Rather than continuing the thread drift from the previous thread, (here: https://www.jaguarforums.com/forum/m...ing-tin-251123) I thought I should start a new thread to discuss piston to bore clearances and experiences.

To recap: I have experienced piston seizure in two pistons in my 3.8 S Type after a rebuild when going up a hill at about 30% throttle, while using the workshop manual recommended skirt clearances ( 0.0011-0.0017"). A friend with a 4.2 E Type has also experienced piston seizure using 0.0015" clearance. Both of us were using Hepolite pistons and 9:1 compression. One 3.8 the other 4.2 engines. However, Glyn here has used the same clearances and had no trouble. I though this thread might be a good place to consolidate and continue the discussion.

Glyn, you had mentioned in passing exhaust gas temperatures and that made me think of another data point. I have a 1990 Daimler DS420 with the 4.2 engine, but 7.5 compression. It uses a pair of SU HIF44 carbs, and I have tuned them to be about 14.1 at highway cruise, and under acceleration or heavy throttle the AFR goes to 12.5. I have an AEM wideband AFR gauge ( model 30-4110), which is the same as I have fitted to my 3.8 S Type with Megasquirt EFI.

On the Daimler, I have also fitted an EGT gauge, the probe goes into the manifold right where it turns down, so the probe tip is perhaps 4" from the exhaust valve. Watching the gauge, idle EGT is around 200-250C, highway cruise under light throttle is 500-600C, and on the Coquihalla Highway ( BC#5) when I go up the steepest portion it's a section approximately 20 km long where I drive full throttle and watch my speed fall as the grade gets steeper, shift to second when I can and hold the throttle open at around 4000 rpm until I get to the top. EGT's there are about 1100C, AFR is 12.5, coolant temperatures were climbing toward 100C near the top of the hill. The car has 56,000km on it now and engine is as factory. Never a problem with a hint of piston seizing, oil stays clean.

So the things I'm pondering is did Jaguar actually machine different clearances than published in the workshop manual? Does compression ratio matter, as the lower compression engine has been trouble free. Do OEM vs aftermarket pistons have different properties? Or was I simply unlucky with the S Type and it was just a fraction too tight? I've driven the Daimler much harder than the S Type, and it's moving nearly double the weight (curb weight of the Daimler is 2700 kg) yet used similar AFR's, measured with the same gauge. Fuel in all cases was 91 AKI.
Thoughts?

 

The truth of the matter is that we don't know & are starting to compare apples with pears. The 3 carb, straight port head E Type engine e.g. would have far more even mixture distribution. It's 3 untouched carbs would have been tuned for a 70K worn engine. You always seem closer to stoichiometric than I have typically seen on the dyno over many engines in many years. We have seized many engines sometimes purposely. Running both motorcycle & car racing teams for our oil company on top of my day job I have seen my fair share of seizures there as well at mostly known AFR from dyno runs prior to the race. CR will have some influence.

My engine was built to Jaguar published figures/specs as were our thousands of plant engines in SA for local production of CKD cars under our local content programme as previously mentioned. I phoned a retired buddy who used to work at Blackheath who confirms this.

The only way to reach closure on this matter would be in a sophisticated dyno room with accurately controlled IAT, accurate fuel control on a known XK engine at known manifold configuration, clearances & pistons, AFR, CR, fuel, timing ad infinitum & run it to destruction by steady leaning of the mixture at specific load. Our Sasol Dyno rooms in SA are even altitude compensated which is why F1 teams pop out here from time to time. They cost a fortune in taxpayer money. They are at 1,753 m (5,751 ft) and can be pressurised to just below coast.

People should read the previous thread for context. Hepolite might have produced a bum batch of pistons or have differing metallurgy but then they should advise clearances required. To my knowledge they don't. My car is fitted with genuine Jaguar 8:1 CR pistons bumped slightly by head skimming (first time from original). Who manufactured those pistons for Jaguar OE I have no idea. They came in original Jaguar green boxes

From our original old Jose engine thread it would have set off alarm bells with me had I known that you were playing with EFI concurrent with your seizures. I still believe AF ratio aggravated those seizures.

My advice to the OP remains as I said in the previous thread

"Enjoy the project. If all we have really contributed is importance of length of inlet tract to ideal torque production we have helped.

Adhere to Jaguar piston to bore clearances or those recommended by the piston supplier. There is no reason to deviate from specification. I don't know if an engine rebuild is envisaged.

When it comes to initial system set up start at the rich end. Suggest 11 to 1 AF Ratio. Don't threaten the engine chasing fuel economy. You can fine tune as you go & once the unit is fully run in. Then you can start leaning the mixture in your quest but do it in small steps. Let us know how you go.

Watch engine temperature like a hawk initially. If it starts to rise abnormally ~ back off immediately."

