wsm pistons really are that bad!

[drake1]

well i rebuilt my 787 with wsm pistons, i had 150psi before the rebuild but due to a large water leak at the bottom of the jugs i decided to replace the pistons while i had the jugs off. using the lowest crank gasket, 0.3mm i still get 2.1mm of squish when i should have at most 1.7mm my pistons are to short! i did a psi test on the bench and i only get 135psi now. the motor only had 175 hours and i didn't have it bored since the machine shop said the cylinders were still in spec, very little wear. this really sucks. i should have listened to the other post about wsm pistons...plus they were the platnium ones!
jeff

 

[LouDoo]

I don't think its the pistons, you should have at least honed the cylinders and 175 hours is getting close to a full rebuild. You also really need to give the rings a chance to seat, 6-10 hours.

Lou

 

[drake1]

not sure how having the cylinder honed would make them higher in the cylinder, but yes they were honed when i brought them to see if i should have them bored or resleaved, if they never getting to tdc where they should be it will have get full compression. i think the compression will go up when they seat in but the clearance issue will always be there. i didnt have it honed since they were in specs and i plan on sending the motor for replacment in a year or two so no point in doing alot of work when it doesnt need it

 

[chucked]

You cant really go off of that psi number until its broken in. You always hone when you have new rings

 

[Guest_User]

I`m pretty sure the OP is talking about piston height relative to deck height, hence the different thickness base gaskets... for the spec`d compressed volume ...

 

[Dr Honda]

I've never had a squash-band issue with WSM Pistons. SO... I'm not exactly sure what's going on with your engine.


With that said.... you say you are getting .21 of squash???? If you are talking "Inch"... then you have almost a quarter inch of gap... and that tells me that your pistons are just wrong. If you are talking "Millimeters"... then your are too tight. Spec on an 800 engine is 0.047" to 0.063".... OR.... 1.2mm to 1.6mm.


So... the number you posted doesn't even fit.

Finally... the base gasket wouldn't be "3mm"..... if it's a 3 dot gasket... it would be 0.3mm. Those are HUGELY different numbers.


Not to sound like a jack off... but nothing you posted is close to what the REAL numbers should be... so there is no way to help, or even tell whats wrong.


As a final note... since you are trying to "Band-aid" a warn engine, with issues... are you sure that you don't have other case leaks?? are you sure the crank bearings are good? are you sure the rods aren't crushed/bent???

 

[Dr Honda]

OK... I read your post again.. and I can't even tell what the problem is.. (other than low compression)

This is want I'm reading...


1) Rebuilt the engine with WSM pistons
2) Ran the engine, but had a leak so you put in a new set.
3) New pistons have some issue with compression.


I don't think I'm reading it properly... but that's what I'm getting. So... let's start with a real time line... and real measurements.

Finally... since a lower cyl leak isn't a "Normal" problem... what have you done to fix it? And what was the cause of the original problem? (and did you fix that problem)


Here again... I'm not trying to be harsh... but it really sounds like you are blaming the pistons for problems with a old/crap engine... and a tech that has minimal experience.



The pistons could have been machined wrong.... but if they were off by a 1/4" crown height... then WSM wouldn't still be manufacturing pistons, because no one would buy them. Also... if the crown height is off by that much... then make sure you have the right pistons. (call WSM) Remember... the 720 and 800 engine have the same bore... but the pin height is different to accommodate a different stroke.

 

[drake1]

you are correct i had my inches and mm switched thats all, i realize that some people get defensive over products they like but i am just posting my results. i did use a 0.3mm gasket, the original was a 5mm gaskets so i am as low as i can go as far as base gaskets. my squish come in at 2.1 mm and originally they had 1.4mm before i replaced the cylinders. i currenty get 2.1mm of squish in both pistons and this is taken off all 4 sides of the pistons just to make sure i didnt have a low spot. no matter how you look at it it is still low in the cylinder. also yes i did have a water leak from the gasket off of one corner of one of the cylinders, i would chalk it up to that but it was only one cylinder that had that issue.. thanks for pointing out me mistake in the original post i will edit that. im not sure if i have a smaller piston, wrist bearing or just got a bad set
jeff

 

[Dr Honda]

I'm not defending WSM pistons... I'm not a total fan... but I haven't had an issue either. I'm trying to understand the real problem, and help you resolve it.

With that said... if you have over 2mm of squash... I would call WSM and tell them. They could have been machined wrong.


Remember... all we have are words here... so the info needs to be correct, and all info... including a time-line... needs to be clearly posted.


So... I can't tell if you put the engine together... and had the issue... or if you ran it, and then had low compression.

Finally... Why did you do the top end? Did you have running issues... or did you just do it because you had to open the top-end?