Crankcase Clearance

**jack620** · 2nd April 2013, 08:19 PM

Here's a progress shot of my 1cc diesel engine. As you can see, the conrod contacts the side of the crankcase. The engine has been made exactly as per the plans and there is no mention in the plans of machining clearance for the conrod. Anyway, I'm going to have to make room for the conrod somehow. I could dive in with the Dremel, but that has potential for disaster and the results are guaranteed to be ugly.

I'm thinking maybe I could mill it out with my lathe milling attachment. I don't know what type of cutter to use though. Maybe a Woodruff key cutter?

Any suggestions?

Chris

**Michael G** · 2nd April 2013, 08:33 PM

A Woodruff cutter would do it. Have you got one small enough? (What size is the bore?)

Michael

**.RC.** · 2nd April 2013, 08:37 PM

Can you narrow up the conrod like in full size engines?

**jack620** · 2nd April 2013, 08:37 PM

Thanks Michael. I don't have any Woodruff cutters (yet).

The bore for the steel cylinder liner is 12mm and the hole in the rear of the crankcase is 16mm. I envisage going in through the rear of the case.

**jack620** · 2nd April 2013, 08:39 PM

RC,
I can probably get away with narrowing the conrod a bit, but I'm pretty sure I will still have to machine some clearance in the crankcase.

**Steamwhisperer** · 2nd April 2013, 08:53 PM

Hi Chris,
Could you use a small boring bar held in a collet chuck.

Phil

**malb** · 2nd April 2013, 08:56 PM

It's fairly common to have a relief carved into crankcase to provide clearance with commercial engines. In some cases they will notch the bottom of the cylinder each side as well. Some of the larger high performance motors have a shallow half round relief right the way arround the crankcase to clear the big end.

In general, keep the crankcase volume as small as possible to maximise secondary compression, as this is what pumps the air/fuel charge into the cylinder, and it effects the charging efficiency.

**Michael G** · 2nd April 2013, 09:01 PM

I have one that is 1/2" diameter, 1/8" thick. PM me with an address if you would like to borrow it.

Michael

**jack620** · 2nd April 2013, 09:41 PM

Phil,
That might just work. I have a small boring bar I ground from 6mm HSS round blank stock. I also have a 6mm ER40 collet. I'll have a go with that tomorrow.

Michael,
thanks for the offer. I'll keep that in mind if the boring bar doesn't work out.

Chris

**jack620** · 3rd April 2013, 11:33 PM

None of my boring bars were suitable for the job. I found a Dremel bit in my set that is very much like a tiny woodruff cutter. It was very aggressive but it did the job, albeit a bit roughly. Thanks for the help guys.
Chris

**Steamwhisperer** · 4th April 2013, 06:22 AM

Well done Chris.
Nothing like a good dose of lateral thinking to get the job done

Phil

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