Rod bearing stampings...C2

[Jim V.]

I have been told original rod bearings are date stamped. I realize this is truely anal, but I am trying to determine if the motor ever saw any lower end work. The stampings found on the back of the bearing cup are as follows:

GMM400
58736

153(Upside down A)5

Can anyone decipher...?

[Clem Z.]

Re: Rod bearing stampings...C2

my guess they are originals because most engine shops used after market bearings because of the cost savings over GM bearings

[Mike M.]

Re: Rod bearing stampings...C2

originals were definitly date-stamped in the 50's and 60's per my experience. mike

[Duke W.]

Re: Rod bearing stampings...C2

Morraine 400 - could be a Julian date, 153rd day of '65, which would jibe with a late build '65. The "53736" doesn't correlate with any part numbers for replacement bearings with are seven digit numbers mostly beginning with 370 and 371 and some with 391 (from my 70s vintage parts catalog). My guess is that they are original.

Are they standard size and what kind of condition are they in?

Do you have a mic and caliper that you can use to measure the thickness of the removed head gaskets and piston compression height? Do the head gaskets have any alphanumberic codes? I'd like to get this data.

Duke

[Terry M.]

Re: Rod bearing stampings...C2

I found date stamps on the reverse of main and rod bearings on both a 1967 SB and a 1976 BB. Neither was in a Corvette. Some of the mains were different size on the top and bottom +/- .001, but I can't recall which engine that was on. It was neat to know I was the first one to be there since the engine was assembled.

[Michael H.]

Re: Rod bearing stampings...C2

Jim,

I have a complete set of new main and rod bearings that were removed from a brand new, never run, 327 service short block in early 1962. The block cast date was November of 61. The info on the back of one of the rod bearings reads as follows; GM M400 8736 on one end and 305D1 on the opposite end. (but not upside down)

The GM M400 and 8736 is on every bearing but the number on the opposite end changes from bearing to bearing. I suppose that would be a date?

305D1, 304H1, 305B1 etc etc......

I'll unpack the main bearings if anyone is interested in the info.

[Clem Z.]

a lot of the factory bearing had odd ball

under and over size stamped on the bearings. over the counter bearings come in +.001,-.001 etc but the factory ones were sometimes stamped +/-.0007,.0012 so the factory had special bearing made to get the proper clearance when new engines were assembled. most after market bearings have the date stamped in mm/dd/yy

[Jim V.]

Duke....Other numbers

Stamped Head gasket
thickness .023
Mc6631 (on one side near top ear)
3830711 (other side near top ear)
GM

Rod ID w/ bearing 2.004
Bearings condition....look like even wear

Piston Heights
1.380 from crown to top of ID of pin
.600 ID of pin
1.380 + 1/2 of .600 = 1.680 Piston Heigh

All measurements were taken with digital caliper of unknown accuracy..not to mention by a novice machinist..(me)..

[Duke W.]

Re: Duke....Other numbers

Thanks for the measurements. The nominal piston compression height is probably 1.675", which is the same as the SHP domed pistons. I've never been able to find a spec for the 250/300 HP piston compression height.

Pin OD is .9270-.9273", so if you measure to the top of the pin and add .486 that should give you another check. The OD is machined to a very tight tolerance for proper fit in the piston. ID tolerance is not critical.

The head gasket seems thick, but maybe GM increased the material thickness to cut CR. SHP engines were double gasketed beginning as a '62 running change, but it's not clear to me when the practice ended, if it did. The Chev. Power Manual says the production steel shim gasket is .018" compressed thickness and I think some aftermarket choices are as thin as .015".

The 3830711 is listed in my '77 vintage parts book as the replacement for all 55-69 265,283,327 (different number for 350s), so it must have been used in production going back to the time your engine was built.

Did you measure it in several different places. Best is to measure several places on several bore hole edges and average the results.

Duke

[Jim V.]

Recheck and Deck Height calc...

OK Duke..

After measuring Head gaskets in 8+ places...the average is .025

The pistons heights are:
2.135 (piston crown to BOTTOM of pin) MINUS .465 (.9271/2) = 1.670

Seems to me in some of your prior help you gave me a nominal for the 300hp at 1.671 which seems to be confirmed....!

SO...given these numbers my computed deck height is:
Bank Piston to Deck + Piston comp Ht + Rod L. + crank throw = Deck H.
Left .029 1.670 5.7 1.625 9.024
Right .023 1.670 5.7 1.625 9.018

Given a nominal of 9.025 seems measurements are very realistic....

[Duke W.]

Re: Recheck and Deck Height calc...

The F-M Speed Pro forged flattop pistons (2165s) are speced at 1.671" compression height and -5.4cc volume. With a composition gasket and typical as machined deck height the CR only comes in at about 9:1, which is waaaaay to low for a high overlap SHP cam - unless you want to run it on regular unleaded, which is not a good idea for a performance engine.

The combination of high overlap and low compression will make for a very poor torque curve. SHP engines can handle a true 10-10.5 with today's unleaded premium. That's why I don't recommend the 2165s.

For a 300 HP engine the OE type eutectic or modern hypereutectic pistons are cheaper and should be fine as long as you don't set the ignition map up to detonate all the time. The F-M OE replacement type 327 pistons seem to be available only in a low compression height, low compression version, so hypereutectics are the way to go to get the CR up into the 9s.

My hunch is that the compression height for the OE cast pistons is the same as the OE domed pistons - 1.675", which is about the average of your two measurment methods. I still don't know the net piston volume, but with a compostion gasket these pistons should yield low 9s and mid to high 9s with a shim type gasket, depending on machined deck height. Yours are pretty close to nominal, but I think most are 5-10 thou high, which is about 0.1 to 0.2 less compression than if deck height is the nominal 9.025", which would yield .025" deck clearance with a 1.675" compression height piston.

Your measurement of the gasket thickness is a lot thicker than the .018" specified in the Chev. Power Manual, but I'll take your measurements at face value.

Duke