British Seagull 4 Stroke Idea

Discussion in 'A Work In Progress' started by cox24711, Nov 10, 2018.

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  1. Nov 10, 2018 #1

    cox24711

    cox24711

    cox24711

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    Recently in a daydream during assembly at school I had the idea of bringing British Seagull back, with new model, a single cylinder 102cc ohv 4 stroke using mostly the same parts as the original as possible. like the gearbox, props, mounts, flywheel, water pump, carburettor and fuel tank just to name a few. Oh and it will certainly be rope start with no recoil! :)

    (they have certainly proven themselves reliable) due to their outstanding reputation, and knowing that many people are fed up with the maintenance of new small outboards from the regular scumbags (Mercury Yamaha Evinrude etc) with the stupid cowlings getting in the way, the recoil starters watch spring snapping, the annoying impellers that wear out or go hard and their unnecessary complexity. They certainly aren't built to last.

    The one trait that you want in an outboard is for it to be reliable and that is what british seagulls nail. Also the ease to work on and accessibility. The idea is to make maintenance of it as simple and easy as possible, so it isn't a chore.

    Because of making new 2 strokes is now outlawed the only option I am left with is a 4 stroke. My aim is to make the simplest design possible, with as many off the shelf components as possible, too keep the costs down.

    I am not sure yet what the bore and stroke should be to keep it in 4hp-5hp range possibly the same as the original 57mmX40mm. I know that with the original gearbox the RPMs need to be quite high ~4,000 so we are looking for peak power at High Rpms.

    Thanks Greg
     
  2. Nov 10, 2018 #2

    cox24711

    cox24711

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    Also I forgot to mention on the crankcase it will have one of these
    500-1001_large.jpg
    that will screw into the water jacket (with a 1 way check ball behind it) so you can connect your garden hose to it to make it easy and simple to flush out after use!
     
  3. Nov 10, 2018 #3

    XD351

    XD351

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    I think the outlawing of new two strokes is for commercial manufacturers and not something that will effect you . There are plenty of parts on ebay and i use the pistons and rings off the 50cc minibike because i can get a piston and rings for a couple of bucks . You could use parts from a briggs and stratton motor and you can pick up things like pistons fairly cheaply .
    I remeber when i was at high school someone gave me an old clinton outboard and to this day I don’t know how it ended up here in Australia as no one i talked to from just about every boat shop in sydney had ever heard of one ! I always wanted to stick a victa motor on it but that was way beyond my skills at the time . Victa also made outboards and occasionally they pop up on ebay .
     
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  4. Nov 10, 2018 #4

    cox24711

    cox24711

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    Thanks mate I was thinking of the same kind of idea using off the shelf bits and bobs. Ideally I would like to make this commercially. But first I need to design it and build a prototype and see where it goes from there.

    Greg.

    p.s that is a nice pic of an XD falcon you have got there. if I am not mistaken it looks like tru-blu, and that would make sense with your username.
     
  5. Nov 10, 2018 #5

    XD351

    XD351

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    Yes Tru blu lifelong DJR fan .
     
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  6. Nov 28, 2018 #6

    JOHN DUNCKER

    JOHN DUNCKER

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    You probably know of this site already http://www.saving-old-seagulls.co.uk/

    He is the goto guy on all things Seagull.

    There would be some interest in a roller bearing 2 st Seagull that could run on 59 to 1.
     
  7. Nov 29, 2018 #7

    cox24711

    cox24711

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    hello john,
    thanks but I am already aware S.O.S I have personally bought parts from John (the owner) and I have spoken to him about my idea, and he likes the idea and is interested with where it goes.
    Greg
     
  8. Nov 29, 2018 #8

    lennardhme

    lennardhme

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    Thanks for the thread fellas.
    I have an old seagull & you've given me the inspiration to "get into it "
    cheers,
    Lennard
     
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  9. Jan 1, 2019 #9

    cox24711

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    Well, with another year done and dusted am back with the basic layout of the seagull 4 stroke.
    Screen Shot 2019-01-01 at 12.04.58 pm.png Screen Shot 2019-01-01 at 12.05.11 pm.png Screen Shot 2019-01-01 at 12.05.39 pm.png Screen Shot 2019-01-01 at 12.06.00 pm.png images.jpeg

    It is single overhead cam (thereby reducing the parts count, reducing the weight and most importantly keeping it simple)
    the arrangement it is done so by utilising 3.5 flat head briggs lawn mower camshaft and (modified valves) and springs etc.

    The piston and rod and crank are your bog standard seagull ones ( I decided on using the standard piston and rod, so it won't mess up the balance of things, otherwise it will probably end up running like a wacker packer :D)

    Because of the shape of the piston, I went with a wedge combustion chamber, and I am shooting for 9.5:1 compression ratio (with a head gasket on it will probably be more like 9:1

    According to john from S.O.S the 102cc models had a compression (when new ) of 55psi that equates to a compression ratio of 3.75:1! :eek: and they even run well with 35 psi! ~ 2.4:1!

    Lets get back to topic
    the camshaft will be driven by a toothed belt (a brilliant idea that I picked up from Malcom Stride's Miniature Internal Combustion Engines Book) thereby bypassing any complicated arrangement of gears, or being worried about chains coming off. (for small 4stroke outboards toothed belts are a common sight.)

