VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Engine numbers

Hi
I know that this is a topic that is well discussed, but I have a problem.
On my 1930 SR, I have found a crack in the block. It may be possible to stitch it, but as I had the chance of another engine I have got it.
The engine number of the existing engine (according to the documents) is R1759. Note it is a RHD car.
I checked the numbers on the potential replacement engine and found
An engraved or stamped no R1508 24
Then cast in numbers 685501-A 81 9. Conv - 8 and finally the bow tie logo and the number 4.
Presumably the R means the engine is out of an RHD car (what's the difference?)
But the big difference in the actual number makes me wonder if
A, my existing number is wrong
B the new engine is for a later year?
Unfortunately I can't get to the car to verify the actual number, so can someone help out here?

The alternate engine is a '29 with block cast January 31, 1929. Yes look closely and see if the 8 is not really a 3. The engine will work but the crankcase breathing system will not fit the parts that you have on your '30 engine. If you plug the hole in the left rear of the block then your stuff will work using the oil filler tube.

Hi,
Thanks for that. I think that I had realised that I had got the numbers slightly muddled, but they were not easy to read the way the engine was positioned. I am gratefull for the comment about the mod that needs doing.
Is that the only difference from 29-30, because I think the 30 had a better h.p. rating. Was that due to compression ratios?

The block casting number should be 835501, not 685501.

The big difference was the '29 head used same size intake and exhaust valves. 1930 had larger intake valves hence, better breathing.