I am about to have my 3.8L engine bored +.020 over and would like to know what the recommended clearance is between the pistons and the cylinder walls. I'm not sure if I understand the information in the service manual and don't want the machinist to make a mistake. Thanks for any suggestions or recommendations.

 

Ted the correct answer depends almost entirely on what piston brand you are planning to use. Follow the guidance of the vendor or you will have issues of either too loose or too tight. The information in the service manual assumes you are using stock +20 pistons. Best of luck!

 

Thank you George. I tried to find Jaguar in the Mahle Application catalog, but I didn't find a Jaguar listing. I have sent their customer service department an email requesting the information I need.

Ted

 

Hello Ted
May I suggest some Ross Racing pistons, you send them one of your old pistons and they make a custom set for you, as per your instruction eg, what compression ratio and bore size.
Had this done on my MK VI Bentley with great apprehension, but it's been 15 years and I've never looked back.
They're forged out of round, so when they heat up, they become round. They have a very short skirt compared to the original, so there is less friction in the bore.
And because they are forged out if aluminum alloy, they all weigh exactly the same.
They're right in California too.

Ross Racing Pistons

Also consider a custom made MLS head gasket, far superior to a fibre one when it comes to mixed metal engines, especially our Jags, they can run pretty hot some times. An MLS gasket can take greater expansion specs then a fibre one.

Don't be afraid to make use of new technology when it comes to older engines.
A guy over at the Rolls Royce forums needed a new head gasket for his Turbo R and he insisted on genuine Bentley parts.
I cautioned him about using new old stock, Bentley isn't interested in improving on older models and sure enough the gasket he got was exactly the same as the one the failed at 50K.

 

I would highly recommend that the machine shop supply the pistons, if they don't know what spec to machine out, then you should find another machine shop !!!

Mahle branded pistons are not always genuine Mahle, so be aware of cheaper alternatives. I had a bunch a machine work done last year, and I worked on the premise that anything that should fit directly into the machined work should be specified with discussion with the machine shop and supplied by them, then there is no blame game to be played if there are any problems.

The guy i used build race spec Jag engines, so knows the engine well, advised me what to do and what to avoid, and would rather not do the work than take a short cut, I had every confidence it what he was doing and he did a superb job.

 

When I had the work done on my Bentley, it was my machine shop that recommended the Ross pistons and they ordered them for me. I was going to source some originals, but they convinced me to do otherwise.

 

Just taken a look at the Ross website, they look really good, by the look they will know what spec to use, although they produce race pistons, they will probably make high spec general use pistons, your machine shop should be dealing with them ideally, the rings should be matched to the pistons too so they should be sourced from the same place as the pistons.

Jeff is correct re weight, density of the casting will be uniform, so when machined they should be equal weight, there could be an odd gram difference, but this should be checked by the machine shop and the pistons then placed on the crank to balance out any differences if there are any at all.

 

QUESTION : what brand of pistons is best to use when rebuilding a 3,8 litre XK engine?

Mahle or Hipolite? I can get either but read a report about Mahle being shipped with deffective rings and I am scared.

I need +.020 9:1 compression.

Thank you for any advice.

 

Jose, I've not reached the stage of rebuilding my engine, so this is opinion on the basis of what I've read and learnt about engines in general and our cars in particular. As I understand it, Heppolite and Mahle supply pistons close to the originals of the 1950s and 60s. No doubt they function well enough, but look very old tech compared with modern practice or even the pistons of the XJ series 3 4.2. Companies like Ross, Wossner, Arias (Terry's), JE (Rob Beere) can supply more modern designs. They will cost twice as much, but can gain some efficiency and performance. If possible, go for an asymmetric design like Rob Beere's Ultra. Most of these companies don't have our cars in their standard lists; you have to contact them or go via a Jaguar specialist.

 

Jose is working on a tight budget. I would go for Mahle after a number of seizures reported with Hepolite. Seems they over expand when hot if running recommended Jaguar piston to bore clearances. Materials change?

 

Yes and also I am not really looking for more performance as such, the 3.8 is peppy enough for the S type.
the engine is getting bored already so I need them a.s.a.p. I decided on the MAHLE pistons.

also replacing the Heater box tubes, blasting inside of the manifolds, everything is so expensive.

 

well I was wrong, the +.020 Mahle are backordered at my price point.

I did check the Ross site and I can understand why they are a lot more expensive, the first difference is Ross pistons are forged, and they are half the length of a cast piston, a modern design. The second difference is that everything from California is always more expensive. Dang!

Don't know how to proceed now. Worldwide Distribution Shortage everybody is saying.

 

Here you go. Mahle, 3.8, +20thou, 9 to 1 CR £395.00

https://www.ebay.co.uk/itm/124309269...IAAOSwyedfQlLp

Suits:
-E-Type S1 3.8
-MK2 3.8
-S-Type 3.8
-XK 150 3.8
-MK9 / MKIX
-MK10 / MKX 3.9
-XKE 3.8

Brand: Mahle
Part No: 279 12 01

Compression Ratio : 9:1

You are buying a set of SIX +20 Thou 87.5mm pistons with Pistons Rings, Gudgeon Pin and Clips!

Bore: 87.5mm
Length: 105.4mm
Compression Height : 41.78mm
Dome Height: 15.2mm
Pin OD: 22.23mm
Pin Length: 76.1mm
Pin ID: 14mm
Piston Skirt Clearance : 0.1mm

Sometimes you have to take what you can get. These are price competitive.
Barratts, Norman Motors, David Manners, Robey all out of stock

 

For a normal street car you don't want forged pistons anyway, cast are better suited. Don't be afraid to go with 8:1 pistons either if they are available and not backordered; if the head is being skimmed that will raise the compression.

 

Could be different materials, or unleaded fuel burns hotter than the leaded when Jaguar specified the clearances, I don't know. Hepolites do well if you give an extra 0.001" skirt clearance over what Jaguar recommends, so instead of using 0.0015" use 0.0025" and it will be ok.

 

It might be worth contacting Ken Jenkins, ukjag@hotmail.com, as he could have some old stock on his shelf. If not, he'll know who has. He and his daughter are lovely people. He enjoys a chat about old Jaguars. He also knows more about them than just about anyone.

 

gents, I may have found a new Mahle set in Michigan USA. Will confirm today otherwise eBay UK will have to do.

All I hope is that the machine shop knows about the required clearances because I sure don't. I did provide them with copies of the engine specs off the Service Manual. The shop I am using builds race and monster truck engines.

 

It's not fuel related or everybody would display the same issues. My guess is materials & Hepolite should warn people to increase PB clearance by XYZ or more realistically take it into account in manufacture. One should not have to change from Jag clearance specs. My engine is to Jag specs & fine.

 

The piston manufacturer knows what engine they made their pistons for and the materials they used in their pistons. They should know and should specify to what size you need to bore the block. They should also supply some ready to use rings.

If they can't do these things, who can?

 

UPDATE:
got lucky and Terry's Jaguar found me the MAHLE set of +.020 9:1 pistons. No other supplier had the specs needed in stock.

Terry's Jaguar has an engine building shop and the sales person went there and found them.

I was getting ready to order from eBay UK link that Glyn sent me when Terry's called. Pricing was very good and free shipping. I also ordered the upper and lower gasket sets and the two intake spacers to make it worth their time.
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Thank you all for all the help and advice.

Another issue has come ip: Engine assembler wants to replace the oil pump, he says the oil pump is the reason the engine failed.

Is this correct?