Flywheel TDC

[Squiffy57]

Can anyone help? I'm just replacing the flywheel on my 05 GS and the Haynes manual tells me to lock the flywheel using a pin/hole with the pistons at TDC.
The only problem is, is that when the notch in the flywheel is opposite the hole the pistons are at BDC! Am I missing something/doing something wrong?

 

[Delboy777]

Can anyone help? I'm just replacing the flywheel on my 05 GS and the Haynes manual tells me to lock the flywheel using a pin/hole with the pistons at TDC.
The only problem is, is that when the notch in the flywheel is opposite the hole the pistons are at BDC! Am I missing something/doing something wrong?

In that case you are a full turn out. Rotate the flywheel one revolution and that should be it........I think !

 

[Kenny Rodkiss]

Someone's been in there before you? Flywheel is held on with five bolts so can't be 180° out? Are sure there is no locking hole when the pistons really are at tdc?

 

[Steptoe]

There is no reason to have the engine at TDC when changing the flywheel.
You're simply changing the flywheel, so just make sure you replace it in the same position - i draw a horizontal line across the face of the one i'm removing and fit the new one as per the old one.

 

[tadhgocuilleain]

In that case you are a full turn out. Rotate the flywheel one revolution and that should be it........I think !

What he said. ^

Remember that in a four stroke engine, each piston does two strokes for one revolution of the engine, which means the crank shaft does two revolutions. Also, you can only be at TDC for one cylinder at a time. Are you sure you're looking at the correct cylinder?

 

[sproggy]

Remember that in a four stroke engine, each piston does two strokes for one revolution of the engine, which means the crank shaft does two revolutions.

Huh? How can the crankshaft do two revolutions for each revolution of 'the engine'?

Also, you can only be at TDC for one cylinder at a time.

That's not true either - one cylinder will be at TDC on the exhaust stroke while the other is at TDC on the combustion stroke.

 

[Squiffy57]

There is no reason to have the engine at TDC when changing the flywheel.
You're simply changing the flywheel, so just make sure you replace it in the same position - i draw a horizontal line across the face of the one i'm removing and fit the new one as per the old one.

As there is a lug on the flywheel I guess that it's impossible to fit it in anything other than the correct position?
Perhaps Haynes got it wrong re: TDC?

 

[Deleted account 190920001]

What he said. ^

Remember that in a four stroke engine, each piston does two strokes for one revolution of the engine, which means the crank shaft does two revolutions. Also, you can only be at TDC for one cylinder at a time. Are you sure you're looking at the correct cylinder?

Seriously?

 

[tadhgocuilleain]

That's not true either - one cylinder will be at TDC on the exhaust stroke while the other is at TDC on the combustion stroke.

That normally isn't the case, as it would cause too much vibration, but it may be possible in a horizontally opposed engine - I'm open to correction on that

Huh? How can the crankshaft do two revolutions for each revolution of 'the engine'?

In a four stroke engine:
Intake stroke - piston moves down and the crank shaft rotates 180°
Compression stroke - piston moves up and the crank shaft rotates another 180°
Ignition stroke - piston moves down, another 180°
Exhaust stroke - piston moves up, another 180°.

180° x 4 = 720° = two rotations, but the engine has only done one cycle.

In a two stroke, crankshaft revs and engine revs are the same.

EDIT: My head is fried. I'm starting to confuse myself. I could be talking nonsense.

 

[Bendy toy]

The boxer twin is like a BSA, Meriden Triumph or old Norton twin. BOTH pistons reach TDC at the same time. The cams run at 1/2 crank speed so the valves on one side are 180 cam degrees (360 crank degrees) out of phase with the other side.

The British parallel twins had no opposing force against the pistons shaking up and down. The boxer balances most of the mass on one piston against the other. Not eniirely because they have separate crank pins but the boxer is much less of a pile-driver than a Meriden Bonnie.

90 degree Vee twin vibrates a lot less because both pistons use the same crank pin and the piston mass can be opposed by the crankshaft balance weights.

A four cylinder radial four with all four big ends on one crank pin is likely to be the smoothest of all. A 2 stroke would fire every 90 degrees of crankshaft turn. A four stroke version gets complicated because the TDCs are always 90 degrees apart so might not be as smooth running.

 

[AberdeenAngus]

but it may be possible in a horizontally opposed engine -

You do know which forum you're on, right ?