic cylinder surface is concerned.

 

3.  Although I will never know what the exact contribution of the new three piece rings was relative to the more aggressive ring seating during my last attempt, I think that replacing the top cast oil ring with a modern three piece ring is a good idea.  The modern design was available for the grooves in the OEM pistons I used, and presumably would be for aftermarket alternatives.  The newer ring design controls oil better.  The older cast rings do not conform to cylinder walls as well as the modern 3 piecers do, and they apply less pressure, less consistently around their periphery than the new ones do.  Thanks here to Dawn at Hastings Rings for patiently investigating options and advising me on their experience with rebuilds like mine.

 

 

Had I to do it all over again, I might choose a new Kubota instead of the rebuild.  The repower would have been less work, hassle, and frustration.  On the other hand, I now have the job done, my costs were less than half that of a repower, and I really like the BMC 1.5 installation.  In the end, I don't regret the decision to rebuild, but I wish good luck to those faced with this dilemma. 

 

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We still have the capability to rebuild these engines (including the fuel systems) but our experience is that unless there is major surgery required to the boat to fit a new engine, then fitting new is the cheaper way forward.

It's long been known round these parts that no diesel should be mollycoddled when being run in, and also that the correct spec. oil (API CC) is crucial.

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Yes to both Tony.  The lower end was completely disassembled, the block bath cleaned etc.  Also, the camshaft, timing gears, timing chain, and the rest of the valve train from cam bearings (new) to valve seats (new) was all replaced new.  The skew gears looked good as new to me.  I am not the original owner, but as far as I can tell from the records, previous owners took very good care of the engine.  It still had traces of cross hatch marks after over 7000 hours, and worked flawlessly.  I would not have bothered with the rebuild yet if the opportunity to have the machining work done locally was not about to disappear. 

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2 hours ago, Rose Narrowboats said:

We still have the capability to rebuild these engines (including the fuel systems) but our experience is that unless there is major surgery required to the boat to fit a new engine, then fitting new is the cheaper way forward.

It's long been known round these parts that no diesel should be mollycoddled when being run in, and also that the correct spec. oil (API CC) is crucial.

Yes, I suspect that the cost of my rebuild would have been a lot higher if I had done less of the work involved.  Getting the engine from the boat to the shop and back was a project, as of course was the removal and re-installation, never mind pulling the thing twice while chasing the oil consumption issue.  If I had paid to have all that done, and had I paid full current price for all the parts, I doubt that the rebuild would have made economic sense.   Had I to make the decision again, and if I did not have a few thousand dollars worth of rebuild parts and spares in hand, I would just install a Kubota.  As far as I can tell from the pictures and specs, the equivalent Kubota would be a drop in replacement if custom engine mounts were used.  I came really close after summer #2...

As for the initial ring seating, there is mollycoddled, and then there is near full throttle for three hours without even warming the engine up.  Neither of my two initial attempts would have been considered shy, but at least the second attempt absolutely failed to seat the rings, and the first attempt was likely marginal.  Were it not for running into the information about the process used by shops with dynamometers, I would never have imagined that I would have to hit the engine that hard initially.  I don't know whether this heavy an initial load is appropriate for a more normal honing end point rather than a ball honed cylinder already polished by previous running, but it seems that it has finally solved my oil burning issue, for which I am very grateful.   If I had to pull that engine for a fourth time, I am not sure it would be going back in again. 

 

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20 odd years ago I bought (knowingly) a 29' yacht with a BMC 1.5 engine that drank oil. The first winter I had it I removed the engine and stripped it down. On taking the block into a nearby re-boring firm that I had used previously and trusted they gave the block a good check over and recommended not to re-bore but to hone and fit new rings. This I did. Once back in the boat the engine still smoked and used oil (should add that whilst stripped down everything else was checked/replaced including valves and stem seals). Went back to re-borers to discuss and see if I had time to remove/strip/have re-bore and get back in boat in time for planned trip. At this stage I was pretty despondent. The fellow there produced a gallon of oil (made by a company I recognized at the time) that felt 'gritty' and a gallon of running in oil. I was told to fill with the gritty oil, run at a fast tickover for 30 minutes drain and replace oil with running in oil together with new filter. Run under medium load for 5 hours up and down the estuary then change oil / filter again.

The result was no smoke, no oil used and an engine that went on giving no problems.

Sorry I can't remember exact details of oil but it was a long time ago.  

 

Frank     

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Interesting experience.  Presumably pretty much the same as the Bon Ami cure, which works by introducing a mild abrasive into the engine to abrade the cylinder walls and piston rings to get the rings seated.  The main difference (at least hypothetically) between introducing the abrasive in the oil, which worked for you, and introducing Bon Ami into the intake air stream, is that, arguably, less of the Bon Ami would make its way into the oil and thereby to the engine bearings and camshaft train. 

I considered the Bon Ami cure when my rings failed to seat for the second time, but decided against it because my machinist advised that if I put the stuff in there, I would never manage to get all of it out.  On the other hand, the tech at Finning said little or none would actually get into the oil - he also confirmed from personal experience that Cat used (and may have developed the process in the first place) the Bon Ami treatment when they had repeated difficulty getting a particular combination of keystone rings and cylinder liners to seat some years ago.  It also occurred to me that the mild abrasive might quickly break down into such fine particles that any residues after the first couple oil changes might be trivial.  This seems consistent with both your experience and that of the Cat people. 

Anyway, in the end, since I had the time, and the cost for a new set of rings and a few gaskets was not that bad, I just re-honed and re-ringed.  Bon Ami might have worked, but I decided not to risk it. 

Cheers,

Allan Edie

 

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