Bore and piston measurements

[Avocet]

Avocet's engine was burning oil last season. I finally got round to measuring-up my piston and cylinder this evening. I'm still somewhat puzzled as to what the problem might be though.

Nominal bore size is 82mm.

I measured it in about a dozen places and the highest measurement I got was 82.09, the lowest was 82.03. Average 82.056mm. I have two workshop manuals. One says the bore should be 82.0 +.01, -0. The other says Max. bore wear 0.15mm. I'm not sure I believe the first manual. I've a feeling those are manufacturing tolerances on a new cylinder rather than permitted wear? I think 0.15mm sounds much more believable for wear?

Anyway, if that second figure is correct, the bore is fine for wear.

I also measured up the piston and was a bit surprised to find it was slightly oval. The nominal diameter from the workshop manual is 81.97mm. I measured an average of 81.85mm across the gudgeon pin and 81.59 along it. Is that normal?

Lastly, I stuck the piston rings down the bores in various places and measured the gaps between the ends. The manual talks about 0.3 - 0.5mm for a new build, with a maximum permissible gap of 2.0mm. The top and middle rings showed gaps of about 0.7mm, but the oil scraper ring had a gap of 1.75mm.

I'd be grateful for any thoughts anyone might have!

(It's a BMW D12, by the way).

 

[Avocet]

Oh, and a supplementary question: The top piston ring has no markings on it. The middle one has the word "top" on it very faintly, as does the scraper ring. The top two have a generous chamfer on their INSIDE top edge. (Or at least, the edge with "top" written on it). I take it the word "top" means that side of the ring has to face upwards? Or does it mean the one with "top" written on it needs to be the uppermost ring?

 

[macd]

Sounds fine to me. What sort of hours has it done?
Pistons ovality, btw, is to allow for differential expansion when hot.

 

[vyv_cox]

Do the bores show honing marks? Bores can be glazed or polished, either of which may lead to increased oil burning. Some explanations at http://coxengineering.sharepoint.com/Pages/Boreglazing.aspx I agree with other posters, wear does not seem excessive.

 

[Len Ingalls]

Since you have eng. apart,Change valve seals(&reface valves). Valve seals are usually the first to "wear out"-(usually dried up from heat).
Symptom of bad valve seals is a puff of blue smoke when starting,after a few hrs shutdown,where as bad rings & u get blue smoke at all times.
Cheers / Len

 

[essexboy]

Pistons are made oval ! The theory at least is a better fit when hot / running. Did you measure both item at the same temperature ? It make a big difference to a piston, the standard is 20c / 68f.

 

[mtb]

rings fitted when doing a ring change in used bores should be ridge dodgers which sounds like what you have. I agree about the valve stem seals but also check for wear in the valve guids
years ago you could get rings made by CORDS which were very hard and would to a certain extent re cut the bores. I went to the factory for a set off a perkins lump the factory was in park royal they would supply any size rings dont know if their still about now but there will be parts new old stock on ebay
Have you checked the bores re ovality the piston wear each side of the bore as it goes up and down re thrust side so worth checking that
You can buy a honing tool for re doing the bores just a simple cross hatch pattern is all they need
which oil were you using Ive noted halfolds sell mineral oil saying its ideal for classic cars HUH more like watch the oil level drop as i think its very poor quality ( just a thought ) so consider the oil as well

 

[Avocet]

Thanks all. I'd heard something about pistons being oval but never thought they'd do it for a 10 horse boat engine! I have another piston from a scrap engine. Sure enough, that too, is oval (though to a smaller extent). Oddly, the bore does have honing marks, but of course, I can't tell if they're deep enough. I have a three-fingered honing tool that goes in the end of a drill and was thinking of poking that backwards and forwards through it a few times. Should I use some sort of lubricant? (thin oil? WD 40? 3-in-1)? The workshop manual helpfully tells me I need a surface roughness of 1.0 - 1.2 Mu. Anyone know what that looks / feels like?!

Haven't looked at the head yet, but will certainly lap the valves in and check the guides.

Anyone know where I can source some piston rings?

