''matching liners and pistons con''

[grnicholson]

Hello

I have had the opportunity to delve into the purchasing of piston/liners and the myth perpetrated by Lotus that the pistons and liners have to be 'matched'.   On my 1971 S2 Europa the liner bore is 77 mm and the piston diameter is 70 mm.  What's to match?  This is a con to make owners pay for a full set of pistons and liners and that only benefits Lotus.  Is this sharp practice still continuing with other models?

There must be thousands of useful pistons and liners in sheds around the country and I need one!!

get in touch!   

[jbcollier]

It is possible to just replace a piston, no problem, if you can find a good used one.

[RoddyMac]

I'm still confused about the 7mm difference in bore to piston.  If the liner is bored to 77 mm the piston diameter should be something around 76.9 mm measured perpendicular to the gudgeon pin.  For instance, the Twin Cam had a piston clearance of .076 mm to .091 mm (.003" to .0036" for the non metric),   having a 7 mm clearance would allow for a bit too much blow-by.

But as John said, you can fit a "non matched" piston to a liner, just make sure the clearances are correct. 

[Bainford]

How thick is a cylinder liner? It sounds like there is a confusion here between piston dia and liner ID vs OD.

[jbcollier]

Liner ID:

81 mm stroke

1470 - 76 mm

84 mm stroke

1565 - 77 mm

1596 - 77.8 mm

1605 - 78 mm

1647 - 79 mm

[grnicholson]

Hello

In the technical manual there is no reference to liner/piston sizes and that is why I measured them.  There is no doubt!  The i/d of the liner, in good condition is 77 mm and the piston, like wise, is 70 mm - a huge difference in engineering terms.  There is no possibilty of having 'matched' pairs, - it's a con!  .................... unless of course you can prove me wrong.

Richard

[jbcollier]

7 mm clearance?  That piston would have self destructed in minutes if not seconds.

You are obviously measuring the piston in the wrong place, or are incorrectly using the measuring tool:

https://www.youtube.com/watch?v=EO4HHuu9rb0

You can use a vernier caliper but it is not as easy to get an accurate reading:

https://www.youtube.com/watch?v=ELKlVpVkX3A

[jbcollier]

Regarding the "myth" of matched pistons and liners, all pistons and liners are matched.  It depends on who is doing the job as to how it is done.

On the manufacturing line, no two bores are absolutely identical and the same goes for pistons. After manufacture and final machining, pistons and bores are graded, often using A, B and C nominclature.  The differences are subtle but real, usually in 0.0003"/0.005 mm steps.  It may not seem like much but if a slightly oversize piston is matched to a slightly undersized bore, disaster in the form of a warranty claim looms large.

On the repair side, bores are machined and final honed to match each individual piston.  When you get an engine back from your machinist, each piston will be marked to its matching bore.  Now you should double check each piston/bore combination to make sure the clearances are correct.  I have built a ton of engines and every now and then you come across a slightly tight fit.  Again, not often, but I have sent back three or so for a wee bit of a tweak.

Engine building is a methodical process of minding your Ps and Qs.  Never take anything for granted.  Always double check everything as you go along.

Now back to the original question.  Are the pistons and liners matched in Renault engines?  Yes, of course.  Renault does not supply oversize pistons as they assume a  failed piston means a failed liner. Therefore they only sell them in sets.  However, there is absolutely nothing wrong with taking a mismatched piston and liner set, carefully check their assembled clearance and lightly honing the liner if required.

[StrawberryCheesecake]

It's less common in these days of tighter machining tolerances, but 'grading' of parts is/was a common industry practice. If you can't produce something to the tolerance you really need for performance and reliability targets, at an affordable price, then you measure and grade the parts into bands within the manufacturing tolerance.

Common practice with things like engine bearings, pistons, etc.

Pistons could be graded by diameter, weight, or both. Performance engine builders would look to remove material from pistons to get them as close as possible to the same weight, so the rotating assemblies can be balanced more accurately, and the engine will rev higher without shaking itself to pieces.

Can you do a budget repair with a single piston of unknown grade, and will it run OK? Allmost certainly.

Will it produce optimum power and longevity? Maybe not, but it if you wanted that, you'd rebuild the whole engine at great expense, or fit a later, more powerful motor.

Is it a con? No.
Is it 100% necessary to used matched parts in every repair? Probably not.