Pistons and rods: what owners fitted on the Turbo and NA

The short version

• Turbo: owners said the standard Z20LET piston is cast and the Astra VXR (Z20LEH) piston is forged, and several owners preferred the VXR one to an aftermarket forged set.
• The balance of the rotating parts matters as much as the piston choice. Owners advised balancing pistons and rods, and one found a matched set out by a couple of grams.
• NA: the Z22YH piston has a raised dome, giving higher compression, but owners said it needs the Z22YH rods as well.
• Owners said you can change standard-size pistons in a Turbo with the head and sump off, without taking the engine out.

Turbo (Z20LET): which piston

The advice from 2009

• An owner asked in 2009 whether changing pistons and rods on a Turbo was an engine-out job. Another said that if they are standard size you can just take the head and sump off.
• The owner was going for forged Woessner pistons. A racing owner said he had chosen Arrow rods (with grooves that act as piston cooling jets) and VXR pistons, reasoning that the VXR ones have a lot of development behind them. He had raced most of a season with them and was running over 400 bhp.
• Another owner agreed the standard parts are better than many people think and cheaper. He said cars were running about 350 bhp on stock VXR engines, and that decent balancing would be worth thinking about.

What a Mahle contact said (2011)

A seller found his old Z20LET pistons were Mahle and asked what that meant. A long thread followed, in which a poster who said he worked for Mahle (unofficially) gave these details:

• Mahle pistons are either forged or cast. A "P" in the number marked inside the piston means forged. He said there is no such thing as "semi forged".
• Z20LET: OE supplied by Mahle, cast, and available aftermarket (part number series 012 20, with the last two digits the size).
• Z20LEH: OE supplied by Mahle, forged, and not sold in the aftermarket. Its alloy is stronger and more resistant to detonation damage than the C20LET's. The skirt has a Grafal coating, the factory piston-to-bore clearance is 0.05 mm, and the ring pack is lower friction.
• C20LET: forged, sold aftermarket (part number series 011 79). It uses a different alloy and is tin plated all over. Its bowl is about 5 cm³ smaller than the Z20LEH's and its ring layout differs. Many people use C20LET pistons in the Z20 engine, which he believed needs the bowl modified.
• The Z20LET cast piston's bowl is about 0.5 cm³ bigger than the Z20LEH's.
• Replacement Mahle pistons are made to the OE specification with the same tooling. There was no Motorsport range for these engines then, and he said they do not make one-off custom pistons.
• Forging does not itself need extra piston-to-bore clearance. That depends on the alloy. Many aftermarket forged pistons use 2618 or similar alloys that expand more, so their makers specify larger clearance.
• His view: if the bowl gives the compression you want, the Z20LEH piston is the better option unless you want really big power or nitrous.

Other owners in that thread said:

• The standard cast Z20LET pistons break the ring lands under higher cylinder pressure, or with detonation on a poor map. One owner thought the cast pistons were good for about 320 bhp, which he said he had heard, not proved.
• One owner argued that there is a need for forged pistons depending on use, and another that they should be avoided unless you accept more frequent re-honing, as some forged sets need nearly double the piston-to-bore clearance. He said a friend's 550 bhp engine with forged pistons needed new rings after 20,000 miles. That is one anecdote.
• One owner said he had run C20LET pistons in a Z20LE-based block for over 5,000 miles at 500 bhp with good compression, little blow-by and no oil consumption.

Checking new pistons (2016)

An owner waited two months for a Wiseco forged set and found minor marks on the outer edge of three crowns. The advice:

• Send photos to the supplier first, so there is a record of the damage, and ask about it.
• Check the marks haven't left raised material on the top land, which would score the bore. One owner suggested knocking down high spots flush with a diamond nail file. Two owners said they would use them, after rounding off the marks.
• One said that area is not a high-temperature part of the piston and that sharp edges are tolerated there. Another said the manufacturer would blame transport, so don't expect much.
• Weigh them. An owner found his matched Wiseco set out by a couple of grams. The owner of the marked set planned to balance both the pistons and the rods.

NA (Z22SE): raising the compression

The question (2013)

An owner wanted 11:1 pistons for standard rods and could not find any from the usual makers. Another said Wiseco do custom 11:1 pistons to order, and that he had a spare set. Another had looked at a firm that could make 4032 alloy pistons at 10:1 for a Harrop-supercharged Z22SE and asked how to fill in its order form (no answer was given in the thread). One owner called Wiseco pistons "rattling oil consumers", which was his opinion.

Skimming the head

• An owner said the Wiseco 10:1 pistons are 10:1 with a 1 mm thick gasket. With a Z22SE gasket (about 0.5 mm) you would get about 10.5:1, and skimming the head another 0.5 mm would give 11:1. Another suggested a Z22YH head gasket, which he put at about 0.4 mm.
• Two owners worked it through. The standard chamber is about 61 cm³ including the gasket (549.5 cm³ swept per cylinder and 10:1). Taking 1 mm off a round face 86 mm across removes about 5.8 cm³, giving 55.2 cm³ and roughly 10.95:1. Another owner doubted a 0.1 mm = 0.1:1 rule of thumb and said the head is domed, so each cut changes the volume by less than the last. A third said he would take the calculation with a pinch of salt unless it had been done and measured.
• One owner preferred a high-compression turbo engine with a proper map to a high-compression NA.
• The owner who asked did not want to skim the head.

