The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: coater87 on 29.07. 2009 00:51

Title: Crank grind
Post by: coater87 on 29.07. 2009 00:51

 I got my crankshaft back from the machinist, I had trouble finding someone in my area that would do the job. The problem comes with the size, I had it ground 10 under- and with careful measuring the crank pins are .0006 over size exactly, on both sides.

 Without trying the plastigage, does anyone foresee this being a huge problem on the bike? I am not sure what type of tolerance "fudge factor" I have here- and there is no way I can ask this guy to try again. I almost had to beg to get him to do it in the first place.

 The rods are stock, and the bearings are Glacier (NOS) if any of that matters in this.

Title: Re: Crank grind
Post by: BSA_54A10 on 29.07. 2009 10:57
You can always buy +20 slippers and have them reamed to fit the big end.
Much easier to cut down the slippers than regrind the crank
Title: Re: Crank grind
Post by: chaterlea25 on 29.07. 2009 12:54
Hi Lee,
If you have access to plastigauge I would do a trial assembly with some and checkthe clearance.
Watch out for any high spots on the rods and caps especially arount the bolt holes
there should be zero gap when the cap is pushed on.
You can always assemble a rod and shells and inside mike them!
Did your machinist put the correct radaii between journal and crank web? This is vitally important!!!!!!
or the crank will break in short order!!!
Use the old conrod nuts for the trial assembly and new ones for the final assembly
Measure the lengths of the conrod bolts to make sure none have stretched compared to the others
look at the threads under magnification and check for damage/distortion
Personally I dont think 0.0006 is going to be an issue
Good Luck
John O R
Title: Re: Crank grind
Post by: MikeN on 30.07. 2009 12:13
Lee,Sorry for asking this.
Have you checked the 1-2" Micrometer that I assume you are using, against a setting gauge?
Title: Re: Crank grind
Post by: coater87 on 31.07. 2009 01:27
 Hi guys and thanks for nthe tips!

 The guy did a beautiful job of grinding this crank, it has the correct radi on the pins and the journal- its just fat. At first I thought it was my mic, and I did indeed check it. After that I started to think my mic AND the known rod were off (got desperate to find a mistake I was making), so I borrowed another mic and got the exact same measure.

 I no longer trust my bore gauge, as the ends of the fingers are pretty beat and I have never replaced it. I do have a haynes manual for the bike, but I have little confidence in those measures compared to what the people on this board know.

 Does anyone have an opinion on what the bare minimum clearance I could get away with would be here?

 Thanks again,

Title: Re: Crank grind
Post by: Brian on 31.07. 2009 02:35
Rather than get into just how much clearance it should or shouldnt have, and you will get a dozen different answers, there is another way.

Firstly .0006" is a bees whisker over 1/2 a thou, in lay mans language, bugger all.

Clean it all and put a very thin coat of oil, crc will do, on the journal and assemble the rods on the crank. Hold the rods out at right angles to the crank (do one at a time) and let go of the rod, if it falls to the bottom it will be fine. If not its too tight. I have been using this method any time I thought the clearance may not be enough all my life and on many motors and I have never seized a bottom end.

Now this may not seem very scientific but was shown to me by a gentleman with a lifetime of experience in building motors and who's opinion I regarded very highly and as I have said has not failed yet.

Your lucky he left it slightly oversize and not undersize.
Title: Re: Crank grind
Post by: beezalex on 31.07. 2009 16:12
You need to do the plastigauge.  Diametrical clearance is .001-.0025".  This is what ultimately matters.
Title: Re: Crank grind
Post by: Brian on 01.08. 2009 02:45
Here is a scan out of Munro's book on BSA twins. You will see that he recommends that when clearance reaches .002" (two thou) the bigends need replacing.

If a conrod will fall under its own weight then clearance is sufficient.

I am not prepared to name the gentleman who told me this method as he is deceased and unfortunately the following info will not mean anything to overseas members but for the benefit of Australian members in the latter part of his life he ran a book shop called "Owl's Rest Books" in Warrandyte, Victoria. BSA_54A10 I am sure you will know who I am refering to and why I have faith in his advice.
Title: Re: Crank grind
Post by: A10Boy on 01.08. 2009 19:22
I agree, stick it together using old nuts and see its they are tight or not. Is there any point using plastigage to measure a minus tolerance?

Also, who measured after the grind, are you sure their micrometer is in calibration?
Title: Re: Crank grind
Post by: RichardL on 01.08. 2009 20:03

On this issue of watching the rods drop, the idea is that they are free to drop, as mentioned, but they should also show some reluctance to dropping. In a previous thread I posted a couple of links to videos I made when seeking advice because the "machinist" decided (without asking me) that he should hone-round my rod-bearing housings. That caused my rods to wobble, for which he ended-up providing new MAP rods to rectify his error. In the links below, the first shows the wobble, the second shows the new rods with a bit of reluctance to dropping. I wouldn't mind them being a bit tighter, but he also ground my crank to the bottom of the tolerance (for these reasons, quotation marks aroung the word "machinist").  My engine has run a little over 800 miles since reassembled. There was third video inbetween these two, but it shows a misstep by me, so I have not included it here. Finally, I have included a link to the rod wobble thread, though, it is long and probabaly not applicable to your questions.

Link to rod wobble topic:

Richard L.