The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: MG on 04.09. 2010 15:51

Title: My engine rebuild / crank balancing considerations
Post by: MG on 04.09. 2010 15:51
Hello all!

After some delay (delivery of the SRM rods and machining the t/side bush) I finally got around to start rebuilding the A10 engine after the the small end bush failure I had (reported here: http://www.a7a10.net/forum/index.php/topic,2893.0.html (http://www.a7a10.net/forum/index.php/topic,2893.0.html)).

I started yesterday by dynamically balancing the new LJ crank (kindly provided by one of the forum members  ;)), which looks like a Swiss cheese now, owing to the slightly heavier con-rods, pistons and different balance factor. Next time I'm going to machine material off the sides of the flywheel, that would definitely look better. It had been reground when I bought it already and I had it nitrided, hence the greyish colour.

(http://imagizer.imageshack.us/v2/100x75q90/661/Q6rDOE.jpg) (https://imageshack.com/i/idQ6rDOEj)

I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).
For an engine with 42mm crank throw radius and a rod lenght of 165mm, using 54% results in the lowest average value for the resulting force over the crank angle (Fmittel/Fmax/% in the spread sheet, sorry it is in German). The polar diagram bottom right shows the force progression in vertical and horizontal direction, the red graph representing 54% and the green one 58%.
The average resulting force is slightly better with 54%, but the peak values are smaller at 58% (graph not visible on the screenshot, sorry), furthermore the amplitude is smaller in vertical direction, while the bigger amplitude in horizontal direction isn't felt that badly than vertical vibration.
I found 58% to be the best compromise between peak and mean amplitude, experience will show how well it is going to perform in the BSA frame. I will let you know once I've covered a few miles.

(http://img691.imageshack.us/img691/1808/5458percent.th.jpg) (http://img691.imageshack.us/i/5458percent.jpg/)


FYI: 70% as recommended for racing compared to the original 54%. The average and peak forces are worse, resulting in higher bearing load, but the deflection in vertical direction is rather small.

(http://img339.imageshack.us/img339/7/5470percent.th.jpg) (http://img339.imageshack.us/i/5470percent.jpg/)


The phosphor bronze t/side bush (supplied by SRM), nicely line-bored with a play of 1.5thou.

(http://imagizer.imageshack.us/v2/100x75q90/537/b0vE74.jpg) (https://imageshack.com/i/exb0vE74j)


The crank inside the cases fitted with the SRM rods. Great stuff, but one word of warning if anyone here intends to fit them:
Check the oil hole in the lhs rod, mine was blocked 7-8mm deep with a mixture of grit and polishing compound. I had a real tough time prising the stuff out with a piece of 1mm dia spring steel wire.

(http://imagizer.imageshack.us/v2/100x75q90/911/czP6Js.jpg) (https://imageshack.com/i/pbczP6Jsj)


Furthermore the recesses in the cylinder spigots have to be enlarged. The SRM rods are wider than the original ones and the edges will hit the spigots. Hopefully I will have time tomorrow to machine the barrels to fit.

(http://imagizer.imageshack.us/v2/100x75q90/911/R6GcWP.jpg) (https://imageshack.com/i/pbR6GcWPj)


As I have everything apart anyway, I'm going to incorporate the cam follower oiling mod. Cutting threads into the rear return oil holes is real fun, but it worked with the setup shown in the pic, this being an M6 tap and a 6BA spanner  *smile*.

(http://imagizer.imageshack.us/v2/100x75q90/911/R6GcWP.jpg) (https://imageshack.com/i/pbR6GcWPj)


I wanted to use hex bolts and lock wire to blank the holes off, but there's not enough clearance to the crankcases to acommodate the heads. So I used Allen grub screws, fitted them with Loctite and peened them into grooves in the base material. There's no way these will ever come loose (and there probably is no way of getting them out again without drilling I'm afraid  *smile*).

