Replacement pistons

[pwhitehouse]

I have now decided to grasp the nettle and rebuild the engine in my Plus 2S 130/5.

Surprisingly (to me) the block and crank are standard sizes although the bores are showing signs of wear (not yet measured). The car shows an indicated 110k miles so I guess this is not the original engine (its number is P 30316)

Any comments on replacement pistons? The manual suggests that this model should have a CR of 10.3:1 rather than 9.5:1. I have tried a number of sources so far and QED offered the most help but it appears that generally 9.5:1 are the only ones available. I have no intention of using this car in competition, although depending on what size rebore I use I might have to use Accralite/Omega pistons or an equivalent.

The block has LB stamped on the front face which I understand limits it to +40 thou rebore, so presumably I can use either +40 on a standard size piston or 83.5mm as an alternative which seems to be the smallest size that Accralite list.

Any suggestions before I take the plunge?

Peter Whitehouse

[andyelan]

Hi Peter

You should still be able to get Accralite pistons in +.020" so I'd go for these rather than stright to +040".

As for compression ratio, the pistons shouldn't make any difference. The 10.3:1 on the original engies was achived by skimming the head rather than by changing the pistons

Regards
Andy

[stugilmour]

Peter, I am pretty clueless on the details, but would mention that the guys at Dave Bean provided me with a complementary thicker head gasket in conjunction with 40 over pistons to prevent too high a CP for todays fuels. I just followed their recommendations when I was sourcing parts for my local re-builder, and it all worked out. Something to have in mind after you have the block measured I guess; others with more knowledge can probably be of more assistance.

HTH

[AussieJohn]

+1 for Andy's post; spot-on.

[JJDraper]

From the archives you should be able to see the original engine number. These engines will last if treated well, so 110k is possible.

If there is no catastrophic damage, its usually the rings that give out eventually, so if the bore is OK (no lip) try re-ringing it & hone the bore. Every re-bore is a life lost - mine is on +60!

Jeremy

[alan.barker]

if you stay with standard bore, try Miles Wilkins for original standard pistons. Not expensive and the same as your old ones, which were good for 110,000 miles
Alan B

[elj221c]

If there is no catastrophic damage, its usually the rings that give out eventually, so if the bore is OK (no lip) try re-ringing it & hone the bore. Every re-bore is a life lost - mine is on +60!

Jeremy

I would agree with Jeremy, although I think you will be lucky if you don't need to go to the next size up at that mileage. It does depend on how the engine has been treated. I had barrel shaped bores after 50K but that was running without air box or filters!

Save the 83.5s until you need them.

[andyelan]

Hi Everyone

When I did an overhaul on my engine, a couple of years back, I measured about 3.5 thou bore wear after 50,000 miles. As I had a new set of standard size pistons on the shelf I'd nothing to loose by honing the bores (to remove any step) and fitting these. They've been fine ever since. Thing might have been different had I had to buy new pistons. In that case I think I would have gone for +.020" and a rebore.

I'm generally not a fan of re-ringing old pistons. In my expierance it's the ring grooves in the pistons which suffer the most from wear so it's usually necessary to replace the lot.

Andy

[avro]

Hi Peter

With the youngest of Twin Cam engines now close to 40 years of age its quite rare to find an engine on its original standard bore size. It may be worth checking before you order your pistons that your cylinder block has not been fitted with cylinder liners. These may have been fitted during the life of the car but its certainly not unusual to find blocks that had these fitted from factory and this tends to be more common on the later engines with the 701M blocks. The liners are quite thin walled and the recomended rebore size is only 20 thou.
I have two in my workshop at the moment that I've recently rebored, both of which have been fitted with factory liners. A single liner on one block and all four fitted on the other.

As the liners on one of these blocks protrude slightly into the crankcase it has enabled me to acurately measure the wall thickness and with a .020" rebore it leaves the thickness of the liner at .048"

Liner material is a better quality and therefore has a better wear resistance than the parent material of the block. Its not unusual to find quite high milage engines on standard bore sizes when they have been fitted.

If your block has liners fitted then it may be worth dropping it in to your local machine shop for their advice.

Nick