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MG MG Y Type - A Question of Balance

I'm in the beginning phases of an XPAG rebuild. My four conrods w/caps and bearings all weigh within 2 grams of each other at 626 grams. Is this close enough, or should they agree even closer to provide proper balance?

What do you think Willie?

Rocky
Rodney C "Rocky" von Dullen

Hi Rocky
Nice to see you haven't been blown away with all that weather you've had over your way-
2 grams is pretty good, hardly worth worrying about, but just depends what you're up to-
If you're just checking then yep all's well but if you're after a project you could perfect them if you wanted
If you are going to do them you need to check the weights of each end of each rod to determine where to remove weight
Also you need to take into consideration if you're going to replace the bigend bolts---I've had these from reputable brands that have ranged over .5 gr difference so that can throw things out as well if you're trying for '0 ish' so if replacing them you need to match balance them before fitting otherwise weight is being taken from the rod for no reason when it's actually the bolt that's out
Even using the old bolts I'd be tempted to get them out and match/balance them first before attacking the rods themselves-
But 2grams is ok, it takes about 15 grams to make a feelable vibration --most modern day cars cranks are balanced to under 5g not many would be under 2g so you're pretty good as is really
Just depends if you want to have a play with them or not
A really good race balance would be under .5grams variation in each end and total balance of the rods but that's getting fairly fussy
Have fun
willy
William Revit

Check too for straightness/true and wear on the ends Rocky. I have plenty of spare used ones if you suspect wear or faults. Check too cam followers for excess wear inside and outside on ends and your underside of the valve train too. Remember to reassemble in same sequence/order for end caps and push rods unless you are fitting new caps, rods and have had cam serviced. Delta Cam here in Tacoma WA do a great job servicing cams and I have sent them a few Y cams over the years for clients from around the world and they were able to provide a faster and cheaper turn around than the original customer host country could get. See links page for a link to their site.
Paul Barrow

Hi Rocky My big end bolts had been overtightened and you could see the threads where narrower in the middle. Head studs where no good ,,Check everything....
Bob Wood

Dear All,

Paul thanks for the kind offer of used rods. I'll see what the machine shop sys after checking mine for warp, cracks, etc.

This is one tired old XPAG: dry liners fitted, bored to +0.060" over; crank turned to 0.040" under and head milled to 73mm thickness with "big" late TD/F valves fitted. The CR must be over 10:1! The piston rings on cylinders 1 and 2 came out in MANY pieces! Rod bearings were pitted like the surface of the moon. Hopefully the crank will pass magnaflux testing!

I had the oil pump, rocker gear, carbs and dizzy rebuilt after I pulled the engine some 30 years ago. Also purchased many new parts for the rebuild at that time including a Crane cam and shortened push rods, tappets, timing gears/chain/tensioner, cam bearing set, clutch/pressure plate and valves/springs. In short, I'll replace every moving part. I have new ARP bolts for the rods caps and wrist pins. I'll order pistons, rod and main bearings and a Cometic head gasket after machining is done.

Willie, this engine will rarely see 4000rpm during my life, so I'll be happy with +/- 2 grams on the rods. The machine shop will balance the crank assembly as part of their work.

Rocky

Rodney C "Rocky" von Dullen

Rocky
It'd be interesting to weigh your new b/e bolts just out of interest---little project and easy to just grind a little off the end to match them up if needed-
Also when you're fitting your pistons to the rods, to tighten the pinch bolt it's best to have something like a large screwdriver or similar in the vice to slide your gudgeon pin over to hold it while tightening up----it's a no no to use the bigend end of the rod for holding it while tensioning up the pinchbolt
just thought i'd spit that out while it was in my head

willy

ps - just out of interest, the pitting of the bearings you mentioned --usually that's caused by leaving an old engine sitting for ages with used oil in it--the acids from combustion that gets into the oil seems to thrive in the tiny spaces like between a crank and bearings, or even the face of cam followers pitted, it's quite common to dismantle an old engine and find the bearings eaten away--i've even seen new/old stock bearing attacked by whatever there is in the greaseproof paper used in the packet to 'protect' them-
something to check if you're buying old stock bearings.
William Revit

Willy,

The ARP b e bolts all weigh the same, up to the limit of my balance. At $180USD per set of eight, they better be! You might enjoy reading about the company and their manf processes https://arpcatalog.com/

I fabricated a pair of brass buttons as diagramed on page 24 of Wood's XPAG Engine booklet http://www.mg-tabc.org/library/the-xpag-engine-wfk-wood-rewrite.pdf

These allow you to grip the wrist pin in a vise while removing/installing the wrist pin bolt. BTW, I have ARP allen socket head bolts for these as well.

Don't know if you are familiar with Cometic head gaskets. The XPAG version is in three pieces: two solid copper figure eight cylinder seals and a composite sheet for the water/oil passages. The copper seals a sized to the exact cylinder [over]bore and thickness based on how much the head has been milled. Cost about $120USD each with 4-6 week delivery. See photos.

Yep, sitting in old acidic oil for 30+ years probably caused the rod bearing pitting. Interestingly, the main bearings were not pitted and showed little wear.

Rocky





Rodney C "Rocky" von Dullen

Just supporting Willy's comment about tensioning the pinch-bolt. I am sure that many rods are twisted because people tension the pinch-bolt while holding the big-end in a vice. Doing that can twist the rod!

My method of checking con-rods for straightness might be a bit crude, (see image) but it seems to work. Whatever your method, do check they are straight.
Bob.

Bob Schapel

Perfect rod checker Bob, Probably more accurate than some of the bought ones which use the tunnel of the be but the side face of the little end--leaving room for error.

Rocky
Yep, well aware of ARP ,I wasn't going to mention the name of the set that was mismatched that i was supplied but I'm glad yours are good- Always pays to check- I don't take anybody's word anymore-check check check.
When I detuned my elan getting it going for road use i went for a Cometic MLS gasket--ordered it on their custom gasket page to suit what was needed -big bore, required thickness in multi layer steel, pressed the button and blow me down it came back instantly with a custom part number and said up to 2 weeks to manufacture- I got a real surprise when it arrived 6 days later down here on the other side of the planet--excellent service and has been faultless.
Cheers
willy
William Revit

Hi Again I had my crank crack tested and machined.. On assemble I found a clearance problem so after checking sizes it was found that the rods where oval on the big ends so they where sized. This fixed the clearance problem Check everything...
Bob Wood

This thread was discussed between 20/10/2022 and 23/10/2022

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