Saving Old Seagulls

This is a project I've picked up and put down several times over the past few years so a lot of the details are forgotten or sketchy but still some of you might find it a quirky project.

In looking to make a 'gull go a tad quicker on a budget, I kept tripping up on the limitations of the bottom end. The solution I couldn't stop considering was to install ball bearings where possible and do away with the plain bearings which, at least on all of the bits I was willing to throw at such a project, were past their best (at best!)

That idea was dead in the water pretty quickly for heaps of reasons so I had a look about at small Japanese bottom ends. Not an idea any purists would consider and probably not kosher among racers but could be a fun engineering project.

It seemed like there were potentially some that might be shoehornable into the space with the same or a similar stroke to the 102. A few £ on eBay got me a spares honda crank, I think it was from an SA50, PC50 or similar.

It turned out to have a slightly shorter throw than the 'gull which was a bit annoying as it brought the engine cc back down to ~97cc and also mean some serious skimming of the barrel and head.

Had to slim down the flywheel a bit and grind some clearance in the crankcases as well as obviously machine recesses for the bearings - who knows how the not fully supported bearing races will fair!

Ended up with one reasonable ball bearing on the beefier lower side and two quite flimsy bearings in the thinner casting at the flywheel end. Above and below the main bearings, I put needle rollers. A comprise, especially as they run directly on the machined, unhardened steel of the honda crank.

The piston was a bit of a head scratcher, I decided I'd have to use an original piston but mounting it was going to be tricky considering the honda little end was expecting a 10mm gudgeon and needle rollers. In the end I pressed in some spacers and reamed them for the 10mm pin. A lot of weight on the end of con rod considering it was designed to take a 50cc piston!!

Filled the head (a lot) and did my best to go through Jennings and another manual (name forgotten) but it's all a bit outside of the scope of such publications written with much higher rpm and loop scavenged engines.

Fitting a press together bottom end freed up lots of space in the crankcase which was filled to what I decided, based on some careful measurement, was a sensible degree.

Some disorganised pics to follow

Gareth what's the center to center length of the conrod?

Ah, good question Charles I of course have that written down but where escapes me from the moment - I will find it shortly

That's all the pics I have I think.

It runs quite nicely, its on av2.5:1 'box with a non seagull prop. I've not tested the latest iteration on the water but have been playing with a baby prop on the pond that really lets it rev out a bit. Could do with finding a propeller somewhere between the ones I have to do some canal tests if I take it out of hibernation again

Con rod 75mm centres (I think!)

Seagull rods are 76mm, if your rod is the same a a Waleroo it will be 76mm.

Did you measure the port timing & the compression?

Those needles have rubber seals, if you didn't polish the crank to size it, will damage the seals, causing a loss of primary compression!

I did do quite a bit with the port timing but my notes are a jumbled mess again so I'd need to do another set of measurements/port map to be sure.

Compression ratio was last calculated at about 10:1 (6.5:1 corrected) but I know I changed head gasket plans since then so will be slightly different. Results in about 120 psi on the cord

You're right about the rubber seals/shaft. And I am relying on them (and the rear ball bearing seals) to keep the primary compression in. Yes, I polished up the shaft as best possible but it's likely not to last like a proper surface would. I'm not at all sure about the longevity of the needles on there, I'm not the best machinist and they deserve better!

It would have been a lot less trouble to use a Kingfisher needle roller crank with new bronze mains.

But some nice engineering, I'd be concerned about the needles on the mains, they're designed to run on a polished hardened steel shaft, where does their lubrication come from?

*Ahem*.. *cough cough*...

viewtopic.php?f=2&t=6034&p=53365&hilit= ... ank#p53365

First and foremost, this is a fun tinkering project.

No budget for fancy pants racing parts. I've spent a bit on one off tools and a few tens of pounds on second hand eBay bits, bearings and propellers but total spend would be less than one of those cranks. Even then, I'd be limited to just roller bearings on the big end (or maybe small end too I can't tell)

Yes, Charles, that is a concern. I was aware of the limitations of rollers on a non hardened, less than perfect shaft. I hope that forces up there will not be excessive and it'll run for long enough to keep me entertained. If I end up needing to replace the rollers, they are very cheap. They are lubricated by the inevitable blowby! But I think I put a little grease in there until it gets washed out.

PS,
I think the rod was 75mm as per this chart for the Lead50/vision.
http://www.mitaka.co.uk/MDP.htm

Your 76mm walleroo/spx50 is listed just below. May have been a better choice!

The Walleroo rod has room for a Pb2 bush in the little end, that would avoid all that piston work.

Haha, the benefit of hindsight!

Maybe that's an idea for another day. To be honest though, while the little end setup I've got there took a bit of planning, actually doing it was straightforward and inexpensive. We'll see if it proves to be sturdy enough or not!