Category Archives: CAD

IRS stage 1 is complete!

OK, I should have written this up months ago, but my time is not my own these days. Despite lock-down my job has kept me very busy. I already work from home, so it’s been no real change for me. Only the occasional hour to work on the Mantula. 

Back before the lock-down came into effect I was able to get the chassis over to George at Arch’s Speed Shop here in Bembridge. It was his job to make sense of my design and the 57 plasma-cut pieces that would eventually become the new IRS chassis update.

Plasma cut parts

The chassis modifications were drawn in a 3D CAD programme, the excellent and versatile Fusion 360.  I really enjoy using it and it’s free for hobbyist users. 

The process of designing the CAD model is my preferred way to ensure that the job is right first time.  I can design everything to the fraction of a millimetre and see the problems before any metal is cut. Perhaps if I had the fabrication skills I would make it up on the car. Maybe for my next build…

Fusion 360 has a sheet metal design mode and I used that to work out the correct bends and then export the DXF files that are eventually loaded onto a CNC plasma cutter and cut out of 3mm steel. 

That plasma cutter is owned by Ray at Northshore Classics and he did a great job for me including the folding according to my drawings. I was very pleased with the quality. He’ll be doing more work for me as the build progresses.  

Plasma cut parts
Plasma cut parts ready for use.

Arch’s Speed Shop

I was very lucky to get the chassis conversion done before lock-down, but it was close. The chassis was collected on 9th March and returned on Friday 20th March. Lock-down started on the following Monday.

Before collection
Chassis ready for collection

I sourced the steel tube which was a perfect match to the original 1 1/2″ x 14 swg. Mine was supplied  by Metal Supermarkets.

First parts added
First webs added

These parts were supplied by Marcos Heritage Spares as part of the seat belt mounting upgrade kit. 

New Type approved style seat belt mountings
New Type-approved style seat belt mountings with improved door striker plates.

Rather than remove the existing door striker plates we decided to double-up.  The plates were spot welded together then welded all-round to avoid water penetration. This adds a little more stiffness in this relatively flexible area. It also ensures that the body will still fit. 

Seat belt mounting
Seat belt mounting

Rear view of the seat belt mounting.  As this area is open to any road debris thrown-up by the rear wheel it was later decided to fill any potential water traps. The welds were rather spattery probably due to contamination.  Maybe not the prettiest welds but we have very good weld penetration – this isn’t going to fail!

Rear cross-member drawing
Rear cross-member drawing

It’s easy to create working drawings from the 3D model in Fusion 360.

Front IRS pick-up tack welded
Front IRS pick-up tack welded

 This is about to be welded-up then top and bottom fish-plates added.

Finished front cross-member
Finished front cross-member

While it would have been easier to make a single mounting structure as we have done on the rear cross-member, there needed to be clearance for the front of the differential. That will make maintenance a little easier if the diff ever has to be removed.

Rear cross-member parts
Rear cross-member parts

The suspension bolts will screw into the steel tubes which are welded through the chassis tube. 

Rear cross-member during fabrication
Rear cross-member assembly dry-run before welding

The plates on the chassis are slightly larger than the tube to protect the poly-bushes. Large fish-plates are to be fitted top and bottom to spread the load from the triangulation tubes and top-rear diff mount.

Cross-members in place
Cross-members in place.

The rear cross-member position is easy to locate correctly and was used as the datum for all IRS components. First attach the rear cross-member then install the lower wishbones and tack the front cross-member where it needs to be.  That ensures no strain on the wishbones. 

From there the uprights are attached and the top wishbone position gives the exact location for the top wishbone mounts.

Top pick-ups in position
Top pick-ups tacked in position

Its very satisfying that the geometry worked out perfectly. 

Before final weld-up
Before final weld-up

Triangulation tubes added along with the rear diff mount pick-up. The rear diff mount will not follow the standard Marcos design “P” bracket because that orients the poly bushes incorrectly.  In fact all of the standard Marcos diff mounting is wrong in my opinion. Watch for an alternative design in a future instalment…

Final weld-up
Final weld-up

The last part of this stage of the build is the final weld-up. Yes there’s more to do but the next part of the design needs some real mock-up work to ensure the new diff mountings and additional cross-bracing work together and are still maintainable. I could do that in CAD of course, but I would have to accurately draw the diff and I honestly can’t be bothered! 

Back home!
Back home!

Here it is taking up a lot of valuable space! Unfortunately around this time despite the start of lock-down I had to start a complex contract and didn’t get a chance to get to grips with the diff mount mock-up.  However I should be back on it in a few days and I can’t wait.

More in the next instalment.


Real progress at last

I’ve definitely been working too much. Seven days a week  with just three days off since the middle of September. One of the joys of being self-employed! 

Now that the various projects are delivered I can give the Mantula the attention it deserves.  This means there’s been quite a lot of progress considering this build normally progresses at a geological pace.

