How to calculate port timings?

[Donnie]

This is odd. It seems as though you have a short barrel! I know I said it was the less likely scenario earlier!

Double checking, at bdc, is the edge of the piston crown perfectly level with the bottom of the exhaust and transfer ports?

Sorry for the delay just finished another 3 long shifts but am off for 8 days now

Right, just gone out and had a look and no, it's not level, I would estimate the crown sits lower than the bottom edge of the exhaust port by 0.75mm and below both transfers about 1.5mm.

I've been told by the previous owner that the exhaust port has been tickled with which could explain the difference.

[Donnie]

What type of casing is it?

There is some variability in the height of the base gasket face between Italian / Spanish /Indian cases, though usually it's too high in the latter examples.

It is a spanish series 2 casing and has had some ally welding to it for adding a boost port etc and refaced though it was done to original height etc if that makes sense.

[Eden]

Is that with the barrel bolted down or just sitting down?

[Donnie]

bolted down with no head on

[dickie]

I'm guessing here, but the probable reason for this is that the previous owner of the barrel had a 60 stroke crank so he lowered the exhaust port but left the transfers.

If it were me, I'd put a thinner base gasket in so that your piston crown is no more than 0.5mm below the lower edge of the exhaust port. From memory of what you said you had in place I think this means you'll have no base gasket at all, in that case just use hylomar blue.

The I'd use whatever thickness head gasket you need to achieve at least 1mm squish; 1 to 1.5mm is the target.

After you do this, you may find that your port timings are a little on the tame side, but only a little so I wouldn't sweat about it.

Alternatively you could buy a 60mm stroke crank instead of a thinner bas gasket, but then you'd need an even thicker head gasket and deeper pockets.

Personally, I wouldn't mess with the transfer ports as these need more skill than I have to get right.

I know this is in complete contradiction to what I originally told you, but with the info we now have, this is the way to go.

In a nutshell, no base gasket (hylomar blue instead) and whatever thickness head gasket you need to get 1.0 to 1.5mm.

[Donnie]

So youre saying to lower the cylinder farther, ie no base gasket so that the piston is more in line with the bottom of the exhaust port I assume?

I don't have any hylomar blue but would assume that any jointing compound would do? ie I have some wellseal, some threebond 1104 and some loxseal 18.10, though i think the loxseals a pipe sealant.

[dickie]

So youre saying to lower the cylinder farther, ie no base gasket so that the piston is more in line with the bottom of the exhaust port I assume?

I don't have any hylomar blue but would assume that any jointing compound would do? ie I have some wellseal, some threebond 1104 and some loxseal 18.10, though i think the loxseals a pipe sealant.

Aye, the piston crown should be level with the bottom of the exhaust port.

I think wellseal is good but I've never used it personally. Just make sure it's got a good temperature range and petrol resistant.

[Donnie]

interestingly just had a look at my other engine on the bench, standard 175 and the piston crowns about 2mm higher than the exhaust port.

[Dimitrios_231]

That's pretty normal,
the exhaust port floor (bottom) is most of the times (if not always) lower than the bottom of the transfers.
Ideally,if the piston crown at BDC is level with the bottom of the transfers,
it will always be exposed at the exhaust port.

[Warkton Tornado No.1]

Sorry but the mention of Hylomar fills me with bad memories.

I am sure that Three Bond et cetera are all good with the price you pay, as are the old school Green & Golden Hermetites. However Black bathroom silicone (other colours are available from Screwfix, Toolstation) is very good. Think of Silicone as a coloured liquid glass & you can equate with it's temperature capabilities, even though labelling may say 200°C maximum.

I think people buy Hylomar because there's mention of Rolls-Royce but I've never found it to be stable in a two stroke engine having had it migrate & it rolls off like the snot from somebody that's been snorting powdered blu-tack.

[dickie]

I think people buy Hylomar because there's mention of Rolls-Royce.

Or because they nicked a tube from work a few years ago and they've still got some left