Noisy Engine 1949 A7 Long Stroke

[Ron B]

I need a bit of advice.  My 1949 A7 is noisy.  The drive side ball bearing is good, the timing side bush and the rods measure .003" clearance and the pistons are have .005" skirt clearance.  The cam is pitted but serviceable  and the cam bushes are good.  Will this amount of clearance on the rods or bush or piston make noise?  It runs very well but at 55 mph it is noisy.  To me a cast iron engine should be quieter.   Any advice will be appreciated.  Thanks Ron B.

[1660bob]

Hi Ron, .003" on the rods (i presume this is the big end clearence) is toooo much-i have read somewhere that once the clearence on a shell type bearing such as fitted to engines of ours gets to .0015" they are clapped and will start to knock.I think the bush sounds kaput to me as well but i don`t know what the service limit is for that, but it must be in good order to maintain vital oil pressure to the big ends. Piston clearences vary according to application but .005" is a lot- I would have said between .0015"-.0035" is the range most aircooled road engines run on. Looks like time for a rebuild before the oil pressure drops too low and starves the driveside big end furthest from the pump, it seizes up, and the engine "puts a leg out of bed", taking out the casings.......  .Sorry i can`t offer good news, but think how nice it will be when all done up.... Regards, Bob.

[terryk]

Piston clearance of .005 is fine if it is solid skirt piston. Split skirt pistons are only about .002

[Triton Thrasher]

I would have said between .0015"-.0035" is the range most aircooled road engines run on.

I'd say one and a half thou is a range British twins seize their engines on.  Where on Earth did you get info like that?

[muskrat]

 I agree with terry, split skirt pistons only need 1 1/2 thou".
3 thou" on the bush & big ends is about at the end of their life. I'd throw some thick (40/75) oil in to see if it makes much difference.
But a rebuild is on the cards.
Cheers

[trevinoz]

Have a good look at the cam followers too. They're usually pretty sad.

  Trev.

[1660bob]

I would have said between .0015"-.0035" is the range most aircooled road engines run on.

I'd say one and a half thou is a range British twins seize their engines on.  Where on Earth did you get info like that?

Hi Thrasher, Just general knowledge to illustrate that .005" is "on the limit " and not condusive to a quiet engine, as for my range .0015-.0035" piston clearence- I was referring to the general range for aircooled engines, not specifically any particular one. As commented above, piston design can require more or less clearence with split skirt types needing less.In the case of wire wound types (admittedly unobtainable now) apparently even less: .0007"! The following reproduced from AJS, single cylinder motorcycle factory w/shop manual:
Single cylinder, 1948-1958 350cc:
Bore finished size:                     2.7187" (tol:+or-.0005")
Bottom of piston skirt dia:           2.7180"
Regards, Bob

[kommando]

Here are the recommended BSA figures for freshly installed pistons, the clearances will increase once the engine runs so do not use these figures for wear limits.

[trevinoz]

Don't even consider using the RGS clearance shown!
Unless you enjoy having a seized engine.

  Trev.

[1660bob]

I would have thought that one a misprint surely? Of all the models listed, the one likely to need the MOST clearence would be the RGS, destined for a likely thrashing from the word go

[Triton Thrasher]

I guess that demonstrates where the odd info comes from.

It's complicated by having split skirt types and possibly low-expansion alloys not readily available now.

The best guidance would come from the piston maker, rather than from BSA.

[kommando]

BSA made their own pistons for a while, the B44 slipper pistons where made by BSA and were finished with diamond tipped tools so they were advanced in their know how. During the late 60's they started to use Hepolite instead but when they started making their own is the info I do not have.

[terryk]

I've pulled BSA engines apart a number of times because owners have only allowed .002 on solid skirt pistons like some modern bikes. The bike might start alright but when they get down the road a bit the engine locks up. When I hone it out to about .004 its fine so .005 clearance on a solid skirt I feel still has plenty of life left in it. The clearance decreases because the piston expands.
I cant see in this service sheet how .002 for solid skirt is correct. I would like to know what brand of piston is used if it is.