Cylinder bore taper

[Paul]

I am at the stage of making pistons for my 3 ½" gauge loco, and on checking one of my gunmetal cylinders for size it appears slightly tapered by about 0.002".
I don't have a bore gauge and using other techniques I can't get a definitive error.  I made a plug that tightens about half was along the bore.
I understand a small amount of clearance is needed between the bore and the piston.

Being a novice/learner, I would appreciate advice as to the way forward. Do I hone, Lap or beg a 1 ⅛" dia hand reamer?  I have found some reference to honing and lapping on the forum but remain uncertain as to the way ahead.

I haven't made any decision regarding O rings or graphite yarn. 

Any advice would be appreciated.

Paul

[ettingtonliam]

I have no experience of honing or lapping on a soft material such as gunmetal. I've used 3 leg cylinder hones on cast iron quite successfully up to 2 1/2" inch diameter. If you are not used to them, its quite easy to bellmouth the bore and make things worse. My own preference would be a reamer, which at least you can be sure will give a parallel bore, which after all you are trying to acheive. Now what type of reamer depends on how close to finished size you are. Obviously, if you are 5 thou over size a solid reamer is no use to you. If you are more than a few thou undersize, a solid hand reamer might not be the easiest thing to use. I do have an adjustable reamer which covers 1 1/8" +/- 1/16", but these things have to be used with care, and adjusted for the very lightest cut each time, otherwise they tend to chatter or grab. I see you are in Poole (Dorset?) and I'm in Worcestershire, but you are welcome to the loan of it for the cost of the postage.
If you opt for O rings you need to get the bore pretty much spot on 1.125". Graphited yarn doesn't care, +/- 1/16" and it will work fine, just needs the piston to be a good fit.

[Paul]

Thank you for your advice and the generous offer of the reamer. I am going to have a go at honing the bore with the help of a friend who has some experience of using a cylinder hone. As your reamer has plenty of adjustment I may take you up on the offer if we are unsuccessful honing, I was interested to read your reference to O rings and graphite yarn.
Many thanks for your response.

[don9f]

Hi, I agree with the honing approach.....it would be very easy to damage the bore with a reamer if the setup wasn’t rigid / true etc. and make things much worse. The loco I’m working on has gunmetal cylinders and came into my possession with about .003” taper in both bores. Honing managed to successfully reduce this and I’m using graphited packing on the pistons.

Cheers Don

[springcrocus]

I'm with Richard on this one. Two thou is way over what hones are able to correct. Reaming may bring the bores parallel but I would be tempted to rebore them with a between-cntres boring bar and make new pistons to suit whatever size the bores end up at.

Regards, Steve

Edit: I missed Don's note about honing three thou successfully but it's still not an amount of work I would want to undertake. Resizing is, in my opinion, far easier on both the arms and the nerves.

Regards, Steve

[miken]

I've trued up slightly tapered cylinders with an adjustable reamer. A little bit at a time to just take a tiny scrape out each pass. Holding the reamer in the lathe chuck or vice and holding the cylinder in my hand to wind it on.

[Paul]

Thank you all. Plenty to think about.

[Roger]

With that much error, I'd be inclined to put it on a faceplate and take a fine cut with a boring bar. It's probably too difficult to set it up again close enough to use a boring bar between centres.

[92220]

One problem with honing is that most people don't know what lubricant to use. It makes a massive difference. I used to work as a design draughtsman at Delapena Honing Equipment Ltd, in Cheltenham, back in the 1970s. We obviously supplied a special honing fluid for use with the hones, but basically, honing fluid is a mix of equal parts of EP90 gearbox oil and paraffin.

Bob.

[Deleted]

Oct 14, 2021 8:38:12 GMT 92220 said:

One problem with honing is that most people don't know what lubricant to use. It makes a massive difference. I used to work as a design draughtsman at Delapena Honing Equipment Ltd, in Cheltenham, back in the 1970s. We obviously supplied a special honing fluid for use with the hones, but basically, honing fluid is a mix of equal parts of EP90 gearbox oil and paraffin.

Bob.

Hi Bob

Except in the case of Alusil... I've known a number of Porsche engines destroyed by so call engineering shops using normal stones and fluid for both reboring and then honing of cylinders.
Firstly, they machine to the wrong tolerance (following normal boring practice), and then damage the bores beyond repair from using the wrong stones/fluids, worse still they try to get a crosshatch pattern which no Alusil block has.
The correct paste/stones need to be used on these engines to expose the silicon crystals in the alloy followed by a 3 stage honing process which gives these engines such a long service life.

Pete

[92220]

Hi Pete.