(http://imagizer.imageshack.us/v2/100x75q90/537/mDoqEB.jpg) (https://imageshack.com/i/exmDoqEBj)


Machining the slots in the followers is on the to-do list for tomorrow, will keep you updated.


Cheers, Markus
Title: Re: My engine rebuild / crank balancing considerations
Post by: A10Boy on 04.09. 2010 16:34
Blimey Markus, it looks good, I wouldn't have recognised it.  *eek* Keep us posted on progress.

Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 05.09. 2010 20:14
Some more work done today.

Cut-outs, enlarged to clear the new SRM rods:

(http://imagizer.imageshack.us/v2/100x75q90/538/BIAibY.jpg) (https://imageshack.com/i/eyBIAibYj)


Grooves cut in the followers. It's incredible what one can do with an angle grinder and a steady hand  *smile* *work*

(http://imagizer.imageshack.us/v2/100x75q90/631/ntWDRK.jpg) (https://imageshack.com/i/hjntWDRKj)

(http://imagizer.imageshack.us/v2/100x75q90/537/IWErPt.jpg) (https://imageshack.com/i/exIWErPtj)


The Wisecos fitted

(http://imagizer.imageshack.us/v2/100x75q90/540/VyfIpS.jpg) (https://imageshack.com/i/f0VyfIpSj)


Engine back in frame.

(http://imagizer.imageshack.us/v2/100x75q90/673/bdrxiT.jpg) (https://imageshack.com/i/ipbdrxiTj)


That's it for today, unfortunately I'll have to wait until next weekend to finish it, too bad I can't retire already  *smile*.
Title: Re: My engine rebuild / crank balancing considerations
Post by: mike667 on 07.09. 2010 00:23
Markus
 i am already jealous - can't wait to see more as you progress!
m
Title: Re: My engine rebuild / crank balancing considerations
Post by: muskrat on 07.09. 2010 08:25
 *clap* *respect* Great work Marcus. I just use the seat of my pants to test my work.
Cheers
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 07.09. 2010 09:24
Thanks chaps, I shall keep you updated as work progresses.


Quote
I just use the seat of my pants to test my work.

That's about what it's going to boil down to.  *smile*
At least I know now that this crank must have been balanced to 54% originally (there was only one hole drilled in the flywheel, so it obviously hadn't been messed with, well, up 'til now that is  ;)), hopefully some test rides this weekend will show how it will perform using 58% on our twisty mountain roads here.

I found it interesting that the BSA engineers obviously have chosen the balance factor that results in the smallest average resulting force for a given stroke and con-rod length.
The calculations I have done are based on basic geometric correlations of the crank gear known since mankind has built the first steam engines (probably), so I guess the guys from BSA had well considered what they were doing.

Eddie Dow stated a balance factor of 54% as standard and recommended 65% (and above) for racing, and from what I've measured now I'd say this data seems to be correct, unlike some other balancing information circulating on the internet.

Cheers, Markus
Title: Re: My engine rebuild / crank balancing considerations
Post by: muskrat on 07.09. 2010 10:08
Yes it's all give and take. You can make it smooth in one part of the rev range but it will be worse at another. My race A7 was at about 75% and was smooth at top revs (7500) but would rattle your bones at 3000.
I just got a LJ crank that has been stroked by about 2mm (4mm on stroke) and the flywheel was cut away greatly adjacent to the crankpins. I will do a lot of checking before I use it in the next motor.
Cheers
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 11.09. 2010 18:27
IT'S DONE!

(http://imagizer.imageshack.us/v2/100x75q90/907/PT0EiC.jpg) (https://imageshack.com/i/p7PT0EiCj)

She started first kick, I just did a short test ride down the street and back, everything seems fine so far. Tomorrow I'll go for a longer ride to see whether my balancing efforts were successful. I'll report back.


