308 Piston advice

SuperDave

For you guys with engine rebuilding experience. I am in the process of rebuilding a 308. The third owner said he thought it had 100K miles. The Odometer says about 69K..It had a leaky head gasket and some pitted valve seats. Other than the rough idle due to the leaky valves it was as quiet as I have ever heard a 308. Looks like I am the first one to break it open. Other than an aftermarket timing sprocket and chain, all seems to be OEM. The pistons look great, almost like new. The bore has wear at the top compression ring only. After removeing the carbon, It appears to be worn in about .009" larger at the wear area as compared to the rest of the bore. Thats a groove of about .0045". I can't find any wear anywhere else. No taper or belling at the bottom like I have seen on other brand X engines. The question is..should I bore it out and replace the pistons or just hone and replace the rings ? One interesting thing is the bearings.. The rods are FM (Federal Mogul) I always thought that FM was the supplier to Hudson in those days.with date code 11 49 and the mains date code 12 53, with the exception of the number two main which has no stamping like the others, has a slightly differe nt shade of metal on the backs and I could read faint ink printing that said "001 USA" on one shell. I measured all the mains and they all mic the same! I have no dea what's going on here. I have heard that the factory saved cranks that had been off sized, But .001 ? and the diameter of the throw is the same as the others.. Things that make you say...Hmmm.I will plastiguage just for the heck of it.

Park_W

With .009 of "flare" at the top, you're a bit over the typical guidance that says to bore of there's more than .006-7. That said, if the rest of the engine is OK I'd try to get by without boring it. Hastings has a ring design that is ideal for this situation. The ring faces are "barrel faced" (their terminology), that is, the outer face is somewhat rounded. Seems logical these would seat and wear better in a cylinder with some flare. Also, Hastings' improved oil ring design is such that, with it in the third groove, you don't need the bottom oil ring at all. My Hornet is running this way, and doing great (a fellow Hudnut near here was instrumental in the design of the Hastings barrel-faced rings). The bad news is Hastings doesn't have the pinned type rings, so you'd either have to have some, get some, or get a machine shop to carefully remove the pin from the upper grooves without scarring the bottom ring groove surfaces (must be able to make a good seal with the bottom surface of the ring).

Unknown

Park



Bernie S used to take a cut off wheel and cut the pins out of the ring lands so he could use unpinned rings. He said if the rebulder was worried about the top piece coming out to peen it with a punch.... one way to get the pins out.



Good Luck

[Deleted User]

I've had more experiance with antique tractor engines that with the pinned Hudson piston, but I'll toss in my opinion. The best luck I've had with cylinder wear issues is to make sure the cylinder walls are paralell with as little deviance as possible. The best money you can spend is to have the block professionally honed to make sure that is the case. With as little top ridge as you are reporting, that can be honed out by the shop you take it to more accurately and with less chance of belling via a "ridge reamer" by hand. Typical hand hones will not take out the ridge AND keep the cylinder walls paralell, because they typicly pivot or are of "dingleball" configuration. You may think its smooth, but a dial bore guage will disappoint you. Commercial hones keep the stones paralell, it would be the smartest $60-120 you could spend.



Take out the RIDGE! Even if the logic is that the top ring lands in the same place, it doesn't! It doesn't take much interferance to break a top ring. Spend the money to have it professionally honed and make the shop show you the dial bore guage readings before you take it back. Don't buy any rings until you get the block back and know how much it took to straighten the cylinders.



use HASTINGS rings! Once you've got your finished cylinder diameters back and in hand, get the 'one size bigger' ringset and file the gap to your finished bore diameter and Hastings recommended end gap.



The more ring grooves your piston has, the more cylinder to piston tolerance you can handle. They tend to keep the piston from slapping due to longitudinal distance they contact the cylinder wall. Some John Deere 2-bangers actually have extra oil rings located in the piston skirt to create this effect. I wouldn't get excited about boring until the amount needed to straighten the walls got over .014", but that means you'll need all 4 ring grooves occupied.



This may sound tedious, and it is. I work old tractors hard and have refreshed many old tractor engines that are daily workers. Engines that I hand 'reamed' and honed didn't last as long as they should have. The engines I did what I've described here have lasted without exception, some working daily since 1985 with the owner's still not using a drop of oil between changes. The pitiful fact about this is I recently installed a brand spanking new set of sleeves and pistons in one of my own Allis tractors without checking anything of the sort I've mentioned (and used the rings supplied in the assembly) - big mistake! With only 3 years of work, its smoking. It will get the same treatment I've mentioned here and hopefully will be like the others I refurbished without the 'new parts'. The rings will be Hastings - without exception.



Hope this helps,

Mark Hudson

[Deleted User]

Dave,



Bore it. You are looking at .038-.040 gap on the top ring when piston is at top. If you don't bore it, clean your vent tube coming out of the back valve cover for easy access for you "blow by"



Randy

SuperDave

I agree!

Every time I have tried to "save a buck" it bit me! I ordered a set of .020 from Dale Cooper and will do it right the first time! He has copies of the original without pins. Not cheap.. But...what IS cheap now adays?

Thanks for all the advice.

Live in the Past... It's cheaper!

Dave