Yes. That's a very good point. The proper stones do need to be used to get the best results for each material. Another point that comes to mind is the type of 'honing' head that most model engineering suppliers sell. These are stones, backed with a metal backing, and 'honing' pressure is exerted via springs. That is not actually honing. It is more like lapping. The hones that we used to produce in vast quantities, looked a bit like expanding reamers, with a stone in place of the reaming blades, and the stone forced against the inside of the bore, using a very shallow taper. On the opposite side to the hining stone, was a hardened steel slipper, which applied equal pressure on the opposite side to the stone. This is the main reason for requiring a lubricant, as well as washing away the loose grit and metal dust. The gearbox oil, that I mentioned earlier, is what provides the high pressure lubrication (the 'EP' in EP90 gear oil means the oil is designed to work under extreme pressures). The problems you refer to above were solely due to the engineering shop not knowing what they were doing, and what their honing stones were for. Delapena make hundreds of different honing stones, and from all sorts of different materials, depending on what they are to be used for. Honing stones for bronze are quite different to honing stones for cast iron and steel. As far as alusil goes, it is highly unlikely to be used by any model engineers. I agree though, that it would certainly require specialist stones, which I am sure, Delapena could supply....at a price!! I have to admit to not knowing a lot about the technicalities of the stones, themselves,because when I was at Delapena, I was only concerned with the design of honing machines, and the fitting of the stones to the honing heads. A properly honed bore should be parallel, and cylindrical, with 0.0001", or better. The Delapena literature talks about accuracies for precision measurement down to 0.00002" or 0.0005mm. That is what honing can do.

Bob.

[chris vine]

Hi Pete,

I used to have a porsche 928 with the Alusil bores you talk about. It was a fab car!

I thought they were machined, (and maybe honed?) and then were acid etched to take away the aluminium around the silicon crystals to leave the spaces between the crystals to hold the oil. Just like the crosshatch system in other designs.

Chris.

[Deleted]

Oct 15, 2021 13:16:08 GMT chris vine said:

Hi Pete,

I used to have a porsche 928 with the Alusil bores you talk about. It was a fab car!

I thought they were machined, (and maybe honed?) and then were acid etched to take away the aluminium around the silicon crystals to leave the spaces between the crystals to hold the oil. Just like the crosshatch system in other designs.

Chris.

That's correct, Chris

The etching is the 3rd part of the honing process which is done using the special paste and elastic honing stones. These are made from Corundum or SiC grains (had to look that up) in a porous plastic bond whose cutting grains rebound on contact with the hard silicon grains so that they remove more alloy matrix than silicon grains thus exposing said grains.
This results in a relatively smooth bore surface which has a favourable effect on oil consumption, friction loss and wear from piston ring contact.
Alusil is a very strong material, head studs are threaded directly into it, you are very unlikely to tear the thread, the stud itself will shear first.

An ideal material for a certain member's car running on high boost...

Pete

[chris vine]

Hi Pete,

Yes, that design of engine which Porsche perfected is certainly very strong.

I think that the pistons are coated in a sort of "cast" iron. so a sort of inside out engine.

I was told that one of the big American car makers tried to make engines this way, but gave up as they couldn't make it work...

Chris.

[Deleted]

Oct 15, 2021 15:57:44 GMT chris vine said:

Hi Pete,

Yes, that design of engine which Porsche perfected is certainly very strong.

I think that the pistons are coated in a sort of "cast" iron. so a sort of inside out engine.

I was told that one of the big American car makers tried to make engines this way, but gave up as they couldn't make it work...

Chris.

Yep.. can't recall the make (could be Chevy) but think it had an unusual name... vera or mega or something like that...The engines are certainly strong, bulletproof in fact, unlike some of the newer engines from the Marque.... that's what computer modeling does for you...
I can't recall what the original rings were treated in, the pistons also needed to be suitable for Alusil.. I did know what my IASA race pistons are coated in, forgotten now... Moly....something...

The 928 is a great car, a bit heavy for my kind of driving but it can certainly travel long distances at high speed which is what it and the 951 were made for.

IIRC Audi and BMW also used Alusil for their engines... not sure but the Audi R8 v10 units may be in Alusil too. The original 951's (M44/50 engine numbers) were made in a higher density Alusil with thicker cylinder walls, again this was down to Porsche's over-engineer everything approach before computers took over.

It was these engine blocks in the 944 LM that did the Le Mans 24 hrs, winning their class and 7th overall, a very impressive result when running against 200mph+ supercars from many different marques. They were also the most reliable cars spending the least time in the pits..

For those who love nostalgia and motor racing in general, here's a link to the 1981 Le man's race. you'll need to know what to look for to spot the 944 LM, i think it's the second or third car behind the first safety car, it had an impressive body kit, it's white.. The saddest thing is the deaths noted from this race, both I think from the Mulsanne straight where cars hit 230mph+.

www.youtube.com/watch?v=tNp7ZMHdZ-c

There are some great names mentioned in the commentary, of course, you have the winners Jacky Ickx and Derek Bell (who I have spoken to at past Porsche events and who still owns a 924 GTR) Tiff Needle who was driving although not in a 944 turbo (951) ( a car BTW that he's on record of stating to be one of his two all-time favourite cars and the same car that he won the Porsche Turbo cup championship in) and the little Irishman Eddie Jordan who was racing, I wasn't aware of this until now.

I enjoyed the video and it strengthened, even more, my admiration for the 951, I mean it came 7th?... just look at the cars that it beat...

Pete

[Deleted]

Chris

Just checked my mail with IASA from years ago.... after translating from Spanish I get 'molybdenum'.. although there was more conversation in which the tech guy said there are two types, he didn't give a name for the other coating.

Pete

[uuu]

I suspect my brother was in the pits for that race - he was with Charles Ivey.

Wilf