 

[pampas]

Mat or ground appearance

 

[CAPTAIN FANTASTIC]

Yes, a bit of honing and a new set of standard rings. Has the top of the cylinder developed a "lip" or a "step" if so, there is a danger that the new rings will catch and break; you may need to "bend" them in by pushing the piston with the new rings in and out a few times or by smoothing out the "lip" with honing.

 

[Avocet]

Yes, a bit of honing and a new set of standard rings. Has the top of the cylinder developed a "lip" or a "step" if so, there is a danger that the new rings will catch and break; you may need to "bend" them in by pushing the piston with the new rings in and out a few times or by smoothing out the "lip" with honing.

No, I can't feel any discernible lip. The top ring is a fair way down the piston and I can see that there was once water on top of the piston, (some rust marks above the level of the top ring at TDC) but certainly no lip that I can feel.

 

[mtb]

use parafin to lubricate as you hone go for a cross hatch patern their spring loaded so simple enough to do, just be careful as its easy to go to far up out of the bore and break the stones
REMEMBER to carefully clean every thing to be sure any particals rust paint or carbon are removed before rebuilding

 

[Heckler]

Avocet's engine was burning oil last season. I finally got round to measuring-up my piston and cylinder this evening. I'm still somewhat puzzled as to what the problem might be though.

Nominal bore size is 82mm.

I measured it in about a dozen places and the highest measurement I got was 82.09, the lowest was 82.03. Average 82.056mm. I have two workshop manuals. One says the bore should be 82.0 +.01, -0. The other says Max. bore wear 0.15mm. I'm not sure I believe the first manual. I've a feeling those are manufacturing tolerances on a new cylinder rather than permitted wear? I think 0.15mm sounds much more believable for wear?

Anyway, if that second figure is correct, the bore is fine for wear.

I also measured up the piston and was a bit surprised to find it was slightly oval. The nominal diameter from the workshop manual is 81.97mm. I measured an average of 81.85mm across the gudgeon pin and 81.59 along it. Is that normal?

Lastly, I stuck the piston rings down the bores in various places and measured the gaps between the ends. The manual talks about 0.3 - 0.5mm for a new build, with a maximum permissible gap of 2.0mm. The top and middle rings showed gaps of about 0.7mm, but the oil scraper ring had a gap of 1.75mm.

I'd be grateful for any thoughts anyone might have!

(It's a BMW D12, by the way).

The rings are worn especially the oil ring, the top refers to the way up, with top on the top. Shove your tool up and down quickly, to bust the glaze, get some new rings, Break the old rings and use the fresh broken end to scrape the carbon out of the piston ring groves, you will be surprised how much is in there! Then re assemble. Worth doing the valves whilst apart, including seals
S

 

[Latestarter1]

Avocet's engine was burning oil last season. I finally got round to measuring-up my piston and cylinder this evening. I'm still somewhat puzzled as to what the problem might be though.

Nominal bore size is 82mm.

I measured it in about a dozen places and the highest measurement I got was 82.09, the lowest was 82.03. Average 82.056mm. I have two workshop manuals. One says the bore should be 82.0 +.01, -0. The other says Max. bore wear 0.15mm. I'm not sure I believe the first manual. I've a feeling those are manufacturing tolerances on a new cylinder rather than permitted wear? I think 0.15mm sounds much more believable for wear?

Anyway, if that second figure is correct, the bore is fine for wear.

I also measured up the piston and was a bit surprised to find it was slightly oval. The nominal diameter from the workshop manual is 81.97mm. I measured an average of 81.85mm across the gudgeon pin and 81.59 along it. Is that normal?

Lastly, I stuck the piston rings down the bores in various places and measured the gaps between the ends. The manual talks about 0.3 - 0.5mm for a new build, with a maximum permissible gap of 2.0mm. The top and middle rings showed gaps of about 0.7mm, but the oil scraper ring had a gap of 1.75mm.

I'd be grateful for any thoughts anyone might have!

(It's a BMW D12, by the way).

Farymann engine is is a quirky old thing however forget all the advice from gasoline girls about oval pistons (somebody needs to tell Mahle) or Cords style stepped rings never used in diesel engines and 'cleaning out' bore honing marks.

IF you intend to hone you have to use new rings, putting old rings into freshly honed bores does not work.

