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molemot

Cylinder liners and piston clearances

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YOU may recall that I had the misfortune to seize up nos. 3 and 4 cylinders on my 1982 Turbo, back last November. I admit I was going far too fast for far too long, but I had thought that 4000miles of careful running in after new pistons and liners would have permitted giving the old girl a bit of stick. All was well until the jungle drums began behind my head at about 145 to 150.....

Anyway, I'm now in the middle of reassembly, and I have a Cautionary Tale to tell. I thought, once bitten etc.etc. so I bought a dial bore gauge to measure the liner diameters, which, with my digital caliper, meant I could do some accurate measurement of bore and piston and thus the piston/liner clearances. Of the 4 new liners, two were exactly 3.75 inches (my dial bore gauge is in Imperial so I stuck to that) one was 3.748" and one was 3.746". The Omega pistons all measured 3.746"...I spoke to them about the required clearances and they said "About 4.5 to 5 thou" and told me their pistons are manufactured to a tolerance of 5 microns (0.005mm).

Now, that means that I have two pistons with 0.004" clearance (just about acceptable); one with 0.002" clearance (too small) and one which won't fit in the bore without being biffed with a hammer!! So a bit of honing to size is called for.

I then thought of checking the seized cylinders from my last attempt at "new-pistons-and-liners"..and found that one of the seized pair gave me 0.0025 piston clearance, and the one that seized worst gave 0.0015 clearance..so no bloomin' wonder they seized, given that the motor had been giving 200 odd horsepower for 10+ minutes..

THE moral of this tale is...never rely on suppliers supplying you with properly matched parts!! None of the 8 liners I have now bought conform to Lotus specifications, as far as I can measure them; the largest diameter I have measured is 3.75" whereas the Lotus spec calls for 3.751" to 3.7515" (Grade "A") or 3.7515" to 3.752" (Grade "B") all measured at 90 deg to the gudgeon pin, i.e. across the engine, and 50mm down the bore. In most cases there probably wouldn't be any hassle, normal driving wouldn't cause the sort of failure I had...but it would always be better to check the measurements and do the sums to find what sort of clearance you are getting.

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That is a bummer, but like you said lesson learned. To go one step further, if you send your liners out for hone, once they return you should double check the clearances yourself. In the past, dealing with bored blocks I have found egg shaped bores upon re-inspection.

Thanks for sharing, as you have found one cannot be too careful or diligent.

Good luck,

Jeff

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Thought I might try to hone them myself with one of those electric drill driven things...after all, I have 12 liners to play with.....!! Then I can check as I go.

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No excuse for .004 difference in tolerance these days. Shoddy! Where did you source them?

Maybe thats why the originals were only ever supplied in matched sets.

cheers Steve

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They came from a "well known Esprit specialist" whom I have emailed....now waiting for a reply.

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Just a suggestion. The machine shop that did mine specialized in big bore motorcycles. Did a spot on job and wasn't expensive.

cheers,

jeff

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Molemot

I would say speak with Steve from SJ. I looked into this when I redone mine. I checked my pistons and they were grade A. I spoke with Steve he said forget all that bollox that dont matter any more because Lotus blah blah blah. I thought this guy doesnt know toffee. Anyhoo I measured my liners with a bore gage and micrometer (inspection grade, calibrated on inspection grade slip gauges. If you are setting your bore gauge on a digi claliper you cant go on the size because even the Mitutoyo ones are only good for +/- 0.02mm) My liners were the original ones form 1985 and I couldnt work out what size they were ment to be. I cant remeber what they were but they wernt what the Lotus book said they were? Any way I bought the new ones from Steve and guess what? Yep exacly same size to the micron as my old ones. Now before you ask I was not measuring where the wear had taken place!!!

As I say have a word with Steve I think hes put togther enough engines over the years. He does come across a bit short...but I would sayhe know what hes talking about!

All the best with it what ever you decide

Buddsy

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Steve's the man I bought the liners from....!! I appreciate what you say about setting the bore gauge with a digital caliper (and it is a Mitutoyo, by the way, reading down to 0.0005") and .02mm is about threequarters of a thou.. but you can't get over the fact that the liner bores vary...and one of them has zero clearance - proved by the fact that you can't get a piston in it!! Also, I've measured the original Lotus liners and they would seem to confirm the accuracy of my bore gauge setting. I'm convinced that the cause of the original seizure was undersized liners...all 8 of the liners I've so far purchased from S&J have been undersized, the largest diameter measures exactly 3.75", so is undersize to the Lotus specification. It's cost me a lot of time and money...so I thought I'd warn the Forum membership to check liner sizes rather than go on blind faith!!

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Agree entirely with you John, dial bore gauges are not that expensive and give you great confidence that you are measuring the true bore diameter. I've used mine all over the engine and recommend the tool to anyone with an engine in bits.