The dynamo belt drive kit, nice piece of kit, but no one told me I wouldn't get the dynamo back in with the pulley on  ;). The flange is 1.5mm wider in diameter than the original sprocket and I couldn't get the darn thing through the opening in the crankcase. But it runs very very quietly.

(http://imagizer.imageshack.us/v2/100x75q90/901/ND9yrJ.jpg) (https://imageshack.com/i/p1ND9yrJj)


I had the dynamo body sandblasted and chromed over the rough surface, giving a nice dull finish that matches the alloy cases pretty well. Looks good in there, just what I wanted. And it ensures a good earthing connection and a firm grip of the mounting strap on the rough surface. The strap is home-plated with the Caswell CopyCad kit btw. (as are all the nuts, bolts, studs, etc.)

(http://imagizer.imageshack.us/v2/100x75q90/673/ghzy3v.jpg) (https://imageshack.com/i/ipghzy3vj)


This will go on for next year's riding season, a late Super Rocket head (67-1549). It will need some attention though.

(http://imagizer.imageshack.us/v2/100x75q90/901/jPil1c.jpg) (https://imageshack.com/i/p1jPil1cj)


Cheers, Markus

P.S. I'm sure the engine will be okay now, told her she'd end up on ebay if something like that ever happened again.  *lol* *grins*
Title: Re: My engine rebuild / crank balancing considerations
Post by: a10gf on 11.09. 2010 18:56
Congrats. Great topic, great results.
Title: Re: My engine rebuild / crank balancing considerations
Post by: muskrat on 11.09. 2010 21:21
She looks fantastic MG. I'll have to remember that dynamo chroming method for my next time. I have been thinking of a home cad kit for quite some time. Looking at your bike has moved it up the list.
What? A drip tray under her already. Yea of little faith. LOL.
Cheers
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 11.09. 2010 21:26
Thanks for the kind comments gents.

Quote
What? A drip tray under her already. Yea of little faith. LOL.
Just being realistic  *lol*


The CopyCad kit works a treat, I only bought the liquid from Caswell UK though. I'm using an adjustable laboratory power supply I had anyway and free sheet zinc oddments from the roofer as anode. That's much cheaper than buying the whole stuff.
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 12.09. 2010 17:49
I've just returned from a 60 mls trip through the countryside.  *smiley4* *smile* ;D *grins*

What should I say, the weather was perfect and the engine is sweeter than ever. In 4th there's almost zero vibration up to 60-65mph, then at 70 it starts to get noticeable in the footrests, it only gets a little bit worse up to 80. I haven't ridden any faster today, as I tried to keep the revs in reasonable limits while running the engine in.
For highway speeds you might want to go a tad higher with the balance factor probably, about 60-65% I'd say, but for the alpine roads here it is ideal, almost vibration-free between 50 and 70 in 4th gear and 35-55 in 3rd, where it is used most.
The irritating high-frequent handlebar vibrations are almost completely gone, which makes it so much more comfortable to ride.

That's what I call a successful operation.  *smile*

Cheers, a happy A10 owner.
Title: Re: My engine rebuild / crank balancing considerations
Post by: Stu55Flash on 12.09. 2010 19:40
MG

Very nice job well done enjoyed reading the topic.

Stu
Title: Re: My engine rebuild / crank balancing considerations
Post by: trevinoz on 12.09. 2010 22:19
Markus,
               Did you dynamically or statically balance the engine?
Trev.
Title: Re: My engine rebuild / crank balancing considerations
Post by: muskrat on 13.09. 2010 13:41
And with or without oil in the galleries ?
Cheers
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 13.09. 2010 14:09
Stu,
thanks for the nice comment.

