Crankcases and Crankshafts

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fleetingcontact
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Crankcases and Crankshafts

Post by fleetingcontact » Sun Feb 24, 2019 5:52 pm

I compared a Silver Century crankcase to a 102 crankcase and could see no difference except the model stamp. Is there a difference?

I have several crankshafts which I know came from a Century (not a Silver Century, just to clarify). Again. I have compared them to earlier and later crankshafts and apart from the fact that some are welded whereas other are not, is there any material difference between them or to put it another way is there any reason why I can't use the Century crankshafts in either a 102 or a Silver Century (if I wanted to)?

blokewithaboat
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Re: Crankcases and Crankshafts

Post by blokewithaboat » Sat Mar 02, 2019 10:07 pm

Aside from how the tiller is mounted to the cases there's no really big difference unless you include the crank bushes.

Same bolt pattern for both engine types (round block and square) so cylinders and cases are interchangeable from either type which also means that cranks and rods etc. fit either type. Older cranks seem to be welded around the webbing, well i can confirm this as i had an older (welded) crankshaft lose part of its webbing and got wedged inside the piston. Engine carried on running for hours as though it was nothing. Only noticed that something was wrong some time later when i was turning the flywheel by hand months after the event. Good old 102 for you, tough as old boots. I've not had a later crankshaft let go, but there's a first time for everything i suppose.


Transfer ports is where the biggest difference will be. Sometimes with the pistons but less of a problem. You'll only see this "difference" when the engine has been taken apart. Older type engine crankcases seem to have a wide transfer port, and later types seem to have a narrower port. Same sort of thing with cylinders. Trick is to get the crankcase and cylinder to match as best you can.


As far as i'm aware the exception to some of this is with the short water jacket models. e.g anything pre-war and the wartime motors. Bolt patterns are different with these motors cases. Short water jacket cylinders won't swap over to the later cases and vice versa. Different bolt pattern when compared with post war types. For the most part, wartime 102's crankshafts and onward into later types from other models are basically the same aside from the crank webbing on older cranks being welded.

Was that too much info all at once?

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fleetingcontact
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Re: Crankcases and Crankshafts

Post by fleetingcontact » Sun Mar 03, 2019 2:36 am

Not too much info at all - very helpful, thanks. Why I asked is that in putting together this 102 bitsa I came across a pair of crankcases that were fairly late-issue from an SC. Measured all the bearings and all seems well.

Then I compared them to some late-model 102 cases and could see no difference at all - same size transfer port, everything. So I've decided to use them - I guess the proof will be if the bludy thing starts or not.

Also lately took a punt on a Ebay job-lot which turned out to have a goodly amount of usable items, in particular two Century crankshafts in very good condition - again checked the bearing surfaces for wear, all seems well. I didn't know they were Century items until I checked the engine number (they were still in the crankcases) hence the question.

I compared them to a 102 and a SC crankshaft and could find no difference in the angle of the big-end journal to the woodruff key slot - this seemed to me to be the critical factor - but since I was using a mk1 eyeball for this measurement I couldn't be sure.

Anyhow thanks again.

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Re: Crankcases and Crankshafts

Post by african imp » Sun Mar 03, 2019 10:11 am

Guys,

Thanks on this thread as I am hoping to build up a Silver Century crank case to either the 102 engine or the Silver Century motor format.

I now need, crank, conrod and piston, plus the block, I have the head and a copper gasket for the Century motor.

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Re: Crankcases and Crankshafts

Post by blokewithaboat » Sun Mar 03, 2019 2:33 pm

Did you also know that a 40 series crankshaft will fit into your 102/century cases as well.

Timing will be miles off though due to the positioning of the woodruff key which aligns the flywheel in relation to the ignition baseplate so this is something else to be aware of if you're buying bits from the internet with gay abandon.

Yes, true there's a few subtle differences with pistons, some a bit academic. As long as your choice of parts fit together in the best way possible there shouldn't be too much of a problem.

