Piston Question

[trbig]

The guy who is boring my cylinders for the 1179 kit was concerened with the piston to cylinder clearance. I don't know anything about it, but he said because the original pistons are cast, and the Wiseco pistons are forged, they expand differently?? The kit says to bore it to .002" or .051mm. He says that's aweful tight for the rpm's this will be turning and especially being an air cooled engine.

I showed him that those were the original clearance specs as well as Wiseco's. He is boring it to that... but I thought I'd ask someone in here of they have had any conflicting info about this. Thanks.


Tod

[TopCatGr58]

Hey Tod,

That's interesting about the clearances you say were Wiseco's specs, because I found this info in their FAQ!?

03. What kind of piston-to-wall clearance can I run?

A. The factors that affect this are cylinder wall thickness, whether the block is filled, the overall compression height of the piston, piston material and thickness, and whether a marine engine is to see fresh-water cooling. Most small blocks get .004 piston to wall clearance and most big blocks get .005 due to the use of our 2618 high-strength alloy. For heavy blower and nitrous applications, Wiseco recommends adding .001 to the standard clearance. Special note: Clearance numbers are obtained from measuring the largest diameter of the piston, typically at the bottom of the skirt. All measurements should be taken 90 degrees from the pin centerline.

Of course they are talking about big V-8 engines or such, but they do mention that for higher stress/heat applications ie. a BLOWER or NO2 to even add .001 to the amount, but what they say is actually twice what you say the specs said!?

T.C.

[trbig]

the instructions in the handout tell you to bore to the specs listed on the box... and this is what is on the box.





It's the same specs as original. I understand what you're saying. This guy says sometimes on a really hot motor (V-8) he's even bored it up to .01".... 5x what this says to do. That's why I was questioning it. He's a lot more knowledgeable than I am at this and he was shaking his head a little at this. Maybe I'll try to contact Wiseco today.


Tod

[trbig]

I contacted Wiseco today, and they said that .002" is the correct clearance on these.


Tod

[79XS11F]

I had mine bored to the specs on the box and had the pistons MATCHED to the bores. I wrote the position numbers on the pistons and they matched them bore for bore instead of just boring 4 holes to a set spec and calling it a day. The pistons also have tolerances and are not likely all the same so they should be checked and matched bore for Bore if necessary.
Rob

[trbig]

I agree and that's what he's doing. I went a while ago to start this process and get the cylinders done first, but he wouldn't touch them without having the pistons there also.

Tod

[TopCatGr58]

Hey Tod,

Just also remembered about the fun Stralya had putting his together, so you might want to ask that the bottom of the cylinders be chamfered for easier ring/piston fitting....if they aren't already that way!
T.C.

[79XS11F]

Good reminder there TC .. Mine were a Bit*h to get in as well. Gapping the rings is also an important issue and can be time consuming.
Rob

[Stralya]

Why did you need to gap the rings? I didn't, just made sure the gaps were 180 degrees away from each other. Champhering only cost $40 but would have been cheaper if I had it done at the same time as the re bore.

[trbig]

Already talked to him about it and it's being done. The cylinders are already chamfered at the bottom almost to the outter edge, so it may not need any more done. He said "You don't need that much to get the rings up in there anyway.. " To which I told him.. ummm... every bit I can get helps!

The oil ring top and bottom ring are supposed to be 180 from each other and the book says to put the top two compression rings 120 degrees apart... but I always split the oil ring 180, then the next ring 90 degrees to it and the top ring 180 from that one. Just my preference. I also had no plans to gap the rings.. ?


Tod

[79XS11F]

You need to check the ring end gaps Tod and check them at the top, middle and bottom of each bore. Use the pistons to square the rings in the bores for the check. Then adjust for proper end gap at the tightest point. If you don't and any of the ring gaps are to tight the rings when they are hot will expand and close the gaps and then you will have a problem and in a worst case scenario possible a seizure of the expanded ring to the wall.
Rob

[Stralya]

Well I didn't know that. I must have been lucky because I just installed everything as it came and assumed the ring gaps would be perfect. I spaced the gaps on the pistons the same way as you did Tod. Just be careful with the oil rings, it's easy to overlap the ends rather than have them but up against each other.

Dan
Student.

PS
I also found it much easier to apply firm but even downward pressure on the cylinder block once you get the rings seated, I tried tapping the block down but that only made the rings pop out from under the ring/hose clamps.

[79XS11F]

It's in Wicso's instructions and I had to file the ends of about 1/3 of the rings to meet the minimum gap. Unless the gaps are way to tight you may never have an issue unless the engine gets REAL hot like on low oil or in very heavy stop and go traffic on a real hot day. Or if the engine is heavily modified in other ways.
Rob

[trbig]

I hadn't seen that until you wrote that.... so I re-read them. I was going to just throw them in there and go.

This is the only thing it says in the whole pamphlet about it.






Guess I better do it! Thanks Rob.


Tod

[79XS11F]

Yeah they sort of just threw it in there but it would be a shame to have an ring issue given the cost of the kit, the machine shop work and your time. If I am not mistaken I was told to go with the OEM gap specs.
Rob