Douglas - For Sale Items

Douglas 1913 Model P motorcycle

Douglas - Wanted Items

Douglas 1915 3 Spd-Gearbox and Clutch

Quick Comments

Reminders, links to interesting topics, bump your own topic, quick comments or any short message of interest to members. Try it out!
Please note - 500 character current limit


2024-06-11, 20:02:05
Have you tried the new Drafts feature yet? I just lost a long message today and learned my lesson. It is a good idea to save a draft of any long post you are writing. You can then just keep writing and keep saving a draft, knowing you have a backup if there is a glitch. The draft is automatically deleted when you post the message.


2024-06-08, 18:30:04
For Sale
xman has two very nice 1950's machines available - a green 1950 mk4 and black 1951 mk5 - both in good condition and running well.


2024-06-07, 02:13:36


2024-06-03, 08:23:05
For Sale
Duncan has just listed his green and cream 1957 Dragonfly for sale with spares and documents.


2024-06-02, 08:34:05
Parts avalable
alistair still has parts available - barrels, carburettor, castings - see all listings.


2024-06-01, 18:33:27


2024-05-28, 00:09:46
Welcome to the new site!
Recommended viewing for a fast start...
 - Quick Tour of the Front Page
 - Quick Tour of the new Attachments
Learn all about attaching photos in the User Guide. Any problems with anything please Contact us     Faulty links fixed - 01June2024

Douglas S6 ring gap

Started by alan reid, 15 Sep 2023 at 11:00

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

alan reid

Douglas S6 Piston Ring Gap.
I am putting together a 1930 S6 engine and looking through the Douglas Care & maintenance book
it states the ring gap is .004 in!
My question is, is this enough.
Alan Reid


Hi Alan,
Radco says "at least .003" per inch of bore" for a "standard engine"; I wouldn't go below this unless the packaging for the rings expressly says so.

richard s1

Make sure your pistons are the original cast iron ones.  Hepolite and Specialoid made aluminium pistons for retro fitting and they don't fit very well in S/T6 barrels when hot. 

alan reid

Many thanks for your replies, but the pistons are aluminium, not cast iron, so i will run with a 6 thou gap.

Regards Alan


Hi Alan,

The top ring is exposed to the flame/hot gases immediately on start up and will expand initially much faster than the cylinder bore, thus the need for a gap to prevent the ring from jamming in the bore. Regardless of whether the ring is fitted to an alloy or cast iron piston.

Engineers (e.g. M.E. Booth, or my guide Radco in his marvellous book) tell us to leave a gap of at least 0.003-0.004" per inch of bore, which comes to 0.008-0.011" for a 68mm bore S6. Unless the rings are made from a fancy low-ex SG cast iron, in which case they probably come with a recommended gap on the packaging.

One of the interesting snippets from Booth's article is the study that showed that there is almost no downside to a too-big ring gap, within reason.

If I were building the engine, and my rings were unlabelled, I'd gap them to 10-12 thou.

Richard makes an excellent point about alloy pistons in vintage side valves. I live in the home town of JP pistons. I've used them with success in a number of engines, but their maximum recommended clearance is not enough for an air-cooled vintage side valve. They worry about piston slap on a cold start; I promise not to complain and get a little more clearance.




Quote from: cardan on 28 Sep 2023 at 05:32
I live in the home town of JP pistons. I've used them with success in a number of engines, but their maximum recommended clearance is not enough for an air-cooled vintage side valve.

Yes - JP get a bad rap in places. When you quiz why, folks have invariably installed them too tight.
As recommended !  Watercooled clearances in aircooled engines is not good, not good at all.
Folks who have given them sufficient clearance report good results.

I'd agree about your suggestion of 10 to 12 thou ring end gap in this duggie engine.
Insufficient ring gap can result in broken rings.
And thats probably only the start of the mechanical mayhem ...