adequte clearance between cylinder and piston on 30' crank case

[hdhikaru]

hi there,
any idea what is adequate clearance between cylinder and piston on 30's crank case ?
it has gusset plate in cylinder hole so should be wider than average?

tks!!
Shin

[T. Cotten]

Shin!

The 'gusset', or 'baffle' in the cylinders is a good thing, even though modern oil control rings made them obsolete. They serve well to stiffen the case castings. It is 'arm-chair engineering' to declare that it would allow any tighter of a piston fit, but I suspect so.

The factory manual specifications (for quality pistons) apply, until the cylinders are greatly over-bored. Then the spigots and walls can flex, distorting more and more with larger overbores.
The remedy is to merely torque the cylinder base to an accurate plate, so that fastener stress will distort the cylinder as if installed.
If the bore is final hone-fitted while stressed, and then installed to the same torque specification, the bore will return to 'round', allowing much closer specs than an ordinary bore job.

Do you know the expected overbore dimensions?

....Cotten

[hdhikaru]

thanks. I mean " gusset plate" is this one. there is no plate like this on later case. so I thought this might block oil splash from fly wheel . it might cause overheat... to prevent that, might need more clearance.... any idea?5406eb3.jpg

[T. Cotten]

Shin!

Later models have the male and female rods reversed, so the female rod throws more oil forward, and the rear cylinder no longer needs the shield.

Removing the baffle or gusset should not be necessary, however many left cases cracked just to the right of the VIN boss, raising a question of either problems with the casting process, or internal forces related to the baffle.

Its anybody's guess...

....Cotten

[hdhikaru]

Hi Mr. Cotton, thank you for your advice. I was thinking to remove the gusset but I should leave it ... so what is the expected over bored dimension? I normally use 6/100mm clearance .. should be bigger than this? put female rod on rear... male on front. ... right?
tks
Shin

Shin!

Later models have the male and female rods reversed, so the female rod throws more oil forward, and the rear cylinder no longer needs the shield.

Removing the baffle or gusset should not be necessary, however many left cases cracked just to the right of the VIN boss, raising a question of either problems with the casting process, or internal forces related to the baffle.

Its anybody's guess...

....Cotten

[T. Cotten]

Shin!

Different piston designs benefit from different clearances.
The original book spec was only ~.025 to .050 mm, however the manual also mentions slots. "T-slotted" pistons "collapse" upon break-in differently than most modern productions.
Modern cast productions have a stiffening plate, so they are usually set up in the range you normally use.
Beware of some aftermarket productions of the last decade with no stiffening plate; They are packed with instructions for ~.114mm (.0045") clearance, and they needed it.
Please note also that forged pistons are available, that will be specified by the manufacturer, often much 'looser' as well.

Stressing the cylinder with 'torque plates' is prudent with anything 1.25mm (.050") over-bore, and critical at larger bores.
Not only does the fastener stress distort the spigot, risking rubbing the piston skirt, the head bolts pull outward higher in the cylinder, compromising ring seal. Thus OHVs with large overbores benefit from hone-fitting with both top and bottom plates.

And yes, most folks 'update' by reversing the rods with the male rod forward.
Does yours have a "split pinion" right case main shaft? Or the later splined shaft?

....Cotten

[hdhikaru]

thank you Mr.Cotton. will check the shaft later.

tks
Shin

[hdhikaru]

Shin!

Different piston designs benefit from different clearances.
The original book spec was only ~.025 to .050 mm, however the manual also mentions slots. "T-slotted" pistons "collapse" upon break-in differently than most modern productions.
Modern cast productions have a stiffening plate, so they are usually set up in the range you normally use.
Beware of some aftermarket productions of the last decade with no stiffening plate; They are packed with instructions for ~.114mm (.0045") clearance, and they needed it.
Please note also that forged pistons are available, that will be specified by the manufacturer, often much 'looser' as well.

Stressing the cylinder with 'torque plates' is prudent with anything 1.25mm (.050") over-bore, and critical at larger bores.
Not only does the fastener stress distort the spigot, risking rubbing the piston skirt, the head bolts pull outward higher in the cylinder, compromising ring seal. Thus OHVs with large overbores benefit from hone-fitting with both top and bottom plates.

And yes, most folks 'update' by reversing the rods with the male rod forward.
Does yours have a "split pinion" right case main shaft? Or the later splined shaft?

....Cotten

Hi Mr.Cotton, owner before me has already remove the gusset from 37 case... so have to ride gently... IMG_5704.jpgIMG_5705.jpg

[T. Cotten]

It should do fine, Shin!

The Glyptal looks very fresh.

....Cotten

[hdhikaru]

thanks!