A buddy in Dallas has just heat seized one piston on his 4 cyl Borgward Isabella Coupe that has been beautifully restored & engine rebuilt. It makes me fed up because I warned him to start rich. It still runs after cool down but a boroscope tells us the damage is done. Anyway it has given him a new project to fit a brand new (NOS) Borgward 6 cylinder racing engine that he located still in it's crate in Mexico.

One thing about the Daimler's oil. By clean I presume you mean clear & bright. Unless you are doing very frequent oil changes it should not stay "clean". Modern highly detergent oils should darken quite quickly. Then you know they are doing the job they were designed to do.

 

There's been a lot of discussion of Hepolite pistons in the Norton motorcycle community. The company, or more precisely the Hepolite trademark, has changed ownership a couple of times. The locations of manufacture have also changed. The general view is that the quality of the pistons and the rings has gone totally down the drain in the process. Worse still, more recently manufactured low quality products have been sold in old style boxes.

Apart from that, the thermal expansion coefficients of aluminium alloys vary, which is why many piston manufacturers specify clearance or a cylinder bore. With all the ownership and factory moves, it's inevitable that the materials in Hepolite products have changed and quite possibly the clearance required.

Incidentally, the present owner claims to be improving their pistons and rings. Personally, I'm increasingly convinced that it's better to fork out on expensive forged pistons. You get a modern lightweight design too, even asymmetric.

Another thought, if you're using Megasquirt and monitoring exhaust temperature, could you program the ECU to adjust mixture strength or fire an extra injector when the temperature goes high?

 

I had these guys make me some for my Bentley through the insistence of my machine shop, best thing I could of done.

Light weight, forged and a nice modern short skirt.
At the time they were 600.00$ U.S. at a fraction of the cost of new RR pistons. (which were old and antiquated)

Ross Pistons: High Performance Import & Domestic Forged Racing Pistons

 

It has been stated in this & Jose's thread that these were in both cases NOS Hepolite pistons. Hepolite used to have a good name.

One thing I omitted to say was that my NOS Jag pistons were post October 62 solving of the 3.8 oil consumption issues ~ IIRC they were produced in '67. Also the bores still have their Brivadium liners.

VW fires an extra upstream LP injector on their DI models to keep inlet systems & back of inlet valve tulip clean with additised fuel. Ford & Benz multipulse the injector to "wash" the valve tulip when open.

 

Peter. These short skirt, forged asymmetric "racing" pistons from a number of suppliers including Rob Beere. Do you know of someone that has fitted them? I wondered if they suffered piston slap until clearances had normalised or in general. Rob Beere's cheapest set for a 3.8 is £1780.00. That is one hell of a lot of money for 6 pistons with rings.

 

It's still less expensive than a second engine re-build. It might be interesting to hear what Ross and others can provide. It would be a lot easier and cheaper with access to a machine shop ....

Also, I fear there may be new old stock and new old box, which is what some of the Norton owners have been concerned about.

 

I wonder what Barratts experience is. They sell almost exclusively Hepolite pistons with the odd set of Mahle's

 

Piston slap is caused from worn pistons and bores, not from pistons with short skirts.
Pistons with short skirts are better because there is less metal contact on the bores _ less friction, stays cooler.
I was very skeptical when I saw my pistons, but they've been in there 20 years and they work well.

The bores on the Bentley have even a longer stroke then a Jaguar and are 3.5 inches in diameter, it red lines at 4200 RPM.

And I wouldn't go near Egge either.
I member in the RR club had to have his engine redone again because the Egge pistons were breaking.

 

Oh! short skirt pistons can slap from new & do in quite a few modern vehicles until temp/clearance normalisation.

I'm pleased you are happy with your Bentley. Short skirt pistons have many positives. My Benz has rediculously short piston skirts running in Alusil bores. When I look at Jag pistons they remind me of many marine pistons.

 

I thought that's why they are forged out if round to prevent that.
I guess on run-of-the-mill Chevs and such, they're probably not forged pistons.

I don't ever remember hearing piston slap in any new vehicle, but then again the newest vehicle I've ever owned is my 1999 Mazda B3000 truck.
I've done the engine on that with a re-bore and it doesn't have piston slap, or at least I can't hear it.
Quite a short skirt on those, and I don't think they are forged _ nothing special I'm sure _ don't know what brand of pistons they are, the machine got them for me. They used Hastings rings though.

I have an 87 Mazda garden truck, and a I can sure hear it there, but someone over-honed the bores, so it's very sloppy.

If I bought a new vehicle and hard that, I wouldn't buy it.

I had to look "Alusil" up.
Aluminum and silicon, what happens if the block is over heated with that type of bore, is it a sleeve, or is the whole block made of that stuff ?

Vega had this aluminum block with some sort of special treatment and the rings were in direct contact with the aluminum, but they had problems and the engines started burning oil.
So it sound like the treatment didn't stand up, or if it was over heated, even a little, it started burning oil.