With a parameter so important as piston bore clearance it seems risky to rely on anything but a direct measurement of the parts, no matter how many engines the supplier has built you may be unlucky and get a dud.

I have not bought my new liners yet, mainly time/work pressure but also due to some ambiguity over this parameter with the Mahle forged pistons I bought ages ago. I have yet to find a definitive value for this parameter when used in iron liners - Mahle can't tell me so I am contemplating buying another set from someone who can say with absolute certainty what the clearance should be...

Ambrose

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I'm surprised that Mahle can't tell you...I had a nice chat with the man at Omega and he said "4.5 to 5 thou" for the application. I'd be surprised if Mahle pistons were any different..perhaps you ought to have another word with them?

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I agree and disagree with you lol!! All should be the same with in microns. Hitting those tolerances should be simple and honing gives micron accuracey fairly easily. Not knocking you John...well I guess I am in a round about kind of way(sorry) but I would be more likely to think the liners are correct and your method of measuring is more likely to be out? You sure you checked the scale on the dial gauge? If they are that far out I would just send them back. If they were 0.005mm out I would return them. Mine were all to within a couple of microns of each other 0.002mm.

Im sure you will get it sorted

Buddsy :sleeping:

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Scott, mate, contrary to popular opinion I am not an idiot. Using the same dial bore gauge in each cylinder in turn, without adjusting it inbetween, gives the variance I quoted. And I didn't measure it once, but many times, with the same result. I'd like you to explain how my measuring system could be in error, as the measurements are totally consistent for each liner. I have only one certified slip gauge, at 50mm, and the digital caliper I am using to set the bore gauge reads that spot on. Note I am working in Imperial units, as my bore gauge is calibrated in inches. Were my method to be incorrect, then - if all the bores were accurate and the correct size - I would expect that ALL the measurements would be wrong, but consistent with each other...the same wrong answer for each cylinder. This is not the case, and the differences I quoted are real. The cylinder measuring 3.746" is too small for a piston to fit in, it's nearly an interference fit. The pistons fit quite comfortably in the liners measuring 3.75". Measuring the pistons with the digital calipers shows all of them to be 3.746", which is what I would expect, giving a clearance of 0.004 on the liners measuring 3.75", consistent with the acid test of "Do the pistons fit?"

I fully agree that getting cylinders to the correct dimensions isn't rocket science, which is why I just assembled the last set of pistons and liners, assuming as you say that the sizes would be correct. Sadly, they weren't...the fact that I seized two pistons, and when I measure those cylinders they are too small giving unacceptably small piston to liner clearance, wheras the other two give a 4 thou clearance (barely adequate I would say, I'd like to see 5 thou) also shows that my measuring is accurate and it is the liners at fault.

As for sending them back...I've had 8 now, from the same source, and none of them have been to the Lotus Grade A or B sizes; the greatest diameter I have measured has been 3.75" on 4 of them and the others have been significantly undersized. No consistency at all. Also my workshop is in France, and shipping cylinder liners about would be an expensive business with no guarantee that the replacements would be any better. No, I think I'll try honing what I have so I maintain control of the process and end up with something to the correct diameter.

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If I bought new liners I would expect them to be supplied bored slightly undersize ready for honing during assembly to final size to match the actual pistons to be used. This appears to be what you got. it is not economical to hold finished liners in graded sizes to match all the potential piston types and tolerances on the parts market

It just surprises me that the supplier does not tell you this - maybe he assumes you know.

cheers

Rohan

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Rohan there is no way you are going to be able to hone 0.1mm. Grind or turn but not hone. They should be finished to correct size for sure.

Buddsy :sleeping:

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Morning, folks!...O-KAY.What have we got here.....? To start with, my measurements have been made with a 3-legged dial bore gauge, set from my Mitutoyo digital caliper. This caliper reads my 50mm standard length exactly, so I'm pretty confident in it. The dial bore gauge is calibrated down to 0.0005", it's new and has a calibration certificate so I reckon that's good too. Now to measurement...all of these measurements have been made several times with consistent results, and I've converted the Imperial to Metric to help out those of you who are numerate in only one system (!) (as an aside, I was once interviewing youths for apprenticeships in a motor vehicle workshop..one of our questions was always "How many tenths are there in an inch?" Often, we'd get the answer "I don't know, we've only done the Metric system."! Hmmmm..how many tenths are there in anything??!!) so this is what I have:

Latest set of 4 liners

1.........................2............................3.......................4

95.25...................95.25......................95.15................95.20

Original set of 4 liners (3 & 4 seized in use)

1..........................2..........................3........................4

95.25...................95.25.....................95.19..................95.21

All measurements now in millimetres.

To remind you, Lotus grade A is 95.28 to 95.29 and grade B is 95.29 to 95.30. None of the 8 liners fall within the limits.

Several things reinforce my conclusions...two melted pistons, from 3&4 of the original set are a pretty strong argument(!) and the fact that the piston will not fit in no.3 of the latest set..well, if I hammered it, it would go in, but there is less than half a thou of clearance, I would say..