#1 Use a Flexhone to cut new cross hatching, Delapena style hones are perfectly OK following re-bore but take out too much material on a worn bore, spring loaded three leg hones absolutely useless. Flexhone cuts new cross hatching and is totally self aligning. No need for honing fluids 50/50 engine oil paraffin does the job perfectly, washed out with strong Fairey liquid boiling water solution.

#2 Once honed check new ring gaps in the bore. Unless you have a Central Tool or Moore & Wright three point bore gauge you will struggle to get accurate bore measurements for taper without.

#3 Measure piston diameter just below wrist pin bore, nowhere else. Piston is simply a vehicle to carry the ring pack, IF engine has seen abnormal wear, unlikely in marine application you can bore barrel 1.0 mm oversize and fit oversize piston. About £35 for bore and £60 for a piston.

#4 New rings will be marked top, sometimes oil control rings are not marked in that case control spring is doing the job and means ring does not care.

#5 If in any doubt have the jug bored 1.0 mm over-size.

Good luck

 

[Heckler]

Farymann engine is is a quirky old thing however forget all the advice from gasoline girls about oval pistons (somebody needs to tell Mahle) or Cords style stepped rings never used in diesel engines and 'cleaning out' bore honing marks.

IF you intend to hone you have to use new rings, putting old rings into freshly honed bores does not work.

#1 Use a Flexhone to cut new cross hatching, Delapena style hones are perfectly OK following re-bore but take out too much material on a worn bore, spring loaded three leg hones absolutely useless. Flexhone cuts new cross hatching and is totally self aligning. No need for honing fluids 50/50 engine oil paraffin does the job perfectly, washed out with strong Fairey liquid boiling water solution.

#2 Once honed check new ring gaps in the bore. Unless you have a Central Tool or Moore & Wright three point bore gauge you will struggle to get accurate bore measurements for taper without.

#3 Measure piston diameter just below wrist pin bore, nowhere else. Piston is simply a vehicle to carry the ring pack, IF engine has seen abnormal wear, unlikely in marine application you can bore barrel 1.0 mm oversize and fit oversize piston. About £35 for bore and £60 for a piston.

#4 New rings will be marked top, sometimes oil control rings are not marked in that case control spring is doing the job and means ring does not care.

#5 If in any doubt have the jug bored 1.0 mm over-size.

Good luck

Why the pissin contest? He measured the bore, it is within spec, he measured the piston, it is within spec, the only prob is the rings out of spec, he has a honer, (the kit you mention is anything from 87 bucks to over 400!)(Ive done bores before now with wet and dry and paraffin!)
He can do the job with what he has got without all the bull!
S

 

[Latestarter1]

Why the pissin contest? He measured the bore, it is within spec, he measured the piston, it is within spec, the only prob is the rings out of spec, he has a honer, (the kit you mention is anything from 87 bucks to over 400!)(Ive done bores before now with wet and dry and paraffin!)
He can do the job with what he has got without all the bull!
S

From the tone sounds like typical U.S. East Coast grumpy red neck talk.

These little Farymann motors originated before the Japanese taught us how to design small diesel engines properly. Unless done properly and not bodged by well meaning posters with their gasoline heads on, minor repair will turn into a fiasco.

You cannot achieve surface finish with spring loaded three legged hone, never designed for diesel engines where surface finish of bore is critical to obtaining high compression, and good lube oil control. No clue how one achieves correct 45 degree hatch pattern and specified surface finish with paraffin and wet and dry.

BRMB Flexhones of correct bore dimensions are not hugely expensive as suggested. http://www.ebay.co.uk/itm/3-3-4-Eng...t=Motors_Automotive_Tools&hash=item51c77a3e51

Somewhat quirky little motor, for example shimming the piston barrel can be a pain however all requires doing 100% correctly. If that is B.S then so be it...

 

[Heckler]

From the tone sounds like typical U.S. East Coast grumpy red neck talk.

These little Farymann motors originated before the Japanese taught us how to design small diesel engines properly. Unless done properly and not bodged by well meaning posters with their gasoline heads on, minor repair will turn into a fiasco.

You cannot achieve surface finish with spring loaded three legged hone, never designed for diesel engines where surface finish of bore is critical to obtaining high compression, and good lube oil control. No clue how one achieves correct 45 degree hatch pattern and specified surface finish with paraffin and wet and dry.