    The crankcase is bog standard seagull as well (I wish there was room to put roller bearings in but there isn't)
    and the carb will be standard villers as well

    Anyway, Happy New Year Everyone.
    Wishing you all the best with your various projects

    Greg
     
  10. Jan 1, 2019 #10

    cox24711

    cox24711

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    oh and I forgot to mention that the spark plug will be a 10mm, with 1" reach
     
  11. Jan 1, 2019 #11

    BobNZ

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    Crankcase modification for big end, gudgeon plus some bore lubrication, along with the possibility that scavenging of oil, many be needed depending on whether you plan on a "dry" sump with pressure fed lubrication or a wet splash system. Rings will need to be modified to cope either way.
    All good fun
     
  12. Feb 6, 2019 #12

    cox24711

    cox24711

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    hello again fellas!
    After a lot of thinking I have decided to drop using original piston because i wasn't happy with the combustion chamber shape. Thus i have been back to the drawing board.
    the new piston is from the good old Victa lawnmower, at 62.7mm bore it was a easy swap, and by having a more or less flat top, I was able to get a lot more satisfactory combustion chamber shape.
    the cylinder will be a original 'square block' seagull one, but it will be bored and sleeved to cover up the exhaust and intake ports and to take the new piston. thus I wont need to have the hassle of making water jackets etc. the con-rod will also be the original seagull one.

    Screen Shot 2019-02-06 at 1.12.37 pm.png Screen Shot 2019-02-06 at 1.13.17 pm.png Screen Shot 2019-02-06 at 1.13.32 pm.png
     
  13. Feb 6, 2019 #13

    XD351

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    You are better off with the piston out of a briggs and stratton as it is designed for a four stroke -it has an oil ring where a two stroke piston does not. Something out of a 3 - 5 hp briggs should get near the bore size you want . I’m not sure about the newer OHV briggs motors but the old side valve type were all flat top pistons .
     
  14. Feb 7, 2019 #14

    mohavegun

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    Good morning,

    Just a thought... Go to NAPA auto parts and ask them to look in their WISECO book and see if you can find a current production WISECO replacement piston and ring set. WISECO has pulled more rabbits out of the hat for me than you can imagine! WISECO makes pistons, sleeves and rings for virtually anything you are going to come across in the field and their catalog references by bore, size, weight, profile and wrist pin diameter.... One time I was working on a very early Kubota diesel engine and strangely enough WISECO had a piston & rings that would fit it but KUBOTA had discontinued it. It was nearly identical to the Kubota piston and the engine was still in service almost 5 years ago which was after being used for more than 4 years! I did bore the Kubota block .25 mm oversize to accept the new piston and shaved about .020" off top of piston for proper deck height. They specialize in replacement pistons for 2 stroke engines and really do cater to the vintage and out of production designs. Most NAPA and other automotive parts dealers and virtually all automotive machine shops can order WISECO parts if they do not stock them and nearly every motorcycle repair shop that is NOT a dealership shop can help you out, most motorcycle shops that are dealerships won't spend much time looking something up for you (personal experience).

    Rod
     
  15. Feb 7, 2019 #15

    XD351

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    Might have trouble getting stock here in Australia though , you can get briggs pistons off ebay or any mower shop . iirc a 5 hp briggs motor has a bore of 2-3/4 inches or 69.85mm & 3hp 2-1/4 or 2-1/2 inch bore.
     
    Last edited: Feb 7, 2019
  16. Feb 8, 2019 #16

    mohavegun

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    Let me know what bore the engine is, stroke is also needed, if it is a 2 stroke engine then port locations from top of stroke. Then need to know wrist pin diameter and distance from centerline of wrist pin to top of stroke. from that I will look for a number for you here in Arizona USA and post it sometime early next week. I have a couple of friends in auto parts house in Kingman.

    Rod
     
  17. Feb 10, 2019 #17

    cox24711

    cox24711

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    Thank you fellas for the suggestions

    Rod
    to put it simply I'm trying to make a 4 stroke engine out of a 2 stroke.
    I'm just messing around with ideas for now but thank you for your input

    XD351
    Yes I have had I look at the briggs engines and sadly the smallest i can find is ~65mm and that is rather stretching it because really I want to use the original cylinder (and use a sleeve to cover up the intake and exhaust ports)
    I don't really want to mess about carving a cylinder from solid, and mess about making the water jackets.

    the victa lawnmower piston I know isn't ideal, but i will keep on looking for alternatives.
     
  18. Feb 10, 2019 #18

    cox24711

    cox24711

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    good news, the honda gx120 bore is 60mm!
    now I just need to buy a piston and ring set.
     
  19. Jun 3, 2019 #19

    cox24711

    cox24711

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    well hello again fellas, I have been busy with school as of late, with not much time for messing about. but I have been coming up with the camshaft timings.
    Screen Shot 2019-06-03 at 1.04.49 pm.png

    the bore 60mm stroke 40mm compression ratio 10:1, and I am aiming for peak power at 5500 rpm
    As always all suggestions/improvements are greatly appreciated
     
  20. Jun 3, 2019 #20

    cox24711

    cox24711

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    Also, does anyone know what shape/ profile of the cams would be the most suitable for a flat tappet high revving motor?
     

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