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low compression has me stumped

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Norm58mga Norman Ellis
Hornell, NY, USA   USA
I have a 72 b engine with an 18v head that I'm rebuilding for my A. Bottom end bearings, new pistons and rings, new hardened valve seats with the valves lapped and the the head shaved by the local NAPA machine shop. I checked the compression before trying to start the motor - 130# in all cylinders except #3 which is at 70#. I squirted oil in all the cylinders which had no effect on the compression. I took the head back to NAPA and he pulled it back apart, relapped all the valves and said there is no problem here. (he says the hardened seats can't be ground because of "galling" and that the valves will seat fully when the engine is running). I pulled #3 piston to check that there were no broken rings and then reassembled with a new head gasket - #3 cylinder is still at 70#.

I'm going to get the engine running but I have no confidence that #3 will straighten itself out.

Thoughts or suggestions? Thanks, Norm

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Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
Before disassembly you could've filled the offending cylinder with some oil. IF compression jumps, it surely is the piston or its rings at fault.
If nothing changes the valves for #3 are at fault . Why your machine shop did not perform this basic test eludes me... confused smiley

Of course any difference in compression ratios so great is not OK, 10% max difference is the rule, so 130-10%= 117psi minimum

It is still not too late to do the simple test on cyl #3 I described above, you've got nothing to loose IMO and just talking may get you nowhere. If it does not help back to the head and the valves
But if it helps compression jump much higher, a frustrating disassembly is imperative of course eye rolling smiley

In reply to # 3694451 by Norm58mga I have a 72 b engine with an 18v head that I'm rebuilding for my A. Bottom end bearings, new pistons and rings, new hardened valve seats with the valves lapped and the the head shaved by the local NAPA machine shop. I checked the compression before trying to start the motor - 130# in all cylinders except #3 which is at 70#. I squirted oil in all the cylinders which had no effect on the compression. I took the head back to NAPA and he pulled it back apart, relapped all the valves and said there is no problem here. (he says the hardened seats can't be ground because of "galling" and that the valves will seat fully when the engine is running). I pulled #3 piston to check that there were no broken rings and then reassembled with a new head gasket - #3 cylinder is still at 70#.

I'm going to get the engine running but I have no confidence that #3 will straighten itself out.

Thoughts or suggestions? Thanks, Norm



Edited 2 time(s). Last edit at 2018-03-10 11:06 AM by Donthuis.

mowog63 Avatar
mowog63 Erik Brzoska
Schwenksville, PA, USA   USA
What were the compression numbers before the rebuild?
I ask because the issue may have existed before the rebuild....

Did you put in a new cam? Look at the cam lobes for that cylinder

Did you install new valve springs? Are the springs all within spec?

New guides? If they are new, were they installed correctly and the right height?

Did you put in new or reconditioned lifters? Measure and check heights.

Did you replace or recondition the rockers? Did you replace the rocker arm shaft?

The easiest way to pin point the problem may be to do a leak down test.....but you have eliminated the rings as a possible culprit.

Good luck,
Erik

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course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
You could connect your compressor to your compression tester hose in the #3 spark plug hole and listen to see if you hear air leaking out the exhaust pipe (exhaust valve leaking), the carburetor intake (intake valve leaking), or out the dipstick tube (air getting past piston rings). The air has to be escaping one of those three ways, through a crack in the head or block, or via a bad head gasket seal.

Since you had the head redone and had it checked again afterward, it's probably not the valves/seats or head gaskets. Could the rings on piston 3 have gotten stuck into the piston grooves such that they aren't expanding and sealing?

Was the head checked for cracks during the rebuild? I'm assuming that would be done normally by all machine shops, but, it can't hurt to check.

Mustangsix Avatar
Mustangsix Gold Member Jack Collins
Orlando, FL, USA   USA
Did you check lash again on that cylinder?


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dickmoritz Avatar
dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
Shallow-dish pistons?

Double-check valve lash.

Cylinder leak-down test...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
Wild ass guess is a tight valve guide.

Valve seat sealing is easy to check just by turning the head upside down and filling the combustion chamber with fluid.

Leakdown tester is a great tool, and I encourage such a test, but a sticky valve guide will pass the test because it is a static test and the valve does seat eventually.

While cranking it over, put as big of a feeler gauge as you can in the rocker to valve gap and see it if gets loose or stays trapped as the engine cranks. If it gets loose, the guide is sticking.

Pete

RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
If the valves are not seating now, they never will. Running in an engine won't bed them in. Never heard that one before. A tight guide could, indeed, be the problem. Were they replaced with bronze units? If so, they require more clearance than cast iron guides. RAY

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Not lapping a seat insert sounds like hoo-hoo to me. They are not titanium.
Incorrect rings in one cylinder? Ring up-side-down? Just guessing as I am not an engine builder.



Cogito ergo sum periculoso

Wachtmans Avatar
Wachtmans Wouter Strodijk
OVERVEEN, Noord Holland, Netherlands   NLD
1974 MG MGB "The Bee"
1974 MG MGB "The Bee"
1974 MG MGB MkIII "The Bee"
Don't know exactly why, but I was told that measuring compression should be done with the throttle down to the floor. And indeed found all cylinders to be almost equal and up 50 lbs. Sigh!

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Edd Weninger Avatar
Arizona or SoCal, Overgaard AZ or H. Bch. CA, USA   USA
Aviation style leak down cylinder test, all at TDC will provide the answer.

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3694634 by Wachtmans Don't know exactly why, but I was told that measuring compression should be done with the throttle down to the floor. And indeed found all cylinders to be almost equal and up 50 lbs. Sigh!

I have myth-busted this one personally...makes no difference if the throttle is open or not when doing compression test.

oleanderjoe Avatar
oleanderjoe Gold Member Joseph Baba
Fresno, CA, USA   USA
Hardened Seats Cannot Be Ground. ????? I don't think they come already ground,?? Do they??? and how is a "VALVE JOB" done on a head with Hardened Seats ?????? From what I remember they look like a shinny silver steel wedding ring. The head is cut back, the seat hammered in and "STAKED" around the edge, than the seat is "GROUND" in the normal fashion ????


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Rod H. Avatar
Amity, OR, USA   USA
1964 MG MGB
1968 MG MGB GT
In reply to # 3694687 by NOHOME
In reply to # 3694634 by Wachtmans Don't know exactly why, but I was told that measuring compression should be done with the throttle down to the floor. And indeed found all cylinders to be almost equal and up 50 lbs. Sigh!

I have myth-busted this one personally...makes no difference if the throttle is open or not when doing compression test.

This has been my experience also, on various vehicles. And having the throttle closed is certainly not going to cause one cylinder to be low.



I can't understand why people are frightened of new ideas. I'm frightened of the old ones. John Cage

'63 MGB
'68 MGBGT
'80 VW Vanagon Kombi
'09 Mazda 3 with 5 speed manual

Wachtmans Avatar
Wachtmans Wouter Strodijk
OVERVEEN, Noord Holland, Netherlands   NLD
1974 MG MGB "The Bee"
1974 MG MGB "The Bee"
1974 MG MGB MkIII "The Bee"
Probably it depends on the type of carburators. In this case it was with Webers. No offence though if there are different opinions, just my experience.

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