620 S Build

[MV8]

Fuel gauge: check GRN/BLK continuity to ground at the gauge or the terminal on the tank with a DVM. What is the value in ohms? Check for voltage (can use a test light clipped to ground) at the gauge connector with the key on or skip if there is a face indication that power is reaching the gauge. The last check would be to pull the tank pump assy to ensure the float is attached and the arm swings freely.   

Throttle Volt spike: Good to see you have baro and may have MAP which would make this speed density instead of AlphaN (a good thing). The TPS should not spike as it is slowly rotated by hand.  I suggest requesting a replacement TPS and treating it like fine china.

Basic Setup/Tuneup Specs: Ask what the TPS and RPM should be at idle as this is the setting. I would expect (a GUESStimate) to see the TPS rotated to show around 0.32-0.35v when the idle speed screw has been adjusted to the specified idle rpm. If Cat support can't tell you these basic set up specs to check then you may need to go to SBD's MBE Tech Support.

Brake Bleeding: There are topics here about brake bleeding issues.

Throttle response: No kit to reduce off-idle response. The TB quadrant could be machined for a ramp groove transition instead of a steady ratio cable groove. There are ratio adapters for motorbike engines in car conversions but they just reduce the ratio. 

[CarYenta]

8 hours ago, MV8 said:

The TPS should not spike as it is slowly rotated by hand.  I suggest requesting a replacement TPS and treating it like fine china.

The transition by hand does seem mostly smooth, but it's micron level precision to make it so. I'll verify what the reference values should be. I drove around today to test it out, and it's wild that that -0.01 V difference made it so so much more driveable. I am surprised. I could Seattle hill start with a car right on my butt no problem, no hand brake.

 

For the fuel gauge, it does perform a test sweep at ignition set to run. I hope it's not the tank sensor 😬 going to grab another gauge from Bruce to test first.

 

Another thing I noticed, is how close the tie rod end boots are to the brake calipers. These are going to get burned off the first time I get the brakes up to temperature. What do I do about this?? I see a few other forum posts, one was to add a washer (not sure how that would be safe).

FL

 

FR

 

Edited November 22, 2025 by CarYenta

[MV8]

The spindles are a Triumph design that has vented rotors added and the clearance shown is reduced from a solid rotor spitfire application. I don't know what topic/post/forum you are referring to as far as a washer somewhere. There are a number of ways to provide more clearance. Washers in general don't make it unsafe but details are important. It would take specially made washers to move the arm away from the rotor and may require a longer bolt as well and it won't be going far. 0.100-0.125" would be reasonable and nothing would be modified, but you should discuss with CAT before getting creative. There are tolerances with any part.  

[CarYenta]

Westermann half doors needed some cutting in order to fit. I initially sanded away with a Dremel to see how it would fit, but found out it was best to just make a single straight cut to get around the track day roll cage.

 

As it comes door on left, cut on the right:

 

Another view of the cut:

 

View from top when installed:

 

View from side with two poppers in place, planning to add two more and then add on some edging so I stop scratching the crap out of my roll cage:

[CarYenta]

Straightened out my headlights, they were set to UK angle as seen here:

 

Suspension felt good around town, pretty darn smooth at low speeds over terrible roads. On the highway, it's very busy and a bit bouncy. Realized I never actually checked tire pressures and they were at 25-30, brought down to 22 to try that.

 

But that got me thinking that the car felt under-damped on the highway very similar to how my Miata with bilsteins did. Reading online and it seems a common sentiment.

 

The other common sentiment is to buy suspension kits from meteor motorsports. Having been burned with tariffs so much lately I think I'll find a US solution. This leaves Penske, MCS, FOX. Will post some suspension measurements.

 

The wheels:

 

 

Each front stud has 20 mm of thread engagement for the stock front wheels and the lug nuts took about 12.5 turns to remove. The studs appear to be about 10 mm diameter, so with the 1.5x rule I need 15 mm thread engagement and have 5 mm extra.

 

My front spacer adapters are 20 mm thick and the front studs are 28 mm long. The relief in the back side of the front wheels is much smaller than the rear wheels and is 6 mm. This means 28 - 20 - 6 = 2 mm extra. Since I have 5 extra to play with, I can cut 3 and have 17 mm thread engagement when using the stock front wheels.

