Cheap_Cherokee
NAXJA Forum User
- Location
- San Jose
Hey I just scored a '85 4.2l What else should i keep beside the crank assembly before I scrap it? or sell it...
4.2l stroker
- Thread starter Cheap_Cherokee
- Start date
bigdaddyjlove
NAXJA Forum User
- Location
- Atalanta
Hey, Sorry to but in to the thread, but I'd like to know more about those pistons. I've got sveral 4.2 cranks and would really like to use the longer 4.0 rods on the next motor, esp. if they let you get away without decking the block.
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
The pistons he speaks of are KB944 forged pistons. You can get away without decking, but your quench would still be terrible. However they do allow you to use the longer 4.0L rod, and are stronger pistons. http://kb-silvolite.com/forged.php?action=details&P_id=543
those pistons put your deck right around .020. stock is .0215
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
Deck clearance + gasket thickness = quench. The .044" MLS gasket, which is the thinnest I know of currently for the 4.0L will net a .064" quench. Which isn't great.
jeeperjohn
NAXJA Forum User
- Location
- Temecula, CA
What is an ideal quench?
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
.040"-.045" Personally I would stay on the safe side and go with .045". .040" is critically close.
old_man
NAXJA Forum User
- Location
- Loveland, Colorado
At my altitude 5000' I run about .053 quench and can run the cheapest pump gas with no ping.
ChevelleSSLS6
NAXJA Forum User
- Location
- Grand Rapids, MI
mill the head down about .020 for near-ideal quench (.044 inch)
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
You deck the block to reduce the quench, not he head
jeeperjohn
NAXJA Forum User
- Location
- Temecula, CA
Does it become an interference engine at less than .040"?
bigdaddyjlove
NAXJA Forum User
- Location
- Atalanta
I thought this too, but the machinist I use swears that with the design of the I6 head, decking the head would do the same thing.
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
Find another machinist. Shaving the head will only increase the compression ratio. It won't bring the pistons any closer to the head. And won't decrease the quench.
Talyn
NAXJA Forum User
- Location
- Radford, Communistwealth of Virginia
No, but the psitons are getting very close to the head and tolerances must be extremly precise to avoid a piston to head collision. Not really worth it achieve that less quench.
An interference engine is completely different from the above. An interference engine is an engine that the valves open into the piston's path. Normally that is fine because the valve opening and closing events are controlled by the camshaft linked to the crankshaft by a chain or belt. It becomes a bad thing when the timing chain or belt either skips a tooth, breaks or is installed wrong. Then there is a good chance that the pistons will make contact with the valves. The 4.0L in stock for is not an interference type engine. However, using a cam with enough lift it can become one. I don't know how much lift is necessary for that to happen. Any reasonable street-able stroker engine should not have an issue with that.
