'90 Evinrude 150 std piston clearance?

[littleape]

Does anyone know what the piston/wall clearance is from factory on these motors?

I'm rebuilding one of these, the pistons are measuring 3.4945 and service manual states the standard bore is 3.4995-3.5005 which would mean they come with ~0.0055 clearance.

How far could you push the clearance before oversize is really a must. Not building a race motor.

 

[Scott Danforth]

Generally I use .001 per inch of bore as a guide. So I would expect 0.003-.004. The factory manual has min and max. Use the range in the manual. .0055 seams to be on the sloppy side, however look at the manual

Contrary to your comment, race motors are built really sloppy because you dont care about smoke or longevity. You want minimal drag

 

[Faztbullet]

What are the cylinder measurements? If it measures more than 3.504 it needs bored.

 

[littleape]

What are the cylinder measurements? If it measures more than 3.504 it needs bored.

Did you mean 3.504 or 3.5004?

3.504 would mean .010 clearance

 

[racerone]

He means 3.504 "----And yes even new they need / have lots of clearance.------The measurement of your pistons indicates very minimal wear on it.----Have you measured the bores, yes or no ?----And yes new pistons are tapered and the piston expands for a proper fit at full throttle.

 

[littleape]

Bores look to be 3.500 +/- 0.0005 before any work being done. Pretty sure I see some taper maybe .0015 max and some low spots. If I have .004 to work with than I should be able to get away with std pistons.

 

[Faztbullet]

Run a rigid hone not a spring loaded or ball type thru cylinder. Any taper,low spots will show instantly. Piston to wall clearance is not as important as making sure cylinder is round. You don't have .004 to work with ...you have .002

 

[littleape]

So looks like I'll have some mixed sizes.

I have one oversize that I'll have to go with Wiseco, some that cleaned up perfectly 3.504 +/- 0.0005, some may still have a tiny low spot between one set of exhaust ports.

For those that didn't clean up 100% would it make sense to go WSM 0.010 oversize or would it be a waste of money? Or maybe bite the bullet and take them all to .010 since they're all at best at max service limit.

 

[racerone]

Just trying to teach here.----Curious as to what your pistons measure just above the rings.----My guess is that it is approaching 0.020" clearance.

 

[littleape]

Just trying to teach here.----Curious as to what your pistons measure just above the rings.----My guess is that it is approaching 0.020" clearance.

Above the rings its 3.475 and where service manual calls to measure 3.494


I'm leaning towards 0.010 oversize instead of running at service limit edge and have it all mint not regretting after everything is reassembled. Hmmmmm

 

[racerone]

Yes , I know where the manual wants the measurement !----That would be 0.030 " clearance at the rings.--------It has been said that if you don't have time to do it right the first time you will always find time to do the job twice !!

 

[littleape]

Yeap no point, going to bump them all up to 0.010 over and have no regrets later.

 

[racerone]

First you need to find a shop that can line it up and only bore out 0.003 per side to get to 0.010" O/S.----What if it does not " clean up " on your bores.-------Best to go to 0.020" over !!