 

For the rear, the relief in the back of the wheel is 20 mm, the studs are 32 mm long, and the spacer is 15 mm. 32 - 15 - 20 = -3 mm of gap between the end of the stud and the wheel relief, so it should be ok as-is.

 

Do I have any flawed reasoning to think 17 mm front stud-lug nut engagement is acceptable?

 

Front studs:

 

Engagement via antiseize:

Edited November 24, 2025 by CarYenta

[MV8]

https://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

[CarYenta]

4 hours ago, MV8 said:

https://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

Oh nice!

Looks like I measured 10 mm but these are M12x1.5 studs most likely, so 12*1.5 = 18 mm, 1 mm short of the 17 mm I'm thinking about.

But, using the equation in the link,

 

d = diameter of thread

p = pitch of thread

 

thread engagement length = 2 * tensile area / (pi/2 * (d - 0.64952 * p)

tensile area = pi/4 * (d - 0.9382 * p)^2

 

simplify your pi

 

thread engagement length = (d - 0.9382 * p) ^2 / (d - 0.64952 * p)

thread engagement length = (12 mm - 0.9382 * 1.5)^2 / (12 mm - 0.64952 * 1.5) = 10.2 mm

 

Which is 0.8 * d as the bare minimum. That is a lot shorter than 1.5 * d which is probably a real-world safer value. Therefore I think 17 mm is good to go.

 

[MV8]

I don't think so. Two times the tensile area, which is the circular area calculated from the thread root diameter. My point of providing the link was that a "rule of thumb" cannot determine a minimum or maximum.

 

Actual Triumph studs are roughly 9.5mm.

 

Also consider changes to offsets and weight if making these greater. The original wheels for this are already much greater than they would be on a triumph. The already narrowed safety margins are what make these cars so much fun imho.

What you do is up to you as it should be. I'm just typing with zero skin in this game.....

[CarYenta]

19 minutes ago, MV8 said:

I don't think so. Two times the tensile area, which is the circular area calculated from the thread root diameter. My point of providing the link was that a "rule of thumb" cannot determine a minimum or maximum.

 

Actual Triumph studs are roughly 9.5mm.

 

Also consider changes to offsets and weight if making these greater. The original wheels for this are already much greater than they would be on a triumph. The already narrowed safety margins are what make these cars so much fun imho.

What you do is up to you as it should be. I'm just typing with zero skin in this game.....

Thanks, I agree I don't know what the actual mechanical limits are for these studs, and that more is better. I don't think they are triumph studs.

 

Another calculator showing the min to nominal range is 5.5 mm to 16.5 mm for M12x1.5.

 

Yet another rule of thumb I've seen for M12x1.5 is 6 turns minimum, 12 turns nominal. If my 18 mm is as I tested 12.5 turns, 17 mm is 11.8 turns.

 

I am convincing myself :P

[CarYenta]

 

 

 

Only needed to take off about 1 mm for the 4x108 wheel studs to not interfere with the back of the front wheel reliefs. Coated with some zinc antiseize until I get around to painting it off. The wheel adapters have grade 12.9 studs and seem real beefy, whew.

 

 

What I did find out is:

1) I am terrible at measuring - the rear hub sure is 63 mm, but the front is more like 58!

2) neither the front nor rear OEM orcus wheels have any hub centric ring spacers, so the front wheels from factory are lug centric! The rear may be too, I ordered a calipers so I can measure the hub sizes better and get a ring for the front at least.

3) I think the wheels will fit perfectly. Mounting tires Thursday. If either front or rear are too wide, I will swap to 195/50 and 225/45 which should certainly have enough stretch in them. Thanks discount tire!

 

Front wheel fitment:

 

 

 

Rear

Lots of room on the inside, will measure once the tire is on but 1-2 fingers-worth

 

At ride height:

Will have to see what the stretch looks like with 245's. I don't think they will bulge out much. It's possible on a two-wheel speed bump it could compress enough to contact the fender, we will see!

 

Ride height from top:

 

Compressed only on wheel side, tons of room.

Edited December 3, 2025 by CarYenta

[pickles]

WOW! this is a journey! I do not want to know how many $'s you are putting into this  Not to mention the hours! But congrats on getting this far. 

[CarYenta]

45 minutes ago, pickles said:

WOW! this is a journey! I do not want to know how many $'s you are putting into this  Not to mention the hours! But congrats on getting this far. 

It's for the science!

 

Am talking with MCS currently about my options.

Anybody have total unsprung mass values for front and rear and motion ratios?

