22 replies to this topic

[pete l]

So how are you going to correct this then ? What bushes are you going to use ?

[MontpellierVanMan]

It looks like I'll use DAM8889 if it will still fit (front bush, press-fit) provided there is enough meat on it, as Moke suggests there is likely to be, I'm only 5 or 6 thou undersized after all, and standard 22G109 (rear bush, top-hat, press-fit).

 

Then I'll probably line-bore them, either on my mate's Centroid mill since in that way the reference surface of the square thrust face next to the gear can be used too, or failing that his Centroid lathe which is better than mine.

 

Aiming for 5 thou clearance on both.

[Allrounder]

Any update on you 5 thou clearance??

 

Just pulled my 1275 A+ apart due to breaking the gearbox and thought id give the engine a quick once over. Found too ive got an problem with the primary gear front brush as in its no longer retain in the gear. Crank measures 1.263" so should be fine but anyone know what the max internal dia of the gear is before I order a new bush and it wont retain it (DAM8889).

[Spider]

The ID of the 1275 type Primary Gears, for the front bush location is 1.751" ID. This is the ID of the gear after the bush has been removed.

[Allrounder]

Thanks Moke Spider.

[MRA]

The front bush was ordered from MS as a DAM8889 floating one, and invoiced as a floating one, but in fact came as a steel-backed split liner with some sort of pink alloy lining.
So it needed pressing in.
Machining it gave curly swarf and nothing like any sintered bearing material would produce.
I am still prepared to entertain the idea that I fitted it too tight, although I did use Graphogen and oil upon assembly.
Upon disassembly the Graphogen seemed to have been useful only as a glue for the swarf generated by the pick-up of the bush on the crank, and after 1500km I was surprised to see so little evidence of engine oil ever finding its way in.
Did you find a suitable blank from DEVA that provides a good starting point for bushes ?

 

Grophogen ?  I can supply you with a much better option that will not give you the same issues as graphite grease...
 

[MRA]

This is the MS gibberish for the DAM8889 : my comments in bold :

 

running clearance for this gear-front bush running clearance finished size 1.503" +0.002"-0-0" (which implies an ID of 1.503" to 1.505" ? but the FRONT bush zone on the crank is 1.625" !!)

for race 1 thou more clearance with plus 1 thou tolerance obut never undersize. (implies what ? 1.503" to 1.507" ?)

front bush 1.628" +0.002" -0.0" never undersize as it requires clearance. (this is better, a front-bush size, 1.628 to 1.630 ; presumably because 1.625 plus 3 to 5 thou gives 1.628 to 1.630 ?)

For race allow an extra thou 1.504" +0.001" only. (implies ID upper limit of 1.505" ? two lines above it's 1.507 ? - again these are REAR sizes)

if cranks are worn measure crank and allow 3-5 thou running clearance (on which one - front or back, or both ?) and race use 5-7 thou on front bush and 4-5 thou on rear bush

 

ANYWAY

 

I have grasped the nettle and following the advice from my local machine shop I have done the work on my own lathe .......... sorry about the boring bar, it was the only tipped tool I could get into the right approach.

 

 

 

 

The crank has cleaned up at 41.13 - 41.14 mm, 1.619" - 1.620", so the only question is whether a DAM8889 can be used.

 

 

 

Rough finish ?  tip radius is too large, speed to low.... 

[pete l]

Any news on all of this ??