Douglasmotorcycles.net

General => Douglas Motorcycles - General Discussion => Topic started by: Ian on 01 Jul 2014 at 00:58

Title: 3.5hp pistons
Post by: Ian on 01 Jul 2014 at 00:58
A friend here has a really nice 1914 3.5hp that has been destroying pistons. He put modern short skirt pistons in so we assume they are not picking up oil correctly. He is trying to get some made in time for the Bathurst rally. Couple of questions to the experts :

1. What types of pistons have been successfully used in these ?
2. I think there is an oil feed hole in the bore similar to drip feed bikes - should the piston skirt overlap that when at bottom of the stroke ?

Title: Re: 3.5hp pistons
Post by: graeme on 01 Jul 2014 at 11:23
Hi Ian

Both of the 3 1/2s I have been associated with have Hillman Imp pistons fitted - with no issues. I know that I recommended these to Kevin, so I'm not sure what is going on. Maybe there are/were Hillman Imp pistons made with longer skirts? Hillman Imp engines were used a lot in sidecar racing outfits, so perhaps special lightweight pistons were made also, maybe this is what Kevin has ended up with? I have also always used 20/50 oil in the 3 1/2s, ie pretty thin oil.

Cheers,
Graeme
Title: Re: 3.5hp pistons
Post by: oily bloke on 01 Jul 2014 at 12:13
Funnily enough I sprint race a Hillman Imp engined sidecar outfit (Impulse). There are many engine sizes and, therefore, many piston sizes. What is the bore size required?. The engines were available in 875,998,1120,1300 and some 1400 experimental engines.
Andy
Title: Re: 3.5hp pistons
Post by: cardan on 01 Jul 2014 at 15:04

1. Yep, I reckon the original cast iron pistons were OK. I've been putting iron pistons in veteran motors when I can, with excellent results. There seem to be plenty of NOS iron pistons around to suit 500 singles (say 85mm bore), but smaller ones for the 3 1/2 may be difficult.

2. Yep. I feed the oil into the back wall of the front cylinder on JAP twins, and for them it is well and truly on the skirt. I'd be certain the 3 1/2 would be the same.

Good luck!

Leon
Title: Re: 3.5hp pistons
Post by: tom hoek on 01 Jul 2014 at 18:53

Seen at bert pol

http://www.bertpol-vintagemotorcycles.com/parts%20Douglas.htm

Unfinished castings for cast-iron pistons for 2 3/4 HP .   45,- euro's each .
 

Groeten Tom Hoek
Title: Re: 3.5hp pistons
Post by: Ian on 02 Jul 2014 at 03:13
Funnily enough I sprint race a Hillman Imp engined sidecar outfit (Impulse). There are many engine sizes and, therefore, many piston sizes. What is the bore size required?. The engines were available in 875,998,1120,1300 and some 1400 experimental engines.
Andy
Andy, the bore size is 68mm I believe - which ones would be close to that ?
Title: Re: 3.5hp pistons
Post by: oily bloke on 04 Jul 2014 at 09:00
Hi,
The standard 875 engines are 68mm nominal bore size. There are high comp (10:1) and low (8:1). Gudgeon pin size is 15.875mm or 0.6250". I don't have the pin to crown height but this should not be too hard to find on the web. Hope that helps.
Cheers
Andy