Edited December 3, 2025 by CarYenta

[Croc]

2 hours ago, CarYenta said:

It's for the science!

 

Am talking with MCS currently about my options.

 

 

Only thing that concerns me is the front stays once tires are on - it will be close I think.   

 

Who are MCS?

[CarYenta]

14 minutes ago, Croc said:

 

Only thing that concerns me is the front stays once tires are on - it will be close I think.   

 

Who are MCS?

Based on tire section width differences of the conti extreme contact force in 205/50 and avon zzs in 195/50, the 10 mm clearance should change to ~5 mm clearance to the inside stay.

 

MCS are motion control suspension out of Atlanta: https://www.motioncontrolsuspension.com/buyers-guide/3-way/

[Croc]

51 minutes ago, CarYenta said:

Based on tire section width differences of the conti extreme contact force in 205/50 and avon zzs in 195/50, the 10 mm clearance should change to ~5 mm clearance to the inside stay.

 

MCS are motion control suspension out of Atlanta: https://www.motioncontrolsuspension.com/buyers-guide/3-way/

 

Is that before or after tire deformation in a corner?

[MV8]

Clearance is also reduced from the wheel rocking on the bearings under inside wheel cornering load. How much depends on bearing setting, condition, and load. Probably the worst case when cold.

 

The rear will move around on bushing compliance which will change with wear.

 

Are the existing dampers not adjustable for the added unsprung weight?

[KnifeySpoony]

DeDion tube moves laterally quite a bit as well. On the order of centimeters in my experience. 

[CarYenta]

4 hours ago, Croc said:

 

Is that before or after tire deformation in a corner?

Before. This will only be driven to church 😂 Hoping the tires are stiff enough.

 

4 hours ago, MV8 said:

Clearance is also reduced from the wheel rocking on the bearings under inside wheel cornering load. How much depends on bearing setting, condition, and load. Probably the worst case when cold.

 

The rear will move around on bushing compliance which will change with wear.

 

Are the existing dampers not adjustable for the added unsprung weight?

The existing dampers are height adjustable only.

I think the unsprung weight will be nearly the same as the OEM wheels.

 

I suppose I should do the math...

 

Front 20 mm adapters are 1.3 lb each with nuts.

Rear 15 mm adapters are 1.1 lb each with nuts.

(I had these flipped on the first post. These weights are according to my really accurate bathroom scale with me + adapter - me without adapter).

 

Front 15x7 wheels are 11.3 lb

Rear 15x9 wheels are 12.7 lb

 

Front Conti ECF 205/50 are 19 lb

Rear Conti ECF 245/40 are 21 lb

(From TireRack)

 

OEM orcus 15x6.5 are 13.0 lb (https://caterhamparts.co.uk/lightweight-15-orcus/7513-65x15-orcus-wheel-anthracite.html)

 

OEM Avon ZZS 195/50-15 are 20.7 lb (https://www.arielmotor.co.uk/product/avon-zzs-tyre/)

 

Therefore, corner weight (new) - (old) =

Front: (1.3 + 11.3 + 19) - (13 + 20.7) = -2.1 lb

Rear: (1.1 + 12.7 + 21) - (13 + 20.7) = +1.1 lb

 

 

1 hour ago, KnifeySpoony said:

DeDion tube moves laterally quite a bit as well. On the order of centimeters in my experience. 

Good thing the inside has so much room. On cornering, compression should keep the whee at a lot of negative camber. A drift over a speed bump might be the worst case scenario where it would tuck without camber change and maybe hit the fender exterior lip. Would be a shame to have to buy carbon fenders!

Edited December 4, 2025 by CarYenta

[CarYenta]

Front hub bore is 58.3 mm

Orcus wheels are around 63 ish, caliper didn't really fit in there. Will take a center cap off at some point.

 

[CarYenta]

My 10 mm wider tires are 15 mm wider 😂 Contacts cycle stay lightly. Otherwise fits great, easy to mount.

 

Should I:

 

1) shim the cycle stay towards center of car?

2) shim cycle stays up?

3) shim both directions?

4) bend the cycle stay?

 

I'm not sure what the cycle stay holes look like since these were installed already when I received the car.

 

 

 

Rears fit as anticipated, slightly more than a 1 finger gap between trailing arm and tire.

No the stands are not under the exhaust although it really appears as such!

Vs Old

 

 

Edited December 4, 2025 by CarYenta