rambler 195.6 ohv rotating assembly

Crankshaft, connecting rods, pistons, camshaft, tappets, timing chain and sprockets.

new 21 apr 2021

overview

(This section is newly broken out, and is a bit chaotic. I am actively reorganizing it.)


crankshaft, connecting rods, pistons

the crankshaft and connecting rods are forged parts, with very large and nearly overlapping journals. though only four main bearings the bottom end seems more than adequate. be careful selecting or mixing connecting rods; i have found at least two different parts, fully interchangable, with identical part and casting numbers, that were over 100 grams different mass but within each set, 10's of grams difference. more old world engineering. (the difference seemed to be at the little end.) the pistons are heavy, with thin rings, cast aluminum with steel inserts, of a popup wedge design. aftermarket pistons are often of terrible quality. most of the engines i've disassembled for parts were .030" or .040" over, with one .060" over.

pistons

for my 2010 build i got fair quality replacement pistons and rings from kanter. i static balanced those with a gram scale, and they were not bad to begin with. rods and bearings were fine, probably; though connecting rod bearings failed (leading to the 2017 teardown) it seems fairly likely that the failure was due to the collapsed (softened) oil pump pressure relif sprign i bought from kanter.

Early pistons had wider top and 2nd rings. Specifics TBD here, check before buying from eBay. "Most" pistons will be the later, thinner rings.

For the 2017 engine Pete Fleming had custom forged pistons made that accept a modern 81mm ring set. the dull aluminum piston in some of the photos is a 1970's Silv-O-Lite .060" over piston from a set i purchased on ePay years ago and never used.

camshaft

it should be no surprise that there are no (and never were) aftermarket performance cams for this engine. a used cam must be reground, but because the base circle is only .020" or so larger than the rough casting increasing lift or duration is not possible.

If you think about this engine as much as I have you'll eventually stumble upon the idea of puttimg an L-head engine camshaft into the OHV block, since it generates significantly more lift at the lifter. Alas, it is not to be. Here is a quick camshaft comparison.


Tappets

avoid resurfaced tappets

for my 2010 build i bought used but resurfaced tappets. this turned out to be a mistake, as the resurfacing removed all of the hardened surface of the mushroom head, precisely where it is most critical. the working face of all 12 tappets was severely pitted.

the cam lobes were fine, no scoring. you can see one lobe of the cam that came out of the engine with these lifters in the background of the first photo. this was not an oiling issue, but metallurgy. don't use resurface tappets.

for the 2017 build i bought new, nos, tappets from Kanter. they have a faint crown. the mushroom head is black oxide, as is the pushrod ball socket. overall length of the nos tappets is 1.878", i measured 6 of the old ones, all were shorter at 1.864" in height. this implies that the resurfaced parts had .014" removed, assuming all tappets are initially the same height.

there's a discernable (thumbnail) wear pattern on the cylindrical section, up under the mushroom. this appears to me to be the lateral force caused by the cam thrusting the follower sideways during normal operation; that's a feature of the mushroom design (and why amc and everyone else ditched them). as the cam operates, the lobe wipes the tappet crown, slightly off-center, rotates the tappet presumably to distribute wear. the follower "rocks" in it's bore with each cam cycle a tiny amount. this scuffs the cylindrical portion of the tappet top and bottom, the rocking pivot point more or less in the center, but offset towards the pushrod end. it's just the sum of the leverages. this leaves (microinches) of gap, where there's no contact, the metal is darker (in the pic above). none of this is unusual or bad or anything, it's just what happens to metal in use. but after 60 years it's a lot of wear.

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