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Re: Crankcases and Crankshafts

Post by african imp » Sun Mar 03, 2019 3:35 pm

blokewithaboat wrote:Did you also know that a 40 series crankshaft will fit into your 102/century cases as well.

Timing will be miles off though due to the positioning of the woodruff key which aligns the flywheel in relation to the ignition baseplate so this is something else to be aware of if you're buying bits from the internet with gay abandon.

Yes, true there's a few subtle differences with pistons, some a bit academic. As long as your choice of parts fit together in the best way possible there shouldn't be too much of a problem.
Thanks,

No I did not know that and the mention of the woodruff key would never have crossed my mind!

A Silver Century crank case is in the pictures, I have two of them.

Roy
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Charles uk
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Re: Crankcases and Crankshafts

Post by Charles uk » Sun Mar 03, 2019 3:57 pm

Jon it might be as well to mention the conrod differences, please.
Make it idiot proof and someone will make a better idiot.

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Re: Crankcases and Crankshafts

Post by african imp » Sun Mar 03, 2019 4:00 pm

blokewithaboat wrote:Did you also know that a 40 series crankshaft will fit into your 102/century cases as well.

Timing will be miles off though due to the positioning of the woodruff key which aligns the flywheel in relation to the ignition baseplate so this is something else to be aware of if you're buying bits from the internet with gay abandon.

Yes, true there's a few subtle differences with pistons, some a bit academic. As long as your choice of parts fit together in the best way possible there shouldn't be too much of a problem.
To change the timing due to the woodruff key, would it be possible to swing the base plate around to the required position?

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Re: Crankcases and Crankshafts

Post by african imp » Sun Mar 03, 2019 4:17 pm

Charles uk wrote:Jon it might be as well to mention the conrod differences, please.
Is there a chart for all of the Seagull engines which show the bore and stroke with a cross reference to the parts we can mix?

What is the compression ratio of a Seagull engine?

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Re: Crankcases and Crankshafts

Post by Charles uk » Sun Mar 03, 2019 4:55 pm

That will require some accurate machining ability & the amount you have to change the timing.

You will have to remove your recoil base plate retaining casting & use the locking screw instead.
Make it idiot proof and someone will make a better idiot.

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Re: Crankcases and Crankshafts

Post by fleetingcontact » Sun Mar 03, 2019 5:23 pm

And the obvious conclusion is...don't do it. Get the correct part. There's a limit to how far this mix'n'match game makes sense. And at this point in the 21st century, crankshafts are still relatively easy to come by. Or do if you are mad enough. I told you the Harley joke right?

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Re: Crankcases and Crankshafts

Post by Charles uk » Sun Mar 03, 2019 7:34 pm

All the racers play the mix & match game, but you'll notice how little they talk about it.

There are a couple of guys in Bermuda who race Century's that you'd definitely steal (Naughty Bits is one of them) but they don't give secrets away!

If you need to know the compression ratio Roy, treat yourself to a compression gauge with an 18mm adapter, there is quite a variation between seemingly identical Seagulls.
Make it idiot proof and someone will make a better idiot.

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Re: Crankcases and Crankshafts

Post by blokewithaboat » Mon Mar 04, 2019 12:28 am

Well its a good thing we're not talking about anything racing related on this thread or i could get myself into some really hot water.
Don't worry , my only racing interests are with 2 wheels.


Charles
As this discussion seems to have a common link with building a "bitsa" from spare parts there are a few pitfalls to be wary of. Nothing to do with going fast or racing here, merely getting the basic engine components to match as best you can. Recycling/reusing if you will. I hope this will become clear enough.


