E: Engine piston for alloy liners

[stumpy lord]

Hi,
having got cheesed off with the delays and excuses , amongst other things, from the guy who supplied alloy liners. I found a company who manufactured a liner for me in alloy. I was wondering what would be a good piston to use in this set up for my Comet.
many thanks,
stumpy lord

 

[davidd]

I have used CP pistons as well as JE. I would think almost any piston would be fine. Certainly Omega should be included. Often the rings are included and it it the rings that should be selected for the Nicasil bore. I would specify that with the provider of the piston. If the pistons come without rings I use Total Seal and when ordering I tell them it is for Nicasil.

Typically, I would send the piston with rings (and muff) to the company that is plating the bore with Nicasil. In the US, Advanced Sleeve and Millenium Technologies are the sleeve maker and plater respectively. I send the pistons and muff with a drawing of the sleeve to Millenium (they own Advanced Sleeve and order the sleeve directly). They send back a fitted cylinder and piston ready for assembly. It is a little difficult to get them to clearance it at .003" because they are used to doing it at .001", but they will do it. They consider .003 worn out, but for an air cooled single it is quite tight. I would think that your sleeve needs to be rough finished by the plater for the thickness of the plating that will be deposited on the liner. Millenium machines the muff for a slip fit on the cylinder and uses Loctite to secure it. They then plate the bore and hone it to fit the piston.

David

 

[BigEd]

Hi,
Having got cheesed off with the delays and excuses, amongst other things, from the guy who supplied alloy liners I found a company who manufactured a liner for me in alloy. I was wondering what would be a good piston to use in this set up for my Comet.
Many thanks,
Stumpy Lord

This is not directly answering your question about what piston to use but might be of some use. My friend Tony has built a few engines with plated bores. (Note these were not Vincent engines). I asked him what he had done and he sent me this reply. The reference in the sentence starting "As we know Eddy ..." refers to me helping him put a steel liner in an alloy muff but does give an indication of how much the muff expanded when we heated it.

"I have ceramic coated / Nicasiled 5 single cylinder barrels. All were straight onto the LM25 castings, all 86/87mm bore. Two were race engines which had very little running in, if any. As we know Eddy, from the other day, at about 180ºC the bore will expand some .015" so even with the heat at normal running temperature I would think it will expands about .008"- .009" ish no problem, the piston a lot less. An air cooled engine will expand more than water cooled especially running in hot weather in towns so a tighter piston cold is fine. Just warm up a little slower.

Cast pistons expand less then forged ones I have found. All my pistons in the coated bores run about .0015" - .0018" (Under 2 thou ) cold clearance. All have showed no sign of nipping up and all bedded in quickly with no oil burning or smoke. A smaller bore would run much closer clearances. Remember as the barrel gets hot it will expand much larger than the piston does so yes, .003" to start I would say is far to big a gap and waist many miles on the bores and on a modern short skirt slipper piston would not be advisable in my humble opinion.

The rings MUST be made of cast iron in nicasil bores, do not run chrome rings. The company I use in the UK is Poeton Aptec Ltd. They are based in Gloucester Tel 01452 300 800. I believe they do a lot of the F1 motors so they know their stuff. They don't ask for the piston only the size to finish at. I suppose if you have no proper measuring instruments then it's best to leave it to somebody who has and can use them.

I bore the alloy barrel or liner so the piston just fits the bore. I then give them (Aptec) the size I want them to finish at, i.e. measure the widest part of the piston at the bottom over the thrust faces and add the clearance as stated above. They then GRIND the bore out plate it and grind and hone to finish size. They only put .003" - .005" thou of plating on. Whatever you do always run air cleaners. the slightest bit of grit could damage the bore. The bores will last many 1000's of miles as most BMW boxer owners know.

I assume they sleeve the Vin barrels to bring them back to std size because of capacity limits.
The above is just my experience of almost 30 years with this type of bore and I am sure I will be shot at dawn for it. However I do hope it will be of use to some.

Kind regards, Tony"

 

[Peter Holmes]

Millenium machines the muff for a slip fit on the cylinder and uses Loctite to secure it. They then plate the bore and hone it to fit the piston.

Doesn't that cause a problem with the transference of heat from the liner to the muff, when I first took an engine to Tony Maughan many years ago I remember that he was most insistent the interference fit between muff and liner was most important, without it you get hot spots and distortion, has that opinion changed?

 

[davidd]

Doesn't that cause a problem with the transference of heat from the liner to the muff, when I first took an engine to Tony Maughan many years ago I remember that he was most insistent the interference fit between muff and liner was most important, without it you get hot spots and distortion, has that opinion changed?

Peter,

It seems to have no ill effects. Millenium has done thousands of cylinders this way. They plate the cylinder right through the Loctite, so it does not seem to have any impact on either the mechanical bond or the electroplating process. I think that we have done around two dozen Vincent cylinders this way. However, the chaps that do it in the UK may not use this same process. Millenium wanted to minimize the squeeze of the muff on the alloy liner so that the line would not have any excess stress on it and stay true.

When you start an engine with these cylinders you can feel the heat out of the muff immediately. It seemed like a good thing here in Florida. It is also useful if you want to sleeve down to the original size after going to a 600. The thickness of the aluminum liner can take up the difference with good heat transfer as well as light weight. I have used stock thickness liners as well as thick with no noticeable difference, but I think thick is good to support the piston when racing.

David