Trev,
it was balanced dynamically. Usually this is nothing to pay out of petty cash, but my engine reconditioner allowed me to use his machinery during an afternoon when he didn't need it, so all it did actually cost was some hours of work and a crate o' beer for the staff.  *smile* *beer*

muskrat,
I didn't have any oil in the galleries and the sludge trap, but I calculated the weight of the oil in there (volume of the sludge trap hole and galleries less the sludge trap tube and plugs, multiplied by the specific gravity of the engine oil at 80°C) and added it to the rotary weight of the bob weights (more calculating, less oily mess *smile*)
This makes quite a difference, I seem to remember it was about 25 to 30grams.

See, you won't catch me on that one!  *lol*

Cheers, Markus
Title: Re: My engine rebuild / crank balancing considerations
Post by: muskrat on 13.09. 2010 14:13
Smartypants  *smile*
Title: Re: My engine rebuild / crank balancing considerations
Post by: Hubie on 14.09. 2010 02:17
I'm now very curious to see how mine will turn out.  My crank is going in for balancing tomorrow so I am very keen to get it back in and see how it goes.  Balance factor is going to be 60-62%.

Great job mate, you'ev only got me more eager to complete my rebuild!

Cheers,

Hubie.
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 14.09. 2010 11:09
Hubie,

I'm sure it'll turn out good with 60-62%.
But don't forget fitting the sludge trap tube, plugs and flywheel bolts prior to having it balanced.
Plus, like muskrat mentioned, the sludge trap should be filled with oil (the oil holes covered with adhesive film to keep it in there). Otherwise you won't have the rotating masses right and the final balance factor will be lower than the 60% you were aiming for. The other way is adding the weight of the oil to the bob weights, like I did.

Let us know how the engine runs once you got it all back together!

Cheers, Markus
Title: Re: My engine rebuild / crank balancing considerations
Post by: A10Boy on 26.10. 2010 18:31
Blimey Markus she looks great, well done. Now for the CSR...........  ;)
Title: Re: My engine rebuild / crank balancing considerations
Post by: a10gf on 10.02. 2015 21:20
Just a comment about why it's preferable to upload pictures to the forum as attachments > see 1st page of this topic (another example here (http://www.a7a10.net/forum/index.php/topic,2893.0.html)).

When using some image host and then doing some changes there, forgetting about all the links getting killed in the process, informative content is lost \ made meaningless due to the the lack of the pictures which were the basis for the post.

MG, if you read this, can it be fixed ? :O)
Title: Re: My engine rebuild / crank balancing considerations
Post by: MG on 11.02. 2015 20:09
Fixed  ;)
Well, almost, can't find the spreadsheet pics I used back then, but the photos are all still there.

Been checking the forum regularly (more or less), but had very little to no time for writing, sorry chaps! But I am happy to see this place growing and blooming while maintaining the excellent level of quality, helpfulness and friendliness I was used to! Great job, and credit to E and Musky for their admin and mod work!

I am also glad to report that the Beezas are running well and being ridden regularly while giving suspiciously little trouble...  *smile*

Keep them on the road!
Markus
Title: Re: My engine rebuild / crank balancing considerations
Post by: Topdad on 11.02. 2015 21:15
Good to hear from you MG,been wondering where you'd got to, take care BobH
Title: Re: My engine rebuild / crank balancing considerations
Post by: a10gf on 12.02. 2015 18:30
Thanks for the news & picture fix  *smile*
Title: Re: My engine rebuild / crank balancing considerations
Post by: Rocket Racer on 20.03. 2015 07:13
What should I say, the weather was perfect and the engine is sweeter than ever. In 4th there's almost zero vibration up to 60-65mph, then at 70 it starts to get noticeable in the footrests, it only gets a little bit worse up to 80. I haven't ridden any faster today, as I tried to keep the revs in reasonable limits while running the engine in.
For highway speeds you might want to go a tad higher with the balance factor probably, about 60-65% I'd say, but for the alpine roads here it is ideal, almost vibration-free between 50 and 70 in 4th gear and 35-55 in 3rd, where it is used most.
When I had my road rocket (rigid framed and three wheels) my balancer mentioned the figures of the day were based around pre motorway speeds and B roads. For modern roads with higher speeds typically higher balance factors help. I recall we went for 70% and the motor is sweet through to the red line. It does however rarely get used below 3/4000 so I cannot comment on town manners because it doesn't need them. As it had been originally at around 50% it was great for a stationary motor and was breaking engine flanges, head steadies and loosing bolts galore!

