Rod bearing clearance

[In-Tech]

Hiya Dave,
The rods do stretch, my old school blown 11,000 rpm BBC had ~.090" piston to head and you could barely see the very minimal contact. A lot better Alum rods out there now(this was in '94)
They do shrink from work hardening.

I can't advise on clearances for that stroker as I've not done anything like that either. I bet that big stroker crank is moving all over the place though.

[mt-engines]

Hiya Dave,
The rods do stretch, my old school blown 11,000 rpm BBC had ~.090" piston to head and you could barely see the very minimal contact. A lot better Alum rods out there now(this was in '94)
They do shrink from work hardening.

I can't advise on clearances for that stroker as I've not done anything like that either. I bet that big stroker crank is moving all over the place though.

As far as the side clearance. I do agree with Walter, it should be a minimum .040, the rods will swell with heat.

You won't find a shelf aluminum rod that sets up at .025 side clearance unless you custom order them. All the ones I've run are in that .040-.055" they usually even tell you on the instructions also.

I've only built a handful of 706 combos, only 2 were 4.840 BS. The others were 4.900. but I've run 5.800-6" strokes on the bigger bore space stuff. Some of those clearances would make people chuckle.

Edit, some small block and small crank journals i have run .020. But we were talking about a 2.200.. just in case anyone reads this for a different application

[Baprace]

Why do you recommend .040 side clearance , that seems excessive to me , I would not want to exceed .025 to keep the rods from dancing from side to side , what would you suggest for vertical clearance , with what weight oil , also try the right side tomorro , your a little grumpy. Have a great day

A "piston guided rod" doesn't dance around like that but, you can believe whatever you want.

No bad mood or anything ... I really don't care what is believed or isn't.

Walter , thank you for the clarification on the rod side clearance , can you give me an idea on the vertical clearance and what rod journal size would you recommend for a 706ci engine NA for starters. thanks again.

[Dave Koehler]

Hiya Dave,
The rods do stretch, my old school blown 11,000 rpm BBC had ~.090" piston to head and you could barely see the very minimal contact. A lot better Alum rods out there now(this was in '94)
They do shrink from work hardening.

I can't advise on clearances for that stroker as I've not done anything like that either. I bet that big stroker crank is moving all over the place though.

Stretch would indicate a permanent result.
The rods did not stay at +.090 or a unscheduled disassembly would have happened the next lap.
I submit that what you saw was heat induced expansion and contraction once it cooled back down.
Don't forget that the pistons expand and move around also.

[Walter R. Malik]

Walter , thank you for the clarification on the rod side clearance , can you give me an idea on the vertical clearance and what rod journal size would you recommend for a 706ci engine NA for starters. thanks again.

I can't recommend anything like that 'cause I don't build them ...
I have witnessed those being built, (and other mountain motor Pro Stock engines), and do know that you want to avoid ANY side loading on the rods.
Also, have witnessed many dyno and other testing on the finished product simply to learn facts and try to understand.