regrinding crank and new rods

T

tomcolitt

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Hi

I am experimenting with building, a relatively low-budget, slightly bored and lengthened stroke engine for a 1960s Mercedes. After I re-grind the existing crankshaft to achieve a longer stroke, I'll end up with smaller diameter "big end" crank journals. I was wondering if people have any ideas how I would begin to find used donor connecting rods at a salvage yard, which I will need, since the old ones will be about 2.5mm too large for the reground crank journals. In other words, if I went to a salvage yard to search for donor rods, I have an idea about the measurements I'm looking for (42-45mm big end, 124mm eye to eye and 22-24mm piston pin diameter) like, but no idea which engines to find these in other than opening each engine (from Accords to BMWs) up. Are there books that show main dimensions of various engines....?

Thanks, Tom
 
The last time I did this I had the crank journals welded and reground so they were the original diameter. I used the original rods and had pistons made with the wrist pin hole moved up to compensate for the longer stroke. Don't forget to make sure that there is clearance between the rods and the bottom edge of the cylinders.
 
Ed G said:
If I read you right, can't you just reconfigure the conn rod bushings into the original conn rods?
Click to expand...

Possibly, but they'd have to weld about 3 to 4mm of material back into the big end and re-bore it. Then it would still be too long by about the dimension of the re-ground crank offset.

Tom
 
m_thompson said:
The last time I did this I had the crank journals welded and reground so they were the original diameter. I used the original rods and had pistons made with the wrist pin hole moved up to compensate for the longer stroke. Don't forget to make sure that there is clearance between the rods and the bottom edge of the cylinders.
Click to expand...

Thanks for the tip about the clearance. If I only changed the crank offset by 2.5mm do you think one could get away with welding and boring the original rod also offset by that much without losing strength? I'd be moving the hole closer to the top of the rod so its not like I'd be thinning out the bearing cap that sees high tension forces on the upstroke.
 
tomcollit has asked: "I am building, a relatively low-budget, slightly bored and lengthened stroke engine..."

There's a contradiction in the above sentence - low-budget and lengthened stroke are mutually exclusive. Why not just rebore the block, regrind the crank, use undersized bearings, and get on with your life?

If you are looking for performance upgrades, head work is probably the first thing to do, while stroking would be pretty far down the list. Milling the head for more compression (or HC pistons that accommodate the overbore) and mild porting better fit the "low budget" criteria than all that would be required to increase the stroke.
 
x_atlas0 said:
How "budget" are you talking here? A set of custom con rods usually goes for <1k$ US for a set of 6.
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Yeah, the things is, I really am experimenting since there was not much potential in these older Mercedes engines. So, I was going to try to put together something using as many stock components as possible, plus maybe $1500 to $2000 for machine work to head, rods, crank and maybe pistons. Then when I can test the concept and the result, I may spring for custom rods and pistons
 
Thanks for all of your answers. It turns out that the crank guy is able to weld and offset grind the crank so that I can use the original rods. All I have to do is take a bit off the top of the pistons to lower the compression ratio.

Thanks, Tom
 
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