Much better than new

In the previous instalment I reported that the engine had been dispatched for a complete rebuild.  Well, it arrived back yesterday looking very smart and probably better than new. It’s still sitting on its pallet,  but I’ll get it mounted on the engine stand tomorrow. There are several important parts to attach too. 

Flywheel and clutch

The flywheel has been modified to accept the ignition trigger ring. Once that’s mounted the up-rated clutch can be installed. 

Flywheel machined to accept the trigger ring

The original clutch was Rover SD1 3.5 litre spec which wouldn’t be able to cope with the much greater torque from the modified 4.6 litre unit.

New heavy duty clutch by Helix Autosport

The front end will get a new water pump along with various brackets and sensors. More on this in a few weeks – I have to modify the serpentine belt routing so that it clears the steering linkage.

Low-profile sump

It was quite some time ago that I had the low profile sump fabricated, so it will be wonderful to finally fit the thing! That can happen as soon as it returns from getting its Zinga galvanising coating.  It’ll also have an abrasion resistant ceramic coating. I’m using the sump as a practice for getting the chassis treated.

Low profile baffled sump

When everything’s fitted I’ll squirt some Double TT into each cylinder to prevent possible condensation rust damage until first start some time next year.

Chassis modifications

The chassis is booked-in for modification during the second week of January. I still have work to do on the rear end design where I’m diverging from the standard Marcos IRS set-up a little. The 3D CAD drawing will be finished next week. I’ll then get the new suspension mounts and various brackets cut out on a CNC plasma cutter.

Inlet manifold design changes

Some time ago I showed the designs for the Jenvey throttle body adapters which I had 3D printed here on the Island.

Original undersized adapter
Inlet adapter in position on the Thor manifold

Unfortunately I made an error by severely underestimating the correct throttle size. Now rather than the original 48mm size I will be fitting 60mm or larger Jenvey items.

Here’s the revised design in Fusion 360. I will probably have to build in some strengthening webs. 

The revised adapter design

These 3D printed items are just for testing, more robust items will be fitted when I’m certain that everything works correctly.

The throttle body size error was noticed by the very helpful Daniel Lloyd from Lloyd Specialist Developments in Warminster. They’ll be setting up the Canems engine management system for me and fine tuning it on their dyno.  

That’s all for now. I’ll try to keep these updates more regular while there’s something going on.



Wow, it’s really been three months since I last worked on the Marcos! Sometimes other things get in the way. So it’s been months of DIY and decorating that has left the Mantula untouched. Hopefully I can get back to it in December. In the meantime here’s a picture…

logo3d (2)

3D printing

Today I collected the results of my experiment with 3D printed parts. In a previous post I showed how I was designing a revised inlet system using two new throttle bodies to replace the standard Range Rover part. The design has matured a little and the geometry is now fixed so it would be great to get the parts manufactured.

The final adapter design in Fusion 360

This is a screen capture from the Fusion 360 CAD program showing the adapters in place on the manifold.


This is the result, printed here on the Isle of Wight by Island 3D Printing.



And here’s one of the adapters with a throttle body attached. The oil filler tube will have to be relocated before I can mount the other side.



I’m still here!

Sorry that I’ve been so quiet for the last couple of months. Progress on the car has been minimal due to other commitments. While I’m out of the country often it been impossible to get anything done.

I’m sure you would prefer to see pictures of the Mantula, but nothing new to show yet. However time hasn’t been totally wasted as I prepare to design the changes for the inlet system.

Fusion 360

The image below shows the work-in-progress on two fronts. The first front is learning to use a 3D parametric CAD system. I chose Autodesk’s Fusion 360 Ultimate. It’s really easy to learn, has an appropriate feature set and is affordable. For any ‘hobbyist’ it is free, at least for now… Better even than that it’s a joy to use. I think I’m becoming addicted.

Second front is the design of the inlet system.


The model still has a long way to go, but I think it’s worth the effort. Drawing the plenum has been a real challenge due to its totally asymmetric, organic design and overall complexity. As you can see I still have three more inlet runners to model, plus all of the fixing points.

The next part is the black mounting adapter with the Marcos logo. This will be 3D printed once the final design has been fixed. The current version needs some rework for clearance.  Of course I will only know the effect on performance once I get to run the engine.

The last part shows one of the Jenvey 48mm throttle bodies. This has been modelled to 0.01 mm accuracy to ensure that the throttle linkage works first time. A really great feature of Fusion 360 is the ability to design joints that are limited to a realistic range of movement. So far I have the throttle opening the correct 83 degrees. This will be eventually joined to the main link mechanism, again with accurate movement limits.

For the next few weeks I will be completing this design work. Then I’ll send the CAD files to a local machine shop for manufacture.

Jenvey throttle  bodies

As mentioned above I’m going to use the excellent Jenvey throttle bodies to replace the original Range Rover item. Here’s a comparison.


Two smaller throttles should give better response than a single large throttle. I can’t wait to find out!