Polyester resin casting new light lens for a bullnose Saab

It is becoming increasingly difficult to find new directional light lenses for the two-stroke bullnose Saabs. Or actually it seems almost impossible. And I need four of them for the Saab Sport – and a few spares wouldn’t hurt.

I have a few lenses that are in more or less decent shape, so I figured I could try and make copies of them. I read about polyester resin casting using a silicone mold, and decided to give it a try. This is a pretty popular way to make hobbyist plastic parts for model railways or scratch built scale models, so i figured it can’t be too difficult.

One new resin cast Saab directional light.
One new resin cast Saab directional light.

I was actually somewhat surprised, but making a polyester resin casting worked out very well as you can see above. Basically what you do, is make a two-part mold (it needs to open up to remove the finished piece) out of silicone mold rubber. The mold rubbers available come in many flavors – soft, hard, heat resistant, not heat resistant, etc. Then you pour in polyester casting resin and wait for it to cure and harden. After it has cured you open up the mold, take out the part and remove any overflow or excess resin. You can buy suitable materials in most hobbyist stores or online.

I used Efco silicone mold rubber and polyester casting resin.
I used Efco silicone mold rubber and polyester casting resin.

As I have not done any of this before I used what the local hobby store had (Harraste, Tampere). Efco Silicone mold rubber and Efco Polyester casting resin. Efco Polyester resin is water clear and should make very nice clear plastic parts. There are a few drawbacks – polyester resin is hazardous to your health when casting and it does shrink a bit when curing ( 6 to 8 %). So you will end up with a slightly smaller part than the original.

UPDATE: I have been recommended UV protected polyurethane instead of polyester. It is more impact resistant and should not yellow over time. I tested the polyester lens and it does break quite easily. I will try out polyurethane and report on the results.

Both – the silicone rubber and the casting resin – are two component chemicals, so they are mixed in a very specific ratios with the accelerator. There is no hurry in the mixing as the working time is usually at least 15 minutes before they start hardening. Read the instructions.

Making a silicone rubber mold

Casting the first mold half.
Casting the first mold half.

First I cast one half of the mold. The inner part of the light lens. You need to make a little tub for the part and design it so that the silicone rubber doesn’t get too much under the part. Otherwise it is hard to remove the original from inside the silicone.

The silicone mold rubber copies all the surface detail on the original piece. Including tiniest scratches.
The silicone mold rubber copies all the surface detail on the original piece. Including tiniest scratches.

As you can see from the photo above the silicone reproduces all the surface detail of the original piece. Including all the scratches and other imperfections.

Even the Hella company logo is reproduced very nicely in the mold.
Even the Hella company logo is reproduced very nicely in the mold.

When the first half was done I applied some mold release (Vaseline grease) on the edges of the first half. I then put the original light lens back in the mold and made a new tub for it, proceeding to cast a new layer of silicone rubber on top.

Both mold halves done. I used regular Vaseline grease as mold release.
Both mold halves done. I used regular Vaseline grease as mold release.

Polyester resin casting

Next I cut a few holes at the highest point of both ends of the “lid”, so I can pour in the polyester resin. The second hole is for the air to escape as the resin displaces it. You know that the mold is full when the resin comes out of the second hole.

Removing the polyester piece from the silicone mold can be tricky.
Removing the polyester piece from the silicone mold can be tricky.

Above you can see the cast part still attached to the mold half. On the left you can see the small square hole where the resin was poured in. There is a similar hole on the other end. The holes need to be at the highest point so you will not end up with a partially filled mold.

As the silicone is quite stiff and the polyester resin shrinks a little when curing it can be quite tricky to remove it from the mold. Despite being careful I tore the mold a little. Nothing to prevent using it, but I need to be more careful with it.

Silicone molds and the resin cast directional light lens for my Saab Sport.
Silicone molds and the resin cast directional light lens for my Saab Sport.

Above you can see both the mold halves and resin part just removed from the mold. Some cleanup required.

Polyester resin cast light lens on the left. Original Saab part on the right.
Polyester resin cast light lens on the left. Original Saab part on the right.

New polyester resin light lens for the Saab Sport on the left and a Saab original on the right. So far looking great.

There is one more downside to polyester resin. Most of them yellow over time if not UV protected. You can coat the piece with UV protecting agent though. Some of the resins also contain UV protection chemicals.

I don’t know how good the Efco resin is in this respect so I’m going to test the part in my two stroke. I’ll install one polyester lens on the car and store another in dark cool place for the same period and see if there is a difference.

Also I don’t know if the resin can withstand all the weather – rain, sun, stones etc. as well as the original. But I think after the summer I will pretty much know if polyester lenses are a go for the Saab Sport.