Roy
In essence it's true you could re-drill the hole in the upper crankcase half for the baseplate locking screw to set your ignition timing to the correct position relative to your choice of crankshaft. That's probably a lot of unnecessary work and as Charles has mentioned will require some very accurate measuring on your part. I do know somebody who's shall we say "adjusted" the position of this screw ever so slightly by a few degrees. Nothing as drastic as what Charles was implying, but this was on an older 40 series engine that was a little shy at starting. You'd hardly notice any difference from where it was originally and would need to know what you were looking at to get an idea.
You can just make out the small hole drilled into the top case half where the locating screw actually comes to rest in these pics as kindly offered by my pal Jeremy. Bearing in mind these are from older "little model 40" cases with a fixed tiller position, the basic timing screw position is pretty much the same for the later 40 series engines with points ignition.
http://www.saving-old-seagulls.co.uk/i_ ... ll/n10.png
If you look closely the hole is positioned just slightly off from centre of the tiller (a bit to the right of centre) whereas a typical 102/century will be positioned over the top of the tiller mounting cast into the lower case half. In other words approx.6 o'clock for the 40 series and approx. 2:30-3 o'clock for the 102/century. Have a look at your century crankcases to see this difference. Both 40 and 102/century crankshafts will show different woodruff key positioning that is relative to this ignition timing point. Does this make sense.

Older 40 series engines from the early 50's are a little different again so i'm not going to even begin to explain or show what the differences are with these.

Keeping things simple then, 102's/century's as well as the typical later 40 series crankshafts both have the same length/throw inside the bore.(forget cylinder bore dimensions here) Same centers for little and big end for both. Crankshafts from both motor types will fit either type of crankcase (other than woodruff key positioning being different)
Biggest and most obvious difference to look for is in the wrist pin. 40's have a smaller diameter wrist pin and 102's/century's have a bigger diameter wrist pin.
Numbers and indeed basic design in con-rods change over the years for both engine types and personally i wouldn't get caught up in that as it makes little to no difference at all in terms of what fits what. Wrist pin dimensions is what you need to be careful with when it comes to con-rods. Remember, we're not going racing here, we're just assembling spare parts from older and newer engines that will still work with one another.

As for compression, get yourself one of those cheaper gauges and start measuring a few engines. As Charles has mentioned they do vary quite a bit, but typically a good running 40 series engine should be happy with 80+ psi even as low as 50 ish psi (as far as i can gather and have tested myself) and a good 102/century is happy with 50psi. If you were lucky enough to buy a genuinely low hours century motor you might see a bit more than 50 psi. but i doubt it would be much higher. Without going down an ugly rabbit hole into another dimension, unless you're a dedicated "racer" there's no need to try and get the compression any higher than this. Too much work. Keep it simple.


Martin and Roy
I believe there is a limit as to how far some of this mix and match will work. Some of it will depend on how good your engineering skills are and how deep your pockets are. Seeing as this topic is loosely related around building "bitsa's" from spare parts or just getting incomplete sets of parts into some resemblance of a seagull power head that are never likely to go that fast any way, i guess there won't be a whole heap of engineering work to do. Essentially if you're building a 102cc "bitsa" then use parts from the same 102cc engine be it from a 102 or century and you shouldn't go too far wrong. Same things apply if you're building a 40 series (64cc) mongrel. Just exercise some common sense.


I hope that wasn't too long. Been told off in the past for lengthy replies :roll:

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Re: Crankcases and Crankshafts

Post by african imp » Mon Mar 04, 2019 11:15 am

To blokewithaboat, many thanks, this is all part of my British Seagull learning process.

I am well used to the various tuning options for the Hillman Imp, from standard Mk1 875cc to full race 998cc motors,

The Seagull is not new to me but the road I have been taking in recent months is :P

I am a blokewithoutaboat by the way.

Roy

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Re: Crankcases and Crankshafts

Post by Horsley-Anarak » Mon Mar 04, 2019 6:19 pm

blokewithaboat wrote:I hope that wasn't too long. Been told off in the past for lengthy replies :roll:
Yes Jon it was a bit long, I would watch out that you don't get chucked off again. Generally most people appreciate concise accurate information, and rolling your eyes at the moderators won't help you to stay on the forum.

Just a little friendly advice.

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