Piston Clearance

[Hamish Coutts]

Just checked the diameter of the bores in my twink and I think I have discovered why I have so much piston slap. The bores are too big.
Before I come to any final conclusions can someone please tell me what the correct clearance is between piston and bore wall. I have heard mention both 2 and 4 thou.

Measuring my piston at right angles to the gudgeon pin about 1/2 inch from the bottom of the skirt, I get 3.267" The bore is measuring 3.274" which seems a bit big to me. All the bores measure the same and they are dead parallel - no wear.
I think the pistons are +20 and are solid skirt.

The engine was rebuilt not long before I got the car 15,000 mls ago. looks like the rebore chappy was a bit too enthusiastic!

Any advice welcomed.

Regards,

Hamish.

[rgh0]

Required clearance depends on piston design, material and type of use (eg road versus racing etc).

The forged JE pistons I use have a specified clearance of 0.1mm ( 0.004 inch) measured at gudgeon pin height at right angles to the gudgeon pin which is the normal point for specifying piston diameter for bore clearance.

The original Lotus piston clearance specifications were given in the manual as below , however no measurement method was specified

Type A pistons 0.0027 to 0.0033 inch
Type C pistons 0.0030 to 0.0036 inch

2 to 4 thou would probaby work with most solid skirt pistons in a road engine of about 3 to 4 inch diameter bore.

0.007 inch clearance sounds to big for any normal piston design but you need to confirm with whoever the manufacturer was.

regards
Rohan

[Hamish Coutts]

Thanks Rohan.

What's the difference between a type A and C piston? Are there any markings on the piston to help identify the type?

My engine is from an +2 S130.

Regards,

Hamish.

[types26/36]

The Wilkins book shows the "A" and the "C" type pistons on page 191, the A type has large valve cutouts and the C type has small cutouts.
The markings on the piston crown normally only show the size but there may be a name inside the piston.
I also think that a clearance of nearly 10 thou is to much, as far as I am aware Lotus only ever supplied 15 thou oversize pistons and any other +20 +30 +40 etc were aftermarket suppliers Hepolite/Malie etc.
Brian