Has my flywheel been lightened?

[chuffer]

I'm in the process of rebuilding my original 3.8 engine and eventually I'll swap it for the one that's currently in the car.

I'm also contemplating going to the 9.5" diaphragm clutch which will necessitate the flywheel being drilled to take it. However, I was reading an earlier thread on lightened flywheels dated April this year and was alarmed at a comment which seemed to imply the original flywheel might weigh considerably more than mine.

I just weighed mine and it is 14lb. It is steel, not alloy.

I'm not really interested in lightening my flywheel and would be happy to have what Jaguar intended but have I got one already? If so, I guess I'll have to use the one off the current engine (assuming that's original!) but then do I have to be worried about balancing the whole crank assembly? How would I do that? Does it matter?

Any help would be appreciated.

cheers
Ray

[christopher storey]

Ray : I have a 3.8 flywheel which was slightly skimmed some years ago. That weighs about 24lbs. Recently I bought a new 4.2 flywheel which from memory weighed 26lbs. At 14lbs yours unquestionably has been lightened considerably , and I think you will find it difficult to get a reasonable idle

[Moeregaard]

On a related note, I'd like to know what the preferred weight is for a flywheel used on a tuned street engine. I've heard that the alloy units are too light for road use. Both E-Types I've owned were 4.2-litre cars and their flywheels were unmolested and, I feel, too heavy. Perhaps David (Heuer) will weigh in on this.

[Heuer]

A lightened flywheel will improve acceleration at the expense of smooth running. However a reasonable compromise can be reached by choosing where metal is removed - take it from the outside and you reduce hysteresis and the engine will become more responsive to acceleration/deceleration, take it from the inside or middle and you will still have less weight to turn but the outer mass will improve the hysteresis effect. If you are looking for neck snapping acceleration tune the engine and lighten the flywheel but don't expect admiring looks from your passenger. Porsche use a dual mass flywheel (basically two flywheels connected by springs) which gives the best of both world's as the springs absorb some of the 'snap' making the car more pleasant to drive. They are also used in diesel cars for a different reason - to smooth out the piston knock.

For a fast road car you could go for a sympathetically lightened flywheel but there will be compromises and it means adapting your driving style to maintain smoothness. I have such a flywheel courtesy of VSE who built the engine. It would not suit everyone as it can make the car twitchy especially at low revs. But on a good hooligan road the result does bring a smile to the face. The alloy flywheels are generally far too light, bad news on a road going E-Type and cannot be used with a T5 or JT5 box, I am reliably informed. We are back to the feeling that Norman Dewis got it right during development and these 'upgrades' should actually be called 'compromises'. Ray - the 14lb flywheel sounds very light, as Christopher suggests it will increase harshness at idle and during small throttle movements. Personally I would stick with a standard flywheel and enjoy the Jaguar levels of refinement.

[Heuer]

Re-reading the thread I just realised Ray has a 3.8 which has a lighter flywheel than the 4.2. The Jaguar spares catalogue lists two flywheels for the 3.8:

C.14921 Flywheel (lightened) - STANDARD FITMENT
C.20310 Flywheel (lightened) FOR COMPETITION CARS ONLY.

Interesting Jaguar list them as 'lightened' even for the standard engine. The C14291 flywheel weighed 18lbs whilst the competition one was a few of pounds lighter (the part numbers are stamped on the flywheel). Alloy flywheels are advertised as being between 9 and 15lbs depending on manufacturer, although many do not give any information (!).

A 4.2 flywheel weighs 26lbs but again Jaguar lists two:

C.18517 Flywheel - NOT FOR COMPETITION OR RACING PURPOSES
C.25662 Flywheel - FOR RACING OR COMPETITION PURPOSES

Open to conjecture as to why Jaguar thought the 4.2 needed a heavier flywheel - longer stroke engine, higher torque, lighter (by 4.5lbs) diaphragm clutch, different gearbox, more refinement for US market etc?

Ray - If you check the part number of the 14lb flywheel you may find it is the 'competition' one.

[Moeregaard]

Thanks for the input, David. I may shave a couple of pounds off of mine, but I think other gains can be made with different cam grinds and exhaust systems while maintaining smoothness and driveabilty.

Regarding the fitment of the heavier flywheel to the 4.2, I believe Jaguar did this to make getting away from rest easier. The early, all-syncro gearbox has a relatively tall first-gear ratio. With the 3.54:1 final drive this is not much of a problem, but with the 3.07:1 I can see a heavier flywheel being desirable in a road car. They may have also done this for the same reason in the heavier saloons, but that's just conjecture on my part.

[chuffer]

Thanks to you all for your valuable input - as ever.

One or two comments have given me something to think about. Firstly, I cannot find the part number on the flywheel David, but if the flywheel has been lightened then maybe it got shaved off. The engine number is stamped on there though, so it is definitely the original flywheel and I'll be surprised if it was bought as a competition car - but no proof. So I'll assume a previous owner has skimmed it. Also on the flywheel, next to the engine number is the letter'B' which I notice is to be aligned in order to get the balance.

120 degrees from the 'B' is stamped an arrow. Relevance unknown.

Second comment to make me think is David's assertion that the 9.5" clutch is 4lbs lighter than the 10", and that the standard 3.8 flywheel is 18lbs. This means that if I re-use the flywheel which is on my current engine (hopefully 18lbs) and a 9.5" diaphragm clutch, this will weigh the same as my lightened flywheel (14lbs) and the 10" clutch. So either way I'll have a lightened assembly! Doh!

This makes me think that in order to get the benefit of easier clutch pedal depression (using the 9.5" clutch) perhaps I should obtain a second hand 4.2 flywheel, but then the ring gear has different teeth so the starter gear will be wrong won't it? :? Is the bell housing any different?

yours, pulling hair out
Ray

[Heuer]

Ray

Don't even think about messing around with a 4.2 flywheel as you will end up changing the starter motor and probably the bell housing. Your standard 3.8 flywheel together with the diaphragm clutch will be fine as that 4.5lb weight saving is not at the perimeter but at the centre. You will be very challenged to notice the differences in driving the car and if they exist at all will be quickly lost in the mists of time.

[christopher storey]

Ray

The 3.8 flywheel is about one-quarter inch smaller diameter than the 4.2, and has 104 teeth compared with 133. You will have endless problems if you switch between the two


This next bit is not for the faint hearted

Moe

You might be interested to know that a party trick I used to do with my 3.07 4.2 S1 with the original close ratio box was to put the lever in top gear, and then press the starter . It would actually drive away from rest like this, albeit slowly - it just shows what torque , and tractability, these engines have, although my main bearings were rather shagged out when I rebuilt the engine!

[Moeregaard]

Christopher, I am very familiar with the XK engine's tractability. I once broke a clutch line and drove home in evening traffic sans clutch. Shifting up and down is not a problem by carefully matching speeds and when forced to stop I would coast up to the light in neutral, shut the engine off and place the shifter in first. When the light turned green I hit the starter and was off and running again. I believe Denis Jenkinson had a similar experience with his 1970 OTS in France.

The point regarding flywheels and starters is well made. There is no interchangeability between the 3.8 and 4.2 liter flywheels and starters.

[chuffer]

Christopher, Mark and David, thanks to you all for your helpful comments. I will now definitely not be changing to a 4.2 flywheel but will have a look at the one on my current engine (which I removed this afternoon) and all being well, will have it drilled to take the 9.5" clutch.

If SHMBO permits it before Christmas, I may separate the box (Getrag) and engine this coming week and remove the flywheel for a look-see.

On a separate subject, that torsion bar reaction plate is a so-and-so to remove isn't it?

cheers
Ray

[kingzetts]

To hijack this thread a little bit, yesterday I removed the bellhousing and clutch from my engine - I found that the car has the (correct for 3.8 ) coil spring clutch pressure plate, with a 10" outside diameter, 6.5"inside diameter on the pressure plate, but the driven plate has an OD of 9.5" and an ID of 6-3/8".

Now I'm not 100% certain but I would have thought the driven plate would be 10"/6.5" to match the pressure plate. Can anyone confirm this?

Also the friction surfaces of flywheel and pressure plate show clear signs of oil contamination and grabbbing, and using a straight edge I think the pressure plate surface is not quite flat. This is likely why I was getting some judder.

As a matter of interest I weighted the flywheel and it is exactly 18lb as standard.

[Moeregaard]

The 10" Laycock diaphragm clutch fitted to the last of the 3.8-litre and early 4.2 cars earned a reputation for being problematic. It became common practice to install the older, arm-type clutch that was standard for much of the 3.8's life, especially for competition use.

Because I didn't know better at the time, I installed a Laycock clutch in my '65 during its first mechanical rebuild in 1983. Even with a refaced flywheel and fresh engine mounts it soon started to chatter and even slipped on one occasion. When I took the car off the road in 1991 for a proper restoration, the guys at a well known Jaguar restoration shop convinced me to fit the Borg & Beck unit from the Series Two. I did and was rewarded with less pedal effort and much more positive take up--and no slipping.

Without having a driven plate sitting on the desk in front of me, I can't answer questions regarding OD and ID, but I do remember the S2 clutch as being a more compact package. I'll do some research and see what I can come up with.

[chuffer]

Two part reply here:
Firstly, Kingzetts, I removed my Getrag gearbox from the engine today and then removed the clutch and flywheel. I already knew the clutch was 10" spring so I can now tell you that the friction plate measures 9.875" outside diameter and 6.75" inside - as you hoped yours would, I imagine. I hope that helps - but beware I do not know if this is standard Jag fitment because I don't know if the Getrag input shaft is the same as the Jag box. It could be a Volvo or BMW friction plate for all I know. It is stamped Borg & Beck with number 47809.

With your inside diameter being less than the pressure plate I guess you will have a ridge on the friction plate. The inside 1/8" will be proud of the rest, if I'm not mistaken.

Secondly, I can confirm that I weighed the flywheel and counted the teeth, and also looked for the part numbers David mentioned. Comparing this flywheel to the 'lightened' one does indeed confirm that the other one has had metal removed. The weight of this one is 26lb (as Christopher suggested, but not as David suggested, although this does include the ring gear). It has 104 teeth and is stamped LB2834 - not one of the part numbers mentioned. So the plot thickens, but no matter, this is the flywheel I will be using in my rebuild. It has a better ring gear and I will get it drilled to accept the 9.5" diaphragm clutch.

Cheers
Ray

[Heuer]

Ray

The early 3.8 flywheels had the starter ring teeth machined into them i.e. no separate starter ring. Given the weight, presence of (I assume) a dedicated starter ring and unrecognised stamped number it could be an after market flywheel and not OEM in which case all bets are off! I have also been told Jaguar stamped the engine number on the original flywheel but there cannot be many of those left still attached to the engine. Original Jaguar replacement flywheels were stamped with the part number.

[chuffer]

Hi David,
Thanks for your latest post - it made me go and look at the ring gear on both flywheels. As a consequence, I think I've finally got to the bottom of this conundrum!

Both flywheels have the ring teeth machined into them, so I guess this means there's a good chance of them being original. Given that the lightened one had the engine number stamped on it (and it matched the engine it came off), rather than a part number, it made me think if the heavier one, with LB2834 on it, also referred to the engine number.

So I pulled out of my Jaguar file a sheet of A4 on which are the numbering sequences for XK engines and there it is..........Mark 2 3.8 engines started with LA, LB, LC or LE. So that's where my 'heavy' flywheel has come from.

I guess it looks like 3.8 Mark 2 cars had heavier flywheels than the E Type, although they seem to be interchangeable. So there won't be any racing away from traffic lights for me, just smoother idling!

Ray

Afterthought: As you said earlier, the standard fitment flywheel on the 3.8 E Type is captioned as "Flywheel (lightened)" in the parts manual, so this is probably a reference to the fact that it is in fact lighter than earlier 3.8 flywheels, such as the Mark 2. It perhaps then follows that if the perceived benefit of going to a lighter flywheel at the introduction of the E Type was not realised, then reverting to 26lb flywheels for the 4.2 was not a reaction to the longer stroke, more torque, US refinement etc but simply going back to what they had always been - 26lbs.

[kingzetts]

Chuffer,
Thanks for the info on your driven plate. Those are the dimensions I'd expect with a 10" pressure plate.

I discussed my plate sizing with Andrew at CMC whose advice mirrored my own thoughts which is that to run a driven plate with a smaller ID than the pressure plate will work initially but may cause problems in the long run when the inner unused ring of friction material starts to form a ridge, as you observe.

Interestingly I spoke to Watjag who (I think) supplied my car's T5 box and they said they only supply the 9.5" driven plate and that this would be perfectly OK in a 10" clutch. They may be right in practical terms but its not a course of action I feel comfortable with.

So I either need to track down a part number for a correctly-dimensioned 10" driven plate to suit the T5 23-spline input shaft, or I need to change to the 9.5" diaphragm clutch. If anyone has any data on the 10" plate for that box, I'd be grateful to hear.....

[Heuer]

John

You could give Uryk at E-Type Fabs a call - 01325 481819. He has a wealth of knowledge on the subject of 5 speed boxes and will no doubt point you in the right direction.

[Heuer]

Ray

Well detected! I think the heavier flywheel was put in the 4.2 to improve refinement for the US market. After all Jaguar only planned to make 500 E-Types and it was originally designed as an out and out sports car. Seems you will get the type of refinement you were after all along.

[pmansson]

I found this old thread while googling the weight difference between 3.8 and 4. flywheels. Why did I google this? Because I drove my recently acquired late 4.2 S1 roadster yesterday, and I think it feels more like my father?s old S-type than the gokart feel of my 3.8 OTS with its 3.54 LSD and T-5 gearbox.
The 4.2 has a Paul Cangialosi box with 3.35 on first and a 3.07 LSD. I am currently not passing 2000 RPM as it is still in the run in phase after the engine rebuild.
The 3.8 feels lighter and more agile in all respects: acceleration, turning, braking and clutching... Much more of a gokart than the saloonlike feeling of the 4.2 which was sold new in the US, and hence, as someone suggested, has a heavier flywheel (now this may have been changed during the engine rebuild. Need to check this with Ken Bell).
This forum is outstanding. Thanks to all and Happy New Year!
Peder