 

Glyn: By "clean" oil I mean no metal particles after the hard drive, there was the usual carbon darkening

Peter: With Megasquirt I believe you could change the fuelling or timing in response to EGT, but I've never tried. My Daimler got hot simply because it was running at full throttle for an extended period.

Rob Beere pistons: I was contemplating building the most powerful 4.2 I could, yet still run it on ordinary pump fuel ( which for me is 91 AKI) and I looked into Rob Beere's redesigned pistons with the rather interesting crowns on them. I contacted RB and he advised me not to use them for a street driven engine, that I wouldn't be happy with them. They are forged and my guess is that the clearances are reasonably loose until they warm up, so probably high blowby and oil consumption until they get quite hot. RB recommended I stay with cast pistons.

 

There are 3 basic piston types, forged, cast & hypereutectic. (e.g. Alusil.)

Most gasoline vehicles use cast pistons.
Forged pistons can be barrelled slightly which would tend to quiet them.

You would have to listen very carefully to pick it up but its there until at operating temp ~ a stethoscope would help. It's not a racket. Toyota calls it clatter. Well if that's clatter then I'm deaf & I'm not. High End Audio is my hobby & I look after my hearing. At races I even wear ballistic liquid filled ear protection. I come from an industry where we are trained to listen for things like high speed knock. I'm pretty sure you would not even hear the slap. Go and listen to a modern new Toyota RAV 4 engine at cold start & tell me if you can hear anything. (better example).

There are no sleeves in a Merc Alusil (hypereutectic) block although it can be sleeved. (I think you call them longblocks.)

The pistons/rings run directly in the block/bore. They have done forever. This is not new. It's as tough as hell which is why so many Merc Taxis run over a million Km's without trouble & you can still see signs of honing marks in the bore.

I've never seen a Benz block distort due to overheating. Heads yes but through fiddling, not left well alone. (e.g. turbo charging & fitting multiple head gaskets to drop CR). So it's outside my experience. I moderate at MBWorld & have never heard of a distorted Benz block. We are the jewel in the Internet Brands Automotive crown. Funnily enough the members there are fascinated by my Jag & endlessly encouraging.

I'm sure it would distort in the end but with so many nannies looking after the modern car it just does not happen. It will first go into limp mode & if it detects further trouble it is just going to shut itself down. I don't open my Benz bonnet between services. It even tells you when your headlight washer bottle is heading for a top up on the display.

BTW ~ I meant to say that since the move to DI they have taken the Alusil bore a step further to reduce friction by twin-wire arc spraying an extremely thin coating based on an iron-carbon alloy to the inner surfaces of the cylinders called Nanoslide. This gives the bore a high shine & highly wear-resistant surface with microporosity. This microporosity holds the oil in place & does away with the need for honing which has been replaced by another machining technology pre treatment that they are cagey about. This friction reduction process has now been applied to their diesel engines as well. The joys of nano technology. Who ever said that F1 technology does not trickle down into passenger vehicles.

 

Understood bud!

 

Surface treatment of aluminium has raises some other thoughts about NOS pistons. What happens to the surface over a few decades of storage? It's not impossible that something goes on at a (sub)microscopic level that turns nasty on heating.

From what I've read, forged pistons usually require more clearance and are more likely to clatter when cold. Reliability before the last word in refinement?

Any experience with other pistons such as Wossner or Arias?

 

I know/knew someone who put forged Cosworth pistons in an E-type (I thought the RB ones were Cosworth with special machining) and he was very pleased. No clatter or knock probelms and he used to drive his car fairly hard. I think the c.r. was raised a little. I don't know how much they cost him, but they were expensive compared with OE. They'd slipped my mind as I tend to concentrate on my 3.4 and for many modern suppliers (Cosworth, Omega .. ) that's not mainstream as they tend to be E-type oriented.

 

Yes ~ that was my understanding re clearance although I've read that there are some methods to ameliorate this. I have no experience of other specialist pistons.

I can't answer your NOS theory but it is an interesting thought.

Thanks! & yes I'm the same & tend to concentrate on my B Head 3.8.

 

So to come full circle here, my machine shop ( a different one that built the engine) recommended slightly larger clearances, 0.002-0.0025" and I have had no trouble since. That extra thousandth of a inch made the difference.

 

So people must make their own decisions whether to listen to Jaguar or piston manufacturer specs or not. And bear in mind that you have a modified car.

 

How do you know if a piston seizure is occurring without doing a borescope inspection down the plug hole?
I would think the onset of seizure would not be detectable in a running engine
Not that I think I have a problem at all.
I fitted a set of Mahle 279 13 02 pistons from SNG Barratt about 3000 miles ago and the engine appears to be running fine.
We used the Mahle recommendation for clearance of 0.004", checked with a feeler gauge.

Bruce M