02J to PYThe last post covered a brief decision on which gearbox I have chosen, and why. This will explain my solution to getting an 02J gearbox bolted to a G40 engine.
Plate vs. Bellhousing
Sorg have offered for a good number of years, the option to mate an 085 and 02A/J bellhousing together. This involves removing sections of the 085 (all 5 mounting locations and the starter housing) and welding them into the 02A/J bellhousing.
This is no trivial task, the tolerances between dowel locations and the gearbox input shaft centreline must be <0.1mm to avoid premature wear on the spline. To add to the difficulty some of the later aluminium casings (02T) which could be used, have higher magnesium content, a very skilled task, and one outside of my ability.
I also didn’t like the thought of having a one off gearbox casing; I’ve damaged two with failures previously, which was fine as they are easily and cheaply replaced. The cost of the Sorg option was roughly £900, plus a custom clutch (I don’t include other custom parts as they are necessary for both adapter plate and bellhousing options, I’ll cover them later).
So, the Sorg option uses the following:
- 085 Bellhousing
- 02A/J Bellhousing
- PY Flywheel
- PY Starter Motor
- Bespoke clutch (200mm with Golf spline)
- Custom adapter for Golf to Polo Clutch release bearing
Total for gearbox mounting, and clutch/flywheel hardware is ~ £1200. I was hoping to achieve everything for under £1k.
Designing an adapter plate allowed me to use a standard 02J gearbox, with no modifications. I could choose to use a Golf flywheel, with an off-the shelf clutch. The main disadvantage of the adapter plate is the extra width it adds to the powertrain. In my case this is 10mm as I have a steel plate, some aluminium solutions in Germany add 15 to 25mm. On a Berg car this isn’t an issue, but I didn't want to encroach on the wheel well too much (despite low offset wheels), I also wanted to keep the driver’s side chassis leg stock.
Adapter Plate DesignWhen designing the adapter plate there were several key considerations to think about:
- Differential positioning remained the same for 085 and 02J installations, this gives the inboard CV joints sensible articulation, and does not necessitate the need to alter suspension geometry
- Gearbox installation angle to be the same in the Polo as per Golf
The above caveats dictate the angle of the engine. I exchanged a few e-mails with a German friend (who I owe a few beers!) regarding his installation, to check I was on the correct track. Although he has a Sadev box in his Polo now, he’d previously done a similar conversion on another car.
With that I set about measuring some of the components. Robin helped with gathering some figures greatly, so thanks to him- I guess he might end up with one of these at some point, so it’s in his interest
The first design looked something like this:
That achieved a differential centre line delta of 2mm(X), 26mm(Y) which I think is acceptable.
To keep this diff centreline, and the gearbox installation angle, means canting the engine over from its original position. My measurements roughly showed the PY block sitting at 12° from the vertical. This would need to be nearer 40°, clearly requiring some inlet/exhaust manifold modifications and some thought around the sump/oil pickup. I’ll cover that at a later date as/when I’ve thought about it!
Clearly the box would require two new mounts (B mount, and C Mount) and with the new engine angle I’d need to cut and shut the old A mount to increase the 12° stock to 40° requirement. I will cover this in a later update, as I’m hoping to fit a smaller/lighter alternator too- so will end up making 3 new mounts, possibly adding a 4th to the subframe at the front.
So, enough waffle about the principle! I mentioned previously a 10mm steel plate- I’d decided on Steel for a number of reasons:
- Plate would be thinner than an aluminium counterpart
- Easier to machine than stainless
- Can be nickel plated for corrosion resistance
- Similar expansion rate to block
- I can machine/drill/tap/weld it
- Cheap!
Aluminium offers a nice solution, but I’d need to double the width and helicoil everything. Now, those of you with a background in engineering will probably be thinking, M12 thread, 10mm plate- yes I realise I’m short of the 1xdiameter standard requirement, my excuses:
- 10mm plate easy to come by (12mm was a longer lead time)
- I like round numbers
- 2 out of 5 fixings I can secure with an additional nut
- Golf box uses 6 M12 securing locations, Polo has 5 M12
- I need to keep the plate diameter as small as possible for starter/flywheel engagement (a topic I will cover later)
- I can switch to a fine pitch thread if more engagement is required (standard M12x1.75 currently)
Adapter Plate MockupWith a design in the virtual world I needed to transfer it onto something physical. Most printers can’t print 100% and suffer from shunt so a plotter is required. A good mate printed a few copies of the design off for me.
I transferred this onto some 3mm ABS sheet I had knocking about, would be strong enough to bolt through with abit of support, and give me a decent idea of what needed to be modified.
I still had the Polo starter motor mountings at that point as I wasn’t sure what direction I wanted to go. Once I’d cut it out roughly it got mounted in the car, the angles aren’t correct, but gave me a flavour of what to expect.
Before I removed the engine and 085 gearbox I did various measurements so have a good idea of where the engine should sit indexed 40° round.
With this done I did several more iterations of the design- to increase output shaft clearance (use the larger Mk4 Golf items) and to delete the Polo starter motor assembly, with some slight tweaking to hole locations, and double checking all 4 dowel dimensions, which are the most critical part.
Although it looks like it would fit in that picture- the crank is hard up against the chassis leg, and has no pulley/FEAD on it, so need to move over another 35mm or so. Chassis must be cut...
Next up is the adapter plate manufacture, and the machine work required post water jet profiling. I’ll save that until next week
Cheers
Pete