Its done four meetings now without needing a spanner laying on it (excepting fuel and fluid dumps)
Title: Re: My engine rebuild / crank balancing considerations
Post by: edboy on 20.03. 2015 10:59
almost 30 years ago my pride and joy was a a65 mk3 spitfire slightly customised and at the time i put the plentiful 11/1 pistons in and it went like a rocket up to 65mph - 70mph and then it felt like a pnematic drill through the handlebars. eventualy bits would snap and pre locktite things would come loose. however on the motorway with the missus on pillion to some rally i chanced the std bottom end up to 80mph-90mph and from the comfort of the middle lane i was in smooth heaven. no vibs for me any longer.unfortunately all the vibration went south and missus went mad punching me in the back and swearing as if it was all my fault. also the fear of a rod banging and through the cases kept my speed down to white finger 65 and 70mph and the missus was content.some time later the frame snapped which i had repaired then sold it.
my conclusion regarding vibration was that piston weight and compression has lot to do with vibration bands and you really have to ride your machine up to top speed before any radical change of balance factor as you may find its smooth enougth to live with at a particular speed.
Title: Re: My engine rebuild / crank balancing considerations
Post by: Rocket Racer on 21.03. 2015 20:34
my conclusion regarding vibration was that piston weight and compression has lot to do with vibration bands and you really have to ride your machine up to top speed before any radical change of balance factor as you may find its smooth enougth to live with at a particular speed.
I agree, higher compression motors are typically "harsher" and reciprocating small end of the rod and piston weights accelerating and stopping each cycle doesnt help. The bigger oversize pistons are typically heavier which also doesnt help
I recently lend my outfit to a relative novice for a couple of meetings as his (a65) rig was in bits and he couldnt believe how smooth my a10 is.
It currently runs genuine BSA 8.25 pistons which are also flycut (to clear the short rods) .
Having said that once I finally can get back to working on my twins (currently distracted by a '72 a75), am planning to try some genuine BSA 14:1's that I have on the shelf  *grins* , but I digress.
Title: Re: My engine rebuild / crank balancing considerations
Post by: edboy on 26.04. 2015 18:39
ah yes_ the legendary 14-1 bsa a10 go-kart pistons. at one time they were the only pistons you could buy for the a10 as 5 star was dropped by the oil companies. they look nice on the mantlepiece though rocket racer, but not so nice hanging from a bent rod if your unlucky. 9-1 pistons aways felt so right to me with an ally head on an a10 but i m sure you know what your doing though.
Title: Re: My engine rebuild / crank balancing considerations
Post by: Rocket Racer on 26.04. 2015 22:42
ah yes_ the legendary 14-1 bsa a10 go-kart pistons. at one time they were the only pistons you could buy for the a10 as 5 star was dropped by the oil companies. they look nice on the mantlepiece though rocket racer, but not so nice hanging from a bent rod if your unlucky. 9-1 pistons aways felt so right to me with an ally head on an a10 but i m sure you know what your doing though.

I'd certainly never claim I know what I'm doing... I was chatting to a club member who is putting 7.25 flat tops in his super rocket for smoothness, not a bad idea for the road and my track bike has been amazingly reliable given the stick it gets. I'm expecting that my next race pistons will be decided by what fits that barrel best. At least  methanol removes the harshness of high comp, but it certainly adds to the odds of having catastrophic failure :(
Was helping a mate with his golden Flash rebuilt where he's gone from +60's back to Std's and the weight reduction in the pistons (both modern 7.25 hepolites) was noticable just picking them up.