Piston to Bore Clearance - the Saga continues

[dlb123]

Hello again. Back this time with another technical question regarding the p to b clearance on the twincam.

The block was overbored by 40 thous about 10,000 miles ago when the engine was rebuilt. All four of the cylinders measure up very well (i used a telescoping gauge and micrometer to the best of my abilities), and are consititently of the diameter 3.288-9" (83.53mm) top to bottom in both planes. Is that consistent with a 40thou overbore? I would be inclined to think so working off the standard out of factory specs of 82.55mm.

The pistons which were fitted at the time of rebuild are branded 'AE', which im told is good manufacturer. They are in excellent condition as you'd expect given how young they are. The largest piston measures 3.286", and the smallest measures 3.2845". With these figures in mind, the largest piston to bore clearance is around 4.5 thous, which is over the recommended 3 thous in the manual. I know that oversized pistons have their own particular specs, but cant seem to find any info on these AE variety.

Now I would not normally be so neurotic about the numbers, but the machine shop I left the block at told me i was going to need a re sleeve at a cost of well over ?500. Given that the block measures up well as per its overbore, and given that the engine ran well with no obvious slap or oil burn, I took the block away for further investigation. My inclination is to put it back together and run it, but would very much appreciate some input from greater minds than my own.

Much Appreciated!

Dave

[rgh0]

Pistons are not straight cylinders and their diameter needs to be measured at gudgeon pin height at right angles to the gudgeon pin. Is this how you have measured yours?

Piston to bore clearance depends on piston design and material and the manufacturer will normally specify the required bore clearance for your application. Typical clearance is around 0.001 inch to 0.002 per inch of bore depending on the design of the piston, material and manufacturing method (forged or cast) and use that affects peak piston temperature ( road, racing turbo etc).

Your clearance is in the ball park and probably OK but you really need to check with AE

cheers
Rohan

[dlb123]

Thanks for the reply Rohan. Thats where I've measured the pistons from, as you suggest.
Trying to get through to AE pistons now but now to see what they have to say. Are there any pistons that come out of the box recommending 4 thou clearance? Seems unlikely.

Should I order new pistons then? I cant see why, given that the overbore is good to +40 thou and the pistons are also to the same spec, the clearance is so large.

Baffled

[rgh0]

The JE forged pistons I use in my competition Elan are designed for 4 thou clearance. i.e. 83.5 mm bore 83.4 mm pistons.

I always buy the pistons first and then bore the block to suit the piston specification

cheers
Rohan

[Craven]

Hi,
I like to use piston ring gap as a guide, if the ring gap is still in spec then fine to reuse.
Don?t mix imperial & metric sizes!!
FWIW
Ron.

[promotor]

You bore size is good so that is a good starting point. 3.250" + 0.040" = 3.290".

I have some new old stock in the box AE Hepolite Powermax +040" Lotus T/C pistons (part number 18950) and the supplied info does not make any mention of clearances it purely states bore size needs to be 3.250" + 0.040" which as said is 3.290". This is because the new piston skirt sizes can be controlled and only ones that pass inspection (IE proper size) will be allowed through and they have to be dead on to work with the specified bore size, so you simply bore the size and away you go. I still don't do that - I check all pistons and then bore / hone each cylinder to the size I have decided on to give the clearance I want.
The info does state that the finished bore size needs to be to a tolerance of -0.000" and can be + 0.001" so that gives you an additional 0.001" in clearance should you need it. I would think that these pistons were designed to use similar clearances as quoted in the Lotus manual as the pistons are the same brand IE made by AE.

I have just measured these new pistons and they all measure 3.2865" (which is pretty impressive!) - this gives a bore clearance of 0.0035" when using 3.290" . Add on the allowed margin and you've got 0.0045" clearance. It's always nice to be at the bottom end of the tolerance to allow for subsequent wear but those figures are really for brand new specs - worn engines can be far in excess of these figures and still run OK. In fact some racing engines run just fine with big clearances as it frees up horsepower! Classic Formula Ford engines (crossflow) run around 0.004" clearance with standard cast pistons as forged ones aren't allowed.

Does the smallest-skirted piston look worn or show signs of "picking up"?

[Davidb]

dlb123: Have you considered getting the pistons knurled? This used to be done a lot and works well. Just for your peace of mind.
The recommended piston clearance in n.america for Formula Ford seems to be 5 thou according to what I have read.

If it ain't broke, don't fix it!

[dlb123]

Hello again. This is a pic of the largest diameter piston which strangely is the only one with scuffs. There is defiantly 1.5 thousands difference between the smallest and largest piston. Surely that's wrong?

Thanks. Dave

[rgh0]

Potentially the pistons and bores were selectively match in the last rebuild. But just as likely someone screwed up or was cheap with a set of odd sized pistons

cheers
Rohan