Please let me know if you have experience in polyester resin casting parts and know suitable resins with good UV properties.

Triple-carburetor engine jetting and lambda

Since I put the lambda gauge in the two-stroke I noticed that it’s running a little on the lean side.

The engine is the 1966 triple-carburetor version. I figured the problem has to be the main jets since adjusting the mixture screw didn’t help. I had 120-135-120 main jets in at the time (that’s 1.2 mm jet in the front and back carburetors, and 1.3 mm in the middle since it runs hotter). With those main jets the lambda was about 1.0 at full throttle on the motor way. And I think that’s too lean for a two-stroke.

I went through a bunch of spare carburetors I have and found a handful of main jets ranging from 115, 120, 130 to 135.
I went through a bunch of spare carburetors I have and found a handful of main jets ranging from 115, 120, 130 to 135.

The factory original jetting for the time was 115-130-120 front to back.

I figured I’ll increase the jetting 10% and see what happens. So I put in 130-150-130 jets. with this setting I’m now getting a 0.9 lambda at full throttle and acceleration.

The performance of the engine is clearly better with this jetting. So I was wondering if I should try and go for even bigger jetting and aim for a lambda somewhere between 0.8 to 0.9. The engine is unported (stock) but I have a modified Sport exhaust system.

I noticed in Niklas’ XP-Manual (www.classicsaabracing.com) that he recommends 125-140-125 main jets for modern fuel (95 octane with 5% alcohol).

Niklas also notes that if you have a “stall” when stepping on gas it may be because of too small idle jet. That’s probably something I need to look into since I seem to have a little bit of that symptom. It’s also clearly visible on the lambda since the value drops to 1.5 when pushing the pedal and the rises to the 0.9 level.

So I would be very much interested on what other people use for jetting on the 1966 engine?

Getting the two-stroke ready for the summer – done

The two-stroke is ready for the driving season. There’s actually a Saab-Club Finland spring meeting a week from now and I decided to take the stroker there.

Since the steering rack was replaced we needed to measure the wheel angels (or rather the toe-in) again - old school style...
Since the steering rack was replaced we needed to measure the wheel angels (or rather the toe-in) again – old school style…
Measuring the toe-in. Should be 2mm +/- 1mm on a two-stroke.
Measuring the toe-in. Should be 2mm +/- 1mm on a two-stroke.
The driver side tie rod is easy to adjust as it is accessible through the engine bay.
The driver side tie rod is easy to adjust as it is accessible through the engine bay.
The passenger side tie rod is not quite as accessible...
The passenger side tie rod is not quite as accessible…
I also bought a nice bracket for the revcounter and attached the lambda gauge on the dash.
I also bought a nice bracket for the revcounter and attached the lambda gauge on the dash.
And I bought a little 2kg fire extinguisher for the car.
And I bought a little 2 kg fire extinguisher for the car.
Ready for new adventures 2014!
Ready for new adventures 2014!

Getting the two-stroke ready for the summer

Just a quick update. The two-stroke is pretty much ready for the summer. I installed all the stuff I was planning on (lambda meter, steering rack, engine and gearbox mounts, revcounter and a backup light). A few photos:

Engine back in the car, with new mounts and the steering rack replaced.
Engine back in the car, with new mounts and the steering rack replaced.
Backup light is in working order.
Backup light is in working order.
The lambda sensor in the exhaust pipe.
The lambda sensor in the exhaust pipe.
The lambda meter (fuel/air ratio meter) and the small revcounter that I will install under the dash.
The lambda meter (fuel/air ratio meter) and the small revcounter that I will install under the dash.

I think between the lambda meter and the revcounter I will get pretty good info on the mixture. Takes the guesswork out of finding the correct jetting.

And here’s a little “advertisement” of a local museum. I just visited the Finlayson Mill “Steam Engine Museum” (here at Tampere) for the first time. I love steam engines, but I just don’t have the time to really get into them. So I like to visit museums and events whenever I find the time. The Finlayson Steam Engine Museum only has one steam engine – the same that powered the factory a century ago – but it’s huge! Two engines – named Marie and Helene – operated a small house sized flywheel. From there power was mechanically distributed throughout the factory. It’s well worthwhile to visit.

finlayson-tampere-finland-steam-engine tampere-finland-steam-engine-museum

Steering rack problems – two-stroke

It’s time to do some spring maintenance on the two-stroke. The most important thing  I had on the to-do list was the steering rack. There was clearly some play on the rack. Not good.

I also wanted to fit the lambda gauge I bought last summer.

The first thing to do was to pull the engine:

Next job was to weld a fitting for the lambda sensor on the exhaust pipe:

Once the steering rack was removed it was pretty easy to figure out what was wrong. But we still needed to take the rack apart to see if there’s anything to be done about it: