RX8 Project – Part 9, Flywheels Part 3

So just to finish of the flywheel section here are the the finished custom parts :

Flywheel spacer on the crank, you can see the black dust seal in the centre covering the new pilot bearing underneath.Duratec V6 Crank Spacer

A wider shot showing the spacer in position among the currently disassembled state of the engine.Duratec v6 Flywheel spacer

And finally the flywheel itself.

Custom Duratec v6/RX8 Flywheel

In this photo the ring gear and location dowels for the clutch basket have been fitted.

The ring gear was actually a lot easier to fit than it was to remove because you can just put the ring gear in the oven (at maximum, in my case 250°C+ off the end of the scale!) and put the flywheel in the freezer for an hour or so as well – this may not actually be necessary but you want the most possible room between the parts when you fit them together. If the ring gear snags on the way down it because there isn’t quite enough space it can be a real pain to get it off again. Before installing the ring make sure it is the correct way round – all the teeth should have a bevel on one side to help the starter engage cleanly this goes towards the position of the starter motor! Take the hot ring out the oven, check it and drop it into place as quickly as possible but make sure it’s right and fully seated to the shoulder of the flywheel. Once touching the flywheel the ring will cool rapidly and lock in place.

The dowels in question turned out to be the wrong size, I specified them as 1/4″ diameter (6.35mm) and this is what is still shown on the drawing but it turns out the ones I measured had more rust than I thought and the holes in the clutch basket are actually designed to locate on 6mm dowels – something I really should have checked! From what I have since found out this is likely one of the many Ford engines which have special dowels which are  (from what I can find out) 8mm on the flywheel side but only 6mm on the clutch side. The correct dowels are actually 6.30mm on the smaller diameter so my original measurement wasn’t actually too far off, I just shouldn’t make daft assumptions! Larger end is 7.97mm diameter by 6.5mm long on the ones I have, overall length is 18mm. Tolerances and fits are not my strong point but I’ll probably start with a 7.9mm drill and hope to press fit them.

For simplicity I recommend buying something like this available via eBay as Cosworth clutch dowels by x-power engines:Xpower Flywheel Dowels

I’m planning to modify the appropriate holes on the flywheel to use the correct dowels I just haven’t quite got round to it yet!

I should probably also take a moment here to mention flywheel bolts. The Duratec crank has a slightly unusual thread which is M10x1.0mm (M10 Extra fine). This is as it happens the same thread commonly used on brake hydraulic components like bleed screws. Needless to say the stock bolts are far too short as the engine originally just had a thin flex plate so longer bolts were needed. Now various companies will sell flywheel bolts for almost any engine but not for something like this and they rarely specify the actual sizes of the bolts in a kit so I can’t just buy one for something else that will fit very easily. My solution was find the best standard bolt I could and so I am using some 12.9 high tensile socket cap bolts which I managed to find from a bolt supplier on eBay with the right thread. For anyone who doesn’t know 12.9 rated bolts are the highest rating before getting into one off special items (usually using exotic materials) and they really are very strong. As a comparison ARP gives their flywheel bolts as having a tensile strength of 180,000 PSI. The 12.9 bolts are rated to have a minimum tensile strength of 176,900 PSI – a number close enough it makes me think they are likely the same material! The strength figures for these bolts mean at the size I will be using each bolt can be safely loaded to in excess of 7000kg of tensile load indefinitely with no deformation. Their failure point being somewhere north of 9500kg each! Some time in the future I will do a full write up of nuts bolts and other fixtures it’s worth knowing about.

So that’s my shiny custom flywheel, next time you see it it should be bolted to a rebuild engine with a whole host of custom or cobbled bits on it!

10 thoughts on “RX8 Project – Part 9, Flywheels Part 3”

  1. Hello, I’m a young mechanic looking to do the same conversion and I’m fascinated by your approaches to the issues. But in reguards to the flywheel, if you were to obtain a jaguar s type with a manual gearbox as a donor, could you use the flywheel and clutch assembly and fit it into the rx8’s gearbox.

    1. Hi, I’m glad you’re finding it interesting!

      So you have a perfectly valid thought about the s-type flywheel but there are a few issues which need to be considered. The first being positioning a starter motor sensibly because with an s-type flywheel the ring gear will not engage with the rx8 starter – most similar swaps seem to have a cut out on the gearbox adaptor plate to make this work which is a valid solution in most cases but here none of the positions looked ideal because of the engine I’m using. I’m not trying to say it couldn’t work with enough thought but to me it wasn’t very easy. The other problem is the pilot bearing which supports the gearbox input shaft. By adding an adapter plate of 10mm (what I used but if I was doing it again I’d go thicker but more on that in another update!) the input shaft would locate but probably only be about 1-2mm inside the stock bearing which I don’t think I trust that but it could be ok! Plus you’d still have to offset the flywheel so the clutch fully engages on the gearbox input splines and then you’ve got to move the starter again.

      So far as I can see if you either need to do what I did and go for a custom flywheel and position everything to make it all work or use a thicker adapter with the starter offset into the adapter(if you want to keep it engine side) so it will still engage. Either way a pretty big job unfortunately!