Z22YH pistons (the direct-injection 2.2 from the Vectra C and Signum)

• One owner said he had built several engines with Z22YH pistons and rods to raise the compression, and that they work very well. He said the pistons need the Z22YH rods, because the gudgeon pin is different. A second-hand Z22YH engine was under £200 in 2013, and you can either swap the crank or the rods and pistons into your engine.
• The Z22YH is used in the Vectra C and Signum from 2004, and he believed the later Zafira. Other pistons from Saab 2.0 and later variable-valve-timing engines have a different crown and are not ideal, he said.
• An owner said Z22YH pistons and rods are heavier than the Z22SE ones. A 2021 owner who weighed them disagreed on the pistons (see below).
• An owner said he had been told you can reuse your Z22SE rings and avoid honing. Nobody confirmed it.
• Opinions differed on whether the Z22YH piston is a "drastically different" direct-injection piston. Others said the YH was an early design, close to a normal piston with no combustion chamber in the crown, and that the Saab ones are the odd ones out. One owner found the Z22YH's compression advertised as 12:1 in places. We can't check any of that.

A 2021 comparison

An owner who bought a set of Z22YH pistons (0.5 mm oversize, 86.5 mm) compared them with the Z22SE ones, picking the YH for its higher compression and better skirt design. What he found:

• The Z22SE crown has a flat ring and slight dish that makes squish at top dead centre. The YH has a raised dome that raises compression, and a rebated tongue at the back for the direct injector. This is also a quick way to tell if a piston is fitted the wrong way round.
• The YH skirt has a screen-printed black anti-friction coating that wears to conform to the bore and helps cold-start lubrication.
• The YH gudgeon pin is shorter, thinner-walled and, he thought, better made. Both are 20 mm. The pin bosses are closer together on the YH, so it needs a narrower rod.
• With the pin, the YH piston was 387.8 g, just under 2 g lighter than the SE piston and pin (clips not included). His YH pistons were oversize, so standard ones would weigh slightly less.
• He said the YH is not just a reworked SE piston.

He later posted pin and dome heights, but only as images, so we don't have the numbers.

The small 20 mm pin

A later poster asked whether anyone had broken these small pins, since most four-cylinder engines use larger. The owner with the comparison said the 2.0 turbo and supercharged variants (such as the LK9, LNF, LQ8 and LSJ) have a 23 mm pin, and the 2.2 Z22SE and Z22YH and the 2.4 have the 20 mm one. Another poster said a racing team had told him their engines had gone a little over 8,000 rpm without trouble. The first owner also said the Roush NA race engines based on the L61 (Z22SE) used Eagle rods and an aftermarket 20 mm pin, and made about 340 bhp on a long-tube exhaust. GM reportedly tested the standard Eagle 2.2 rods to 8,000 rpm and 500 bhp, though he added you can break any part.

The 2013 group buy: Eagle rods and Wiseco pistons from the USA

In 2013 a group of NA owners bought rods and pistons together from a US supplier. What the thread shows:

• Prices. One set was $804.99 plus $99.99 shipping. Three sets were $780 each with $199.99 combined shipping, and five or ten sets $764 each (shipping $249.99 and $350). Separately the supplier quoted pistons at $509.99 and rods at $369.99, cheaper as a combination. The final cost to the group was about $846 each delivered, which they converted to about £525. Each buyer paid £530 to the organiser, who placed one order for five sets.
• Import duty. The organiser expected up to £150 for the lot from a duty calculator. The bill came to less than a third of that. He quoted the official rates as 3.5% duty plus 20% VAT.
• Payment. The supplier would only take one PayPal payment to one address, or separate wire transfers. They chose one payer.
• Size. The pistons were 8.9:1 and either 86.0 mm (standard) or 86.5 mm (0.5 mm oversize). An owner said a new engine wants 86.0, which saves a rebore (£150 or more). A used block that has already been bored needs the oversize.
• Delivery. The parts came by courier in October 2013 and were then passed to the person building the buyers' engines.

Converting a 2.0 Saab engine to 2.2

Some of the group were building 2.2 engines on a new Saab 2.0 block. What owners said:

• To make it a 2.2 you need the Z22SE crank, 2.2 rods (the Eagle ones) and 2.2 pistons at standard bore. A Z22SE bottom end could be bought for about £50 for the crank. Other parts mentioned were a Saab or Z20NET head gasket (about £50) and GM head bolts (about £25).
• One owner said the LSJ rods are rated at 400 bhp or more and are fine on a 2.0. Another pointed out that with the 2.2 crank the stroke is longer, and LSJ rods and pistons would leave the pistons well above the deck, so a 2.2 needs 2.2 rods and pistons.
• To keep it a 2.0 with a Harrop supercharger, the advice was LSJ forged pistons (from the supercharged LSJ, not the later direct-injection turbo LSJ or LNF) and an external trigger wheel and crank sensor.
• An owner who supplies parts for the conversion offered idler gears for the balancer deletion, which is described in Balancer shaft deletion: what owners did on the Turbo and NA.