View single post by Esprit2
 Posted: 04-26-2015 04:24 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 330

The liner bore is to be measured at 50mm down from top, across the thrust axis
Grade A = 3.7510 - 3.7515" (95.275 - 95.288 mm)
Grade B = 3.7515 - 3.7520" (95.288 - 95.308 mm)

The piston diameter is measured 15.0876 mm (.594") up from skirt's bottom edge, and 90° to the wrist pin bore:
Grade A = 3.7467 / 3.7472" (95.1662 - 95.1789 mm)
Grade B = 3.7472 / 3.7477" (95.1789 - 95.1916 mm)

Piston/ liner clearance is obtained by subtracting the piston OD from the liner ID:
Clearance = 0.004 / 0.005" (0.10 / 0.13 mm)

You report your clearance is 0.00787" (0.2 mm), which is too much.

The rings' proper fit is primarily responsible for the compression and oil seal; and the piston/ liner clearances has little to do with that. However, the piston can't be allowed too much room to flop around, or it will result in piston slap and excessive wear to the piston & liner.

Both proper ring fit and piston/ liner clearance are important, each for different reasons. It's not okay to discount the piston's loose fit because your first concern is ring sealing and stopping the excessive smoking due to oil getting past the rings.

I suspect (just guessing) the liners were worn and the previous owner honed them enough to clean up the surface and produce a good cross-hatch pattern; but in the process the liner ID ended up oversize.

What are the dimensions you measured for the piston ODs and liner IDs ?

OEM pistons & liners are no longer available from the factory, either Jensen or Lotus. Your only hopes for 'original' is to find a dealer/ specialist who is sitting on a set of NOS parts, or to buy good used liners.

However, if the liners are worn oversize, they don't have to be replaced. Instead, purchase oversized pistons, and have the liners bored as required to produce the specified clearance fit.

JE Pistons makes nice forged pistons for the 907 that are superior to the original cast pistons. They come with new rings and wrist pins, and can be ordered in several oversize diameters and several compression ratios.

Pistons and liners should be match machined. Don't just order pistons 0.0315" (0.8 mm) oversize from one source, and have the liners bored oversize the same amount by a local shop, and expect the clearance to turn out correctly. At a minimum, order the pistons first. Then take both the pistons and the liners to a machine shop hand have them bore the liners as required to produce the correct clearance fit. Each piston and liner pair becomes a matched set with their clearance near the minimum end of the specified range (above).

If you order JE Pistons from JAE in California, USA, then they work with a machine shop near them. Order oversize pistons, and ship you're old liners to them. They will bore the liners to fit the new pistons. For an additional fee, they will also gap the rings to fit. You receive matched piston, rings and liner sets that are ready to install. On your side of the pond, I 'presume' Garry Kemp in the UK provides a similar service.

Targeting piston/ liner clearance at the minimum end of the specified range give the greatest allowance for future liner wear. If the clearance ends up at the top of the range, then very little liner wear will be required to result in the clearance going out of spec too large.