Firefly .46 crankcase

The series kicks off in 4432 with a description of machining the crankcase, the text states several times that the large hole in the rear of the block is 30mm dia.

The rear view of the crancase on drawing WF46-0003 clearly shows this hole as 28.2mm dia. The plan view shows the cylinder mounting bolt holes spaced wider than this bore and dimensioned at 30mm cts which seems to tie up with the hole being 28.2mm

Can someone confirm if the text is wrong or the drawing, not a good start.

J

Edited By JasonB on 11/06/2012 18:52:46

Having now looked at a drawing of the backplate that fits into this hole, the text is obviously wrong.

Can David, Diane, Alex or Frank comment on this, was Frank building to an earlier revision of the drawings or did he mistake the 30mm bolt cts as the bore?

J

Hi J,

If you have'nt already, you could take a look at Alexe's web site, there's a section on the firefly there.

**LINK**

regards om.

Thanks OM, that is we're I looked at the backplate drawing to confirm its size which confirms the drawing is correct and text wrong as the plate is machines 28.15 for an airtight fit which it won't be in a 30mm hole.

J

Hi There
The drawings are all proven.
There should be no errors in them.
Several engines have been built directly from them.
regards David

Well I had already reached that conclusion but still does not explain why the author is talking about boring a 30mm hole, and the length of another hole to meet this one. You gave the same answer on the Northumbrian frames and we know where that ended up!! I'll repeat my question

"Can David, Diane, Alex or Frank comment on this, was Frank building to an earlier revision of the drawings or did he mistake the 30mm bolt cts as the bore?"

I also see that on Mr whittakers site there is a big note saying "updated plans comming soon" As MHS have stupidly chosen to remove the revision letter from the drawings what actual revision are you publishing is it an old one, the current one or these future updatted ones?

J

Edited By JasonB on 12/06/2012 07:36:02

Hi There

We are publishing drawings as supplied with no redrawing.
We have not removed revision numbers.

regards David

Well whoever added the MHS logo put it in place of the revision details ans I doubt Alex W did that. Its also likely that the same poerson removed the revision letter from the bottom left corner.

Take a look at this drawing it clearly shows revision number, revisions list and the hole as 28.2mm

Hi There

I think I know what has happened.

The drawings were originally supplied to RCM&E. Perhaps they modified them?

I have not had them altered at all.

regards david

David,

Not to split hairs but RCM&E is still MHS, I would have thought parts of the same organistion could speak to each other, or is this just another of your burdens we must live with?

Hi Kwil

Why would they tell me they deleted the issue number?

The drawings were supplied by Alex Whittaker.

I assumed he had some sort of agreement with

MyHobbyStore to display the logo on his drawings.

regards David

Well spotted Jason. This also affects the wall thickness dimension given in the text as 3.75mm. Should be 4.65mm. Drawings seem ok though. Will give this engine a try.

Yes the wall thickness in teh text suits the larger size hole so there is a risk of someone machining the bore to drawing and then the wall thickness to the text and ending up with a small block.

And while I'm on there are not enough dimensions for the two side chamfers, we only have the 6.75mm short side, the drawing either needs the angle as per the other chamfers or the long dimension as they stop just short of the chamfer on the top carb face. I know the chamfers are not critical but the info is not there and I'm surprized that of the six or so builders none noticed this?

I quite fancy giving it a go myself but would like to know why the author says 30mm before cutting metal and I'll likely wait for a few more installments to be published.

David in his intro Alex says that that the proving group builders will "make their expertise available via e-mail" now I can understand them not wanting all and sundry bombarding them with e-mails but I assume you have Frank's e-mail would it be possible to ask him to explain where the 30mm comes from ?

Edited By JasonB on 12/06/2012 13:17:26

Hi Jason,

I haven't had my copy yet (via WHS) but you are quite right about the 28.2mm rear hole. I have just grabbed a moment to look at the full set of drawings on Alex' site and they all show it as that dimension (Rev C). As to email, I suspect that Alex wouldn't like his address being published but you can access him via an email link from his site (link above). It would probably be a good idea to publish that information in the articles.

The only reason that I can think of for such an error is that the hole centres for the cylinder mounting holes are given as 30mm on the plan view and the article is being written well after the original build leading to confusion? As for the angles of the nose, the top and bottom are given as angles while the sides are shown as beong derived from the dimensions (23.5 x 6.75mm)

I have some offcuts of suitable alloy so may have a go at this engine if these niggles can be sorted.

Regards

Terry

Just for a bit of fun have a play with this, hold your mouse button down on the drawing and then move it about to see the engine from any angle. Also in views, click transparent.

David, can a link to Alex's site be added to the next available ME as I'm sure it will help anyone building the engine.

J

Good game!

Its is indeed Ken.

Terry where are you getting your 23.5 (or23.45) from? The top slope that the carb opening is in is 23.45 back from the front of the casting but the side slopes are not quite so long, I would guess at 22mm which would give 17deg near as dam it. Thats why I said we need the length or angle.

Jason

 PS I've just bandsawn a lump of some 2"x4" flat stock that I had

Edited By JasonB on 12/06/2012 17:22:57

Jason,

Not directly related to the engine, but a query about the link showing the 3D model as a .pdf

We received a component drawing from a prospective customer today in this form & would like to get dimensions from it - do you know if this is possible ? Right clicking on the page gets a menu that includes "3D preferences" - in here are settings for 2D and 3D measurement, but there doesn't appear to be a toolbar for measuring ?

Being able to check dimensions on the Cad model would maybe help with the point you initially raised ?

Regards,

Nigel B

I've been using 3D PDFs to send parts to clients for some years; so far I've never found a way to make measurements. The PDF files are smaller than the native part files by about an order of magnitude. I assume the PDFs define the part in a way that does not allow measurement?

Regards,

Andrew

Hi Jason,

I see your point on the 23.45mm (sorry about the typo) dimension on the drawing on Alex' site but the 3d pdf you linked to appears to show the side and top slopes to end at the same dimension if you view it from above.

I have little experience of building i.c. engines and am not sure of the significance or importance of such a discrepancy,unlike that of the conflicting dimensions of the rear hole. I would be grateful for any light you can throw on this. I have a lot of respect for your experience and have been grateful for your observations on the portable engine.

Best regards

Terry