The piston manufacturers, Omega, say that the pistons are manufactures to a 5 micron tolerance..that's 0.005mm.. so I would contend that they can be considered as accurate; my digital caliper measurement has them all at 95.15mm. Omega say that piston/liner clearance should be 5 thou..that's 0.127mm. None of the 8 liners will give that clearance. I would further contend that if you buy pistons and liners from the same source, and the liners are honed, then it is reasonable to expect that the liners have been honed to size to suit the pistons supplied. Since none of the liners conform to Lotus grade A or B dimensions, but are ALL undersized...and vary between each other to such an extent as to reduce piston clearance to zero in one case...it indicates to me a severe failing in quality assurance. And that's another job I used to do...!

Thing is, don't want to slag off one of our best parts suppliers; if Steve has ordered a batch of liners and specified the dimensions he requires, and receives the completed batch, is it reasonable to expect him to measure every liner supplied with a 3 legged bore gauge to check that his supplier has done the job properly? Who is going to get the task of inspection? Who is going to train them in setting and using a bore gauge? S&J are a small niche outfit, and we rely on them a lot for our parts. I have emailed Steve with my findings, and we'll see what he says. My reason for posting this was just to make everyone aware that, before you happily nail things together with blind faith in your supplier, a bit of inspection and measurement is a good idea..and that way you should pick up the problems before everything starts to rattle back there!!

Edited by molemot

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not being funny but if you are building an engine should you not check the parts first not just assume that whats supplied is correct checking ring gaps is the first thing you do then piston to liner gap I may be wrong. it's like not checking the big ends and finding one was ground 10 thou down and the engine rattles on start up.

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Paul, you are quite right, and that's the major reason for this post. If you have machining work done, then checking the results would be normal. Having said that, if you know the machine shop well, and their work had always been good, you might accept that what you received had been done properly. When buying a set of pistons and a set of liners from the same firm, one could reasonably expect that the liners had been bored and honed to a size to suit the pistons supplied, especially as it is obvious that they would be used in conjunction. I have rebuilt quite a few engines and always had the machining work done by a machinist I trusted, who also supplied the pistons; thus the clearances were never an issue. Sadly, that machine shop has gone to the great toolmakers in the sky....

To re-iterate....as Paul says, and as I said in my initial post...Never assume...CHECK!!

Edited by molemot

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Whatever anyone may say about the measurements, if a piston will not physically fit in a liner, something is wrong without a shadow of doubt. I think it should be reasonable to assume that the parts are correct. I wouldn't expect my local Lotus dealer to measure each piston and liner before rebuilding an engine just as I wouldn't expect SJ or Paul Matty or JAE to check to make sure they are to spec before shipping. SJ's suppler is not doing their stuff on quality assurance.

I'm glad it's been brought to out attention so no one gets caught out.

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I've found this out the hard way as well, bought a ford xflow engine that had just been reconditioned as a short engine with head removed. Turned out the guy who built it put the crank thrust washer in back to front with the white metal bearing surface against the block... He was a fellow enthusiast rather than an engine shop, so I should have checked.

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As John points out, .004" is a country mile. You could easily detect it, if not measure it, with pretty with basic tools. e.g. a straight bar and feeler gauges.

What are the options for accurately removing such a small amount of material to get the clearances correct? I assume (maybe incorrectly) that its now going to be pretty difficult to set a machine to get the bore concentric and remove perhaps a 2 thou cut?

(I remember Pete at PNM telling me that he had unsuccessfully experimented with boring out liners by a few thou in an attempt to fit oversize pistons - Although thats exactly what I have in my car, but I sourced them in the States)

Steve

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Steve

Getting them concentric is not the issue. Size and finished with the correct surface roughness would be the harder part...but tat the end of the day if you buy liners they should be right. The issues of size and using a digi calliper in many ways that irrelevant too. The liners should all be at least the same size when a bore gauge is being used as a comparator between the bores!!

Buddsy :sleeping:

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I fully agree Buddsy, after all they don't give this stuff away! The obvious concern would be that the quality control extends to length, OD etc. Interesting to see if 'nip' is all within tolerance.

cheers Steve

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Yes, the liner "nip" was within tolerance, and the bores were nicely honed with a good crosshatch pattern...just the wrong sizes!! I'm going to try honing to size and see how it goes...nothing to lose and I've got 12 liners to play with, if you count the original Lotus ones as well. So should be able to get my hand in before going for gold, so to speak...I'm making a fixture to hold the liners and converting my pillar drill into a honing machine; it'll probably be a bit messy with lubricant everywhere, but it'll be another thing I've never done before... The amount to be removed varies from 0.0005" to 0.002" and I think I should be able to hone off that much, with a selection of different grit stones ending up on fine. Pressure, rotational speed and speed of stroke seems to be the variables...

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