BRMB Flexhones of correct bore dimensions are not hugely expensive as suggested. http://www.ebay.co.uk/itm/3-3-4-Eng...t=Motors_Automotive_Tools&hash=item51c77a3e51

Somewhat quirky little motor, for example shimming the piston barrel can be a pain however all requires doing 100% correctly. If that is B.S then so be it...

And so it begins!

PBO, DIY! Geddit?

Whats with the gasoline head insult? Do you not understand how patronising you sound?

Easy for someone to spout from the perspective of having pro tools!

Read the OP again, his bits are within spec except for the rings, he needs to clean the lands out, bust the glaze, fit new rings and all will be fine. He has tools which are adequate to do the job DIY!
S

 

[Avocet]

Thanks for the input gents. Not sure how Farymanns came into it though? This is a BMW D21 engine. It's a marinised version of a German small plant engine made by a company called Hatz.

I take the comments about the 3-point internal micrometer. Alas, mine is only a 2-point one and difficult to use. That said, I did take a total of 16 measurements and then took them all a second time to see if I could get repeatability. Where the results were more than 0.05mm different, I discarded both. That left me with about a dozen pairs of "good" measurements, which I then averaged. One way or another, I'm reasonably satisfied that the bore IS comfortably within tolerance and that it's not oval, conical or barrel-shaped.

The top and middle piston rings are fine, I can't see any daylight between them and the sides of the bore. The gap between the ends when poked down the bore is only JUST outside the tolerances for NEW rings in a NEW bore! The oil control ring gap is much larger though, for some reason. It's only just within the specified tolerances for a WORN engine.

The bore doesn't LOOK glazed, but I'm not really sure what to look for. I can certainly see fine scratches in the finish, but they're perpendicular to the length of the bore, not at 45 degrees. I might try to see if I can get a decent photo and post it. (Note, I still have to take the valves out and look at the guides, so at this point, I'm not even 100% certain that he problem IS rings / bore related).

I did look at those Flexhone tools some years ago, when I bought my three-legged one and decided they looked like a recipe for disaster in inexperienced hands. They seem like a glorified "flap wheel" that would make the cylinder go barrel-shaped if left in one spot for too long! I reckoned on the straight stones of the three-legged tool providing a better guarantee of getting the wall parallel to the axis of the cylinder. (obviously, provided they weren't pulled out beyond their pivot point at each end of the stroke)! I could certainly be wrong here. I've only every used it on a petrol lawnmower engine (on which it seemed to work fine, but it didn't have a bad oil consumption problem beforehand so I'm not sure it made any difference at all).

Just keeping in mind the "bigger picture" though, this is a small, raw water cooled engine that is getting on for 30 years old. They never sold well even when they were new, and some spares are now unobtainable. It's days are well and truly numbered and it doesn't owe me anything. I'm happy to chuck a few hundred quid at it and if that gets me another few seasons, then great, but it won't be the end of the world if not!

 

[Heckler]

Thanks for the input gents. Not sure how Farymanns came into it though? This is a BMW D21 engine. It's a marinised version of a German small plant engine made by a company called Hatz.

I take the comments about the 3-point internal micrometer. Alas, mine is only a 2-point one and difficult to use. That said, I did take a total of 16 measurements and then took them all a second time to see if I could get repeatability. Where the results were more than 0.05mm different, I discarded both. That left me with about a dozen pairs of "good" measurements, which I then averaged. One way or another, I'm reasonably satisfied that the bore IS comfortably within tolerance and that it's not oval, conical or barrel-shaped.

The top and middle piston rings are fine, I can't see any daylight between them and the sides of the bore. The gap between the ends when poked down the bore is only JUST outside the tolerances for NEW rings in a NEW bore! The oil control ring gap is much larger though, for some reason. It's only just within the specified tolerances for a WORN engine.

The bore doesn't LOOK glazed, but I'm not really sure what to look for. I can certainly see fine scratches in the finish, but they're perpendicular to the length of the bore, not at 45 degrees. I might try to see if I can get a decent photo and post it. (Note, I still have to take the valves out and look at the guides, so at this point, I'm not even 100% certain that he problem IS rings / bore related).

