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500cc barrels

Submitted by david_cooper on Monday, 31 March, 2025 - 22:24
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Heavy Twins

The DSN lists lots of piston changes.  But Haynes notes that the barrel height changed in 1960.  Higher and lower compression pistons continued but the high ratio went up and the low compression pistons now gave CR nearly as high as the high compression pistons did with the earlier barrels.  But does anyone know how much the barrel height changed?
The head design changed many times, so no doubt the CR was also changed because of this...but that's another matter.

Submitted by gordon_johnston on Tuesday, 1 April, 2025 - 05:03

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What changed was the head for the 1960 model year. As far as I know the barrel heights never changed.

Submitted by philip_hannam1 on Tuesday, 1 April, 2025 - 05:04

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The 88 barrels changed in 1961 to cater for the SS version of this engine that used the 650 style  cylinder head with horizontal inlet ports. Some of the first true 88SS engines also had cylinder heads with narrower inlet ports. However, one standard size to fit both 88 and 650 soon became the norm. 
The barrel height, from top flange to underside of the base flange, was reduced from 2.678" to 2.600" to help raise the compression ratio.
The pushrod tunnel exteriors also moved forward by around 0.125" so that they could be widened internally to accommodate the SS pushrods. 
The barrel part number changing from 21712 (standard engine and earlier twin carbs on single manifold heads) to 21712B (SS engine). 

Submitted by robert_tuck on Wednesday, 2 April, 2025 - 05:05

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So the height was reduced by 78 thou . this must have needed a change in the pushrod length too ?. Something new to learn  !.

Submitted by david_cooper on Thursday, 3 April, 2025 - 05:06

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Thanks to Phil for providing the barrel height change for 500cc Dominator.

For anyone interested:
the early Hepolite piston number is 10533
the later (and currently sold) number is 10338.
The late 10338 is 2.4mm taller than early 10533 (my measurement agrees with Hepolite catalogue).

10533 is just level with the top of the 500cc barrel spigots.  I put a piston (with grease round the edges to seal it) level with the spigots and fitted my late 88SS downdraft cylinder head with copper gasket.  Gasket thickness 0.038".

Propped it on a slant, removed plug and syringed liquid until it was part way up plug hole.  This took 37cc
Compression ratio = (250 + 37)/37 = 7.7.

So: early pistons in late barrels and head give compression ratio 7.7

If I subract the volume of 2.4mm tall and 66mm diameter to allow for later 10338 pistons, the volume goes down to 29cc.
So; late pistons in late barrel and head give compression ratio 9.6.    (Haynes quotes 9.45, which is close).

When I then add on  .078" (= 2mm), the low compression 10533 gives compression ratio 6.65.  The Dominator Service Notes quote 6.7 for the early machines.

SUMMARY:
Early LC pistons in early barrels; CR = 6.65
Modern later HC pistons in early barrels; CR = 7.95
Early LC pistons in late barrels; CR = 7.70
Late HC pistons in late barrels:  CR = 9.6

All based on the later cylinder head.

I see Andover Norton lists 10338 (late HC pistons) today.  They quote CR 7.8.  This is close to my value for EARLY barrels.  With late barrels it will give about 9.6.

Hope this is of interest to someone.

Submitted by philip_hannam1 on Thursday, 3 April, 2025 - 05:06

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Robert has raised a good point..........were the pushrod lengths ever changed to suit the shorter 1961  barrel?
Not as far as I know.  But as the change was less than 0.10" did the factory bother???  They did with pushrods after 1967 when the valve lengths were lengthened by 0.10".

Don't forget that the new Norton Model Directory is full of useful facts and figures.

Submitted by robert_tuck on Thursday, 3 April, 2025 - 05:07

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Perhaps we should check the valve / rocker action when working on our Norton's. Poor action was stated as the reason for the later specified longer valves and shorter rods . And we were told the change improved power and reduced wear . Not the easiest procedure to do ,or to understand.

Submitted by david_cooper on Saturday, 5 April, 2025 - 08:15

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The critical angle to define how the rocker tip moves against the valve is between the line of the valve stem and the line from the top of the valve to the centre of the rocker shaft.  The pushrod length doesn't change that angle. Only changes in valve stem length do.  

But changes to valve length could cause the adjusters to run out of thread so pushrods could need changes for that reason.  But if the rockers don't hit the cover and don't run out of adjustment, altering the pushrod length just changes the local angle between the contact surfaces from rocker to stem.

 



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