Screw and bolt Sizes

[Ghosty 3]

Hi All,

I have a silly question about the screw and bolt sizes in the O & R 3/4 hp engines, it may be common knowledge to you but I have only seen one other of these and that was a long time

ago.

The case screws, carby mount screws, and the output shaft screw. All appear to be 32 pitch thread, but I no longer have any reference material to imperial sized screws and bolts as I

have only used metric for the last 35 years.

Any help would be helpfull.

 

Cheers

Andrew

[factory 487]

That's not a silly question at all, there were a few different types of imperial thread commonly used in the UK in the past, but the O&R engines use American UNC & UNF threads which weren't used much over here (apart from some computer parts & connectors, some of which still use them). UNC/UNF is also known as ANC/ANF but sellers often only use the number or fraction and the number of threads per inch e.g. #10 x 32tpi.

 

Below is a chart I use, created from several pages in an old reference book. All the threads apart from the spark-plug are either UNC or UNF, the spark-plug thread is metric.

 

 

The starter/blower housing & carb mounting screws are both #6 x 32tpi UNC, these screws have a Fillister head.

And the power output shaft (tapered version) is tapped #10 x 32tpi UNF, there is also a D shaped output shaft with no hole found on some engines.

 

David

 

Edit

 

Here is a PDF of the thread chart, as the picture above has been automatically reduced in size by the forum software making it harder to read.

UNC UNF Threads & Drill Sizes.pdf

 

David

Edited June 5, 2017 by factory

[Ghosty 3]

Thank you David, Would hate to damage the OEM parts, as it looks like it has done very little running. Will have to hunt down some taps so I will be able to make up a carby adaptor.

I have saved the charts for further support.

 

Cheers

Andrew

[factory 487]

I've built up a good selection of UNC/UNF taps & dies over the years (for repairing my both collection of O&R engines and vintage HP test equipment), most of them were bought second-hand at reasonable prices from the stalls at steam & vintage shows. It can take quite a bit of searching though if they are mixed up in boxes, as they usually are.

 

I have also found a few suitable screws (for the earlier O&R engines that use slotted screws) as well, usually on the ex military surplus stall. Webhead has helped with supplying the Fillister head Philips screws that are found on most O&R engines.

The starter/blower housing screws are the most often to be missing, or damaged by people using the wrong type or size of screwdriver..

 

David