I did look at those Flexhone tools some years ago, when I bought my three-legged one and decided they looked like a recipe for disaster in inexperienced hands. They seem like a glorified "flap wheel" that would make the cylinder go barrel-shaped if left in one spot for too long! I reckoned on the straight stones of e three-legged tool providing a better guarantee of getting the wall parallel to the axis of the cylinder. (obviously, provided they weren't pulled out beyond their pivot point at each end of the stroke)! I could certainly be wrong here. I've only every used it on a petrol lawnmower engine (on which it seemed to work fine, but it didn't have a bad oil consumption problem beforehand so I'm not sure it made any difference at all).

Just keeping in mind the "bigger picture" though, this is a small, raw water cooled engine that is getting on for 30 years old. They never sold well even when they were new, and some spares are now unobtainable. It's days are well and truly numbered and it doesn't owe me anything. I'm happy to chuck a few hundred quid at it and if that gets me another few seasons, then great, but it won't be the end of the world if not!

Not wanting to get in to a "I know more than he does scrap" I have always used a two point internal micrometer, I have found that it doesnt take long to get used to the feel of the drag to get a good reading. The 3 legged honer, I agree with you, seems more stable, Ive had mine for years and it seems to do the job well as long as you dont let it protrude too much out of the bore. The marks in the bore, the text book that I studied years ago talked about angled markings in the bore. Glazed bores look like glazing and you can see the markings through it. You need to "bust" that. The tool you have will do it, up and down with some lube such as paraffin. In the olden days, with bike engines they talked about leaving cylinders out in the garden to let moisture lightly rust it to form microscopic pits! I have dome it with wet or dry and diesel before now, out in third world countries where I didnt have any other options. I have actually repaired cylinders with Bakerlock, a heavy duty epoxy type resin, used for glueing dirty, oily wet production casing strings. This was an extreme emergency to get a job away till we could get spares along the 3 month supply line. A circlip had come out and gouged the cylinder, dresed the gouge, cleaned it with solvent, slapped the jollop on and then dressed that up and polished with emery and wet or dry. It worked long enough to do the job, smoked a bit but hey ho.
The point of my anecdotes? As you quite rightly recognise you have an old engine, you can repair it more than satisfactorily with what you have got, I repeat my view, the oil scraper ring is shot, the compression rings less so. If you can get a set of rings for a reasonable price, clean the piston up, shove your honer up and down the bore to get rid of the shine which will stop the rings bedding in, and away you go
S
PS did someone say 82mm bore? Is this it? http://www.engine-trade.com/product/hatz-piston-ring.htm

 

[Beamishken]

I did look at those Flexhone tools some years ago, when I bought my three-legged one and decided they looked like a recipe for disaster in inexperienced hands. They seem like a glorified "flap wheel" that would make the cylinder go barrel-shaped if left in one spot for too long! I reckoned on the straight stones of the three-legged tool providing a better guarantee of getting the wall parallel to the axis of the cylinder. (obviously, provided they weren't pulled out beyond their pivot point at each end of the stroke)! I could certainly be wrong here. I've only every used it on a petrol lawnmower engine (on which it seemed to work fine, but it didn't have a bad oil consumption problem beforehand so I'm not sure it made any difference at all).

Just keeping in mind the "bigger picture" though, this is a small, raw water cooled engine that is getting on for 30 years old. They never sold well even when they were new, and some spares are now unobtainable. It's days are well and truly numbered and it doesn't owe me anything. I'm happy to chuck a few hundred quid at it and if that gets me another few seasons, then great, but it won't be the end of the world if not![/QUOTE]

I tend to agree with you re the hone tool I've been using a 3 legged hone for 30 years & probably done into the 100's of rebuilds & not had a problem
as you say the bead type will drop into any inaccuracies where the straight stone will bridge them & show up a problem I use a medium stone & wd40 for lubrication & not had any issues
as you say possibly oil consumption could be down to hardened valve stem seals &or worn guides but personally I'd hone it &fit new rings as a matter of course now it's apart
the gear you have is adequate for the job in hand & you sound like you have a pretty good idea what your doing
best of luck with it