Main Cap Query

[AlfaLofa]

I have an old block which I aim to refurbish.

I've just had a look at the underside and only 2 of the main caps are stamped - numbers 4 & 5.

However number 4 cap is fitted to number 2 bearing and number 5 is fitted to number 4.

What (if any) would have been the implications of this??

The caps on bearings 2, 4 and 5 are all cast with the letter "R" as opposed to the "F" and the "C" on the front and centre caps.

PS - can new steel caps be fitted without line boring ??
(Edit - I've just spotted on Burton's web site that line boring is required with new steel caps)

[rgh0]

The caps would have been stamped for the bearings they were originally line bored for. Either your block has been reline bored and the bearings not restamped which would be unusual or the bearings have been put back in the wrong location or the block has been cobbled together from parts. You really need do a test measure assemble to see how well the caps line up with a crank in terms of bearing clearances and whether you need to line bore it again.

Yes the front and centre caps were distinct, the other 3 are identical and the other 3 all have a R cast in them. All the caps also have an arrow with a F indicating the front direction for installation

cheers
Rohan

[AlfaLofa]

Thank you Rohan.

Not knowing what has happened previously to this engine I think I'll just get new main caps.

If it (possibly) needs line boring anyway - I might as well get the boring done with stronger caps?

[oldchieft]

When fitting caps you need to use some judgment.

In my engine when I spun the crank with just the mains tightened there was a bit of drag, after some investigation I found if I swopped two of the caps and put them in the wrong position the crank spun free.

So it was a case of have it look right, or have it work right!

It has been running with the caps in the wrong place for about twenty years.

Jon the Chief

[elan_fan]

That always used to be one of the standard tests when rebuilding an engine and was considered perfectly acceptable. Modern tolerance and methods make all these things seem a bit of a bodge.

[AlfaLofa]

Mmmm !

I only received this engine in early December so......I have no idea as to what has happened to this engine in the past.

So the "misplaced" caps might have been deliberate !!

It's a pity that they weren't re-stamped if it was deliberate.

The "quantity" of information I receive appears to be directly proportional to my confusion

[rgh0]

Ys you need to do some measuring and investigation work to try to determine if the caps as installed were ok or whether they can be made OK by swapping around or whether you need to re line bore.

If you can't get acceptable bearing clearances and a free spinning crank with the caps in any combination of install positions then you need to line bore.

If you decide to line bore then the additional cost of doing it with new steel caps is not great but also not really necessary unless you are developing the engine. I assume the caps you have are the earlier style round caps not the later square caps. Even for a highly developed engine running at 8000 rpm plus the later square main caps are fine

cheers
Rohan

[AlfaLofa]

Hi Rohan - the block is a 701M with square mains

[ricarbo]

When I rebuilt mine, many years ago, I had a small green book by Dave Vizard on the twin cam. Sadly, i can't find it. I'm pretty sure it said to put the crank in (without connecting the rods), wrap string round the fanbelt pulley and tie a 5 pound weight to the string. If that would rotate the crank, then you are good enough. Memory being what it is, I can't be categoric that the weight required is 5 pounds.
regards
Richard

[rgh0]

Hi Rohan - the block is a 701M with square mains

See if you can get OK bearing clerances by swapping the caps around if not then line bore the existing caps.no real need to get new ones

cheers
Rohan

[RichardHawkins]

Steve/Richard

I have the book you mention by David Vizard. Page 10 shows what you describe, but with a 1lb weight.

Richard Hawkins

[AlfaLofa]

Good news - panic over.

I've cleaned a bit more gunge from the underside of the block and I've noticed that the block is stamped with a 4 and a 5 adjacent to main caps 2 and 4 respectively.

(In fact the 5 is just about discernible in the above photo).