      In all seriousness if you’ve got the whole manual Jag your best/simplest bet might actually be to just swap the Jag engine and gearbox as one unit and get a custom driveshaft, although that said as far as I’m aware all the s-type flywheels are dual mass type which have many problems of their own…!

      1. Okay tbh I don’t have any parts yet, I just know what conversation I want to do and am theorising all the steps before I commit. So I see your points there so that’s left me thinking the best plan is to make my own flywheel and I know you just designed your own in the end but what if I made it easier for myself and just took the factory flywheel from the Mazda rx8 and just machined the exact design but with the v6 crank holes but made it out of a stronger matirial like high carbon steel or something along those lines.

        1. It’s always best to plan ahead!

          There are any number of ways to fit it all together once you start thinking about custom flywheels. You could indeed use the layout of the RX8 flywheel with the V6 hole pattern and if so then you could use the larger RX8 clutch. As mentioned on the blog I went with the mondeo ones primarily on a cost basis and originally I wasn’t going to turbo the engine so it wouldn’t really matter. As it turns out that does mean I’ll now almost certainly need to get an uprated clutch but these are still fairly cheap vs the RX8 ones.

          Either way you end up having to make a custom flywheel so you use whichever clutch you want. You could even set it up for a racing clutch such as a multi plate if you really wanted. If you do what you suggest the only other thing you need to be careful of is the offset spacing from the end of the crank which needs to be spot on and will change depending on your adaptor plate thickness…

          1. Thank you for your responses, I never thought about adapting it to fit a better clutch and the spacing from the adapter plate completely slipped my mind haha. But now if got that mostley figured out, just need to learn how to combine the two Ecu’s (jag+rx8) and how to fit the engine itself into the car. So with that I look forward to hearing how your own project goes.

  2. Thank you so much for this. As an X-Type owner I’ve always wondered how the AJ30 would work in an RX-8, and thanks to your fantastic blog I’ll have a good idea of how.

    Could I ask how much it cost to get a custom flywheel machined? And is there any chance you’d make the CAD files available if anyone wanted to follow in your trailblazing footsteps?

    1. No problem, just remember I’m figuring this out as I go along so there’s still a chance of it all going wrong but it does look pretty promising!

      The machining drawings for both the flywheel and spacer are linked in the blog on section two of the flywheel segment (http://www.chamberofunderstanding.co.uk/2018/01/27/rx8-project-part-8-flywheels-part-2/). I’ve yet to upload the 3D models as I’ve recently upgraded my PC and haven’t had chance to sort them out but most machine shops won’t be able to use them anyway. Please be aware because this is still ongoing I can’t confirm the flywheel is actually entirely correct so take use at your own risk! That said it all seems OK so far! Also in an ideal world you would combine the flywheel and spacer into a single part and I haven’t updated the design for that yet.

      Cost on this is a bit of an interesting story, in essence a local machine shop owner that supplied lots of parts to my ex employer offered to do it in return for a stack of pizza for his staff so it actually cost me about £75 but in truth the lump of EN24 steel alone was somewhere about £100. What he did was just get his guys to work on it when they were short of other work to keep them in practise. You would probably be looking at £500-600 to get this made somewhere but your best bet is to just contact machine shops for a quote.


      1. Many thanks for the in-depth reply Jon. I’m actually 1000 miles east of you, where machine work (at least labour rates) are significantly cheaper, but as you say the chunk of steel is a significant cost on its own.

        I won’t be looking to do this swap until my X-type gives in to the dreaded tin-worm, but since the inner sills like to rot out and it’s a 3.0litre manual, an RX-8 swap seems like a great way to re-use what are essentially worthless mechanical bits, so I’ll be following your build with keen interest.

        Please keep your fantastically detailed posts coming.

  3. I have just found your blog and have read it with interest.

    I am fitting the same engine and gearbox combination to a Chrysler Sunbeam as a project. I note the route you have taken with the flywheel and spacer. I managed to find an adaptor plate on the net, i bolted the gearbox to the engine today and interestingly the clutch slave cylinder is fouled by the near side cylinder head! Im not sure yet if this will be able to be overcome…

    For a flywheel i am using a TTV steel ‘standard’ flywheel designed to use the standard jag clutch but i may have to get a hybrid Jag cover/RX8 plate made.

    Keep the updates coming. 🙂

    1. Glad you’re enjoying the blog, it sounds like you have a really interesting project there!

      Unsurprisingly I have the very same problem but haven’t figured out a proper solution to it yet. One option I am considering is a pull type clutch slave cylinder. These allow the slave cylinder to be mounted facing in the other direction (towards the gearbox) and should work ok with a suitable bracket. Option two is that apparently a Saab 900 concentric slave cylinder can be made to fit but I have not tried this yet. I plan to give this a go but as yet just haven’t had chance!

      Do you mind me asking where you found an adapter? I looked for ages so I could avoid all the time and effort of making it but eventually gave up!

      Updates will follow when I can get chance, unfortunately something that